Architectural Overview

Table of contents:

Architectural Overview

This document provides a high-level overview of the various classes which together comprise the Sheets Hypercode Environment. While it cannot claim to be comprehensive or even complete, it should give enough grounding in the basic concepts to allow informed browsing of the code itself. Ultimately, of course, the code is the definitive description of the system.

If you have questions about the code and architecture which are not covered by this document, feel free to ask for clarification. For the forseeable future, many of the original authors will be reachable via

Caveat: The Gwydion project strongly advocates evolutionary design of software systems, and Sheets reflects this in two different ways. Firstly, we have striven to produce an environment which simplifies understanding and extension of complex systems, even when they have not been created via formal design techniques. Secondly, Sheets has itself evolved from a small demonstration project into a full-featured Java environment, and has gradually evolved from JDK1.0 into JDK1.1 (and potentially JDK1.2). This has resulted in a far more powerful system than we could have ever envisioned in a single design phase, but has also resulted in code whose organization is less than perfect. In addition, 95% of Sheets was coded using Sheets. For better or worse, this has allowed us to easily create, maintain, and extend classes which might be quite unwieldy when viewed from a lesser environment. (We don't actually advocate development of 1000 line classes, but Sheets has no problem making sense of them.)

1 Fundamental Concepts

1.1 Fragment

The Fragment class comprises the basic data model for the Sheets environment. Each fragment is a persistent ArchivalObject which represents some user-level program entity. The user may view and manipulate them through Viewers, or create new ones via LanguageExperts.

The exact granularity of Fragments have been chosen on a somewhat ad-hoc basis, but they obey one basic principle: each fragment should be as small as possible while still representing an independently meaningful object. For example, we find it reasonable to divide Java code into distinct methods and fields, but not to further divide into statements and expressions. Similarly, documentation is divided into sections and paragraphs, but not into individual sentences.

The fragment model embraces a fundamental dichotomy. They are typically viewed and edited as text, and can even be changed in fairly arbitrary manners. On the other hand, they are still structured objects, which can store abitrary internal state that need not be displayed to the user. This hidden information can be selectively revealed to the user as attributes or via alternative views or simply maintained as internal state to support other operations.

Given the fundamental nature of Fragments, they have a fairly complex extension protocol. However, there are a small core of methods which all subclasses of Fragment are expected to implement:

Methods which create new Fragments must make sure they are registered with SentinelMgr via brandNewFragment. Note however, that the Replacement mechanism automatically registers all fragments passed to replaceWith, so nothing need be done for replacements.

1.1.1 Viewing

Each fragment supports a number of different views and is responsible for generating Viewers to display the fragment in accordance with these views. In order to support common viewing modes (and auxiliary functions such as searching) they are also responsible for generating Strings representing a set of standard views.

Most fragments will support viewing by extending the methods standardViews, makeView, and toString.

1.1.2 Replacement

Although fragments may be modified by changing attributes or (for container fragments) by changing their contents, most editing operations end up instead replacing a fragment with one or more new (but typically related) fragments. Although this seems counter-intuitive, it is what allows us to split one paragraph into two by putting blank lines in the middle, or to convert a Java method into a comment by adding "//" at the beginning of each line.

Most fragments will support editing and replacement by overriding the replaceWith method. This routine takes the string produced by a user's edit and converts it into a set of fragments. It should then call a different lower-level replaceWith method to perform the actual replacement.

These fragments may also wish to extend pickSuccessor to designate a primary replacement, or the sameInterface and affectedBy methods so that the user can be notified of any potentially global effects the replacement might have.

Fragments which support the "extend-fragment" command or auto-extension should implement newExtension and isEmptyExtension.

1.1.3 Attributes

Each fragment can define arbitrary "out-of-band" data attributes which the user can view and manipulate. These can either be scalar or list-valued attributes. Scalar attributes are integral parts of the Fragment itself (such as the package name for a Java fragment), and are typically viewed and manipulated as text. List-valued attributes contain references to an arbitrary number of other self-contained fragments. They may be constrained to contain Fragments which satisfy certain properties.

Attributes are specified via the supportedAttributes method, which returns a Vector of Attribute objects. Fragments which are created by replacement can copy attributes from the original via the inheritAttributes method, while brand-new fragments can encourage users to specify values by listing them in attributesShownOnCreation.

Although any attribute of a fragment can be retrieved via the supportedAttributes list, the findListAttr method can provide a potentially faster shortcut for retrieving list-valued attributes.

1.1.4 Persistence

Sheets provides two different mechanisms for making Fragments persistent. The load/store mechanism writes binary data to the active database, while the load/save mechanism dumps data to an ASCII interchange format which provides a backup and DB evolution tool.

The first mechanism is based upon the PObject interface, and is typically extended via the loadData and storeData methods. Persistent members will be manipulated via DB.lookupID and getID methods. The second method is based upon the ArchivalObject class and is typically extended via getArchivalType, saveCoreInfo, saveAuxilaryInfo, loadAuxilaryInfo, finalizeLoad, and isDumped.

1.1.5 Notification

Whenever something happens to a Fragment, be it replacement, modification, or simply a change in status, Sheets notifies various other objects by passing them appropriate events. Sentinels, Viewers, and other PObservers can all receive FragmentEvents when fragments are replaced, changed, or edited. Others may receive ContainerEvents when fragments are added, removed, moved, or replaced within FragmentLists.

In addition, Sentinels can receive AcensionEvents or DescentEvents whenever any operation changes a fragment's plane of existence. The planeOfExistence denotes the fragment's reachability and how it is referenced. This indicates whether the fragment is semantically part of the program, or is dead, or whatever. Possible values are IN_THE_PROGRAM, HANGING_OUT, GONE_BUT_NOT_FORGOTTEN, or WAY_TOTALLY_DEAD.

1.2 Attribute

Instances of the Attribute class are used to provide controlled access and viewing of various interesting aspects of a Fragment. ScalarAttributes provide access to various parts of the Fragment, while ListAttributes describe arbitrary lists of other Fragments that are somehow associated with the Fragment. Note that Attributes objects don't actually contain values -- they provide access to the values stored in the Fragment.

Each Attribute is created with a name and an associated Fragment, which may be accessed via getName and getFragment. Optionally, they may cause attribute flags to be shown in the sidebar by extending isInteresting, or prevent modification of the attribute value by extending isWritable. (Note that you typically achieve the latter by simply inheriting from StaticScalarAttribute.)

The ScalarAttribute class provides the ability to display and potentially modify a string describing some aspect of the Fragment. You must override getValue to retrieve the value from the Fragment and may override setValue to modify it when the user specifies a new value. Note that setValue can't be called directly, but can be called indirectly via changeValue.

The ListAttribute class provides capabilities to retrieve and display named FragmentLists associated with the Fragment, or to create them if they don't yet exist. You can often use this class directly without subclassing it. However, you can still extend it to provide specialized behaviors. Most commonly, you will extend makeConstraint to force any newly created FragmentLists to constrain their values via a ListConstraint. You may also override defaultView to specify how the contents of the FragmentList should be viewed by default or isUnordered to specify that two lists can be considered effectively identical even if they are in different orders. Very occasionally, you will need to extend makeList to provide special actions when a new FragmentList has to be created.

1.3 Sheet

The Sheet class embodies the most common variety of collecting Fragments together. It contains a String value and a FragmentList which comprises its contents. Sheets are most commonly displayed and modified through a SheetViewer, which gives a file-like textual view, or a TOCSheetViewer which gives a compact, narrow summary suitable for quick navigation.

1.4 FragmentList

As its name implies, the FragmentList class manages ordered sequences of Fragments. This list may comprise the contents of a Sheet or the value of a list-valued Attribute. The sequence of fragments be explicitly modified (and displayed) via a ListViewer or automatically maintained via a ListConstraint.

Each FragmentList has various simple properties. The name field describes its function, while getOwner returns a unique Fragment which "owns" the list. The list is regarded as "interesting" if the user has explicitly modified it in some way, and may be forced to contain Fragments fitting some arbitrary desciption by associating a ListConstraint. The appropriate methods are isInteresting, makeInteresting, getConstraint, and setConstraint.

The Fragments contained within the list may be accessed via numFragments, getFragment, and getFragments. The contents may be changed via a wide variety of methods which are, for the most part, self-explanatory: addFragment, addFragments, replaceFragment, removeFragment, and moveFragment.

A FragmentList may be "frozen". This is a feature intended for use in version control, and forces the list to contain specific version of Fragments, even if they are later modified, replaced or deleted. The methods isFrozen, setFrozen, getRawFragments, and getActualFragment.

1.5 SheetViewer

The AbstractSheetViewer class is a ViewContainer which optionally shows the value a Sheet, and can be opened to show the contents in a ListViewer. (Like most instances of Viewer, it can also show Attributes of the Sheet.) Usually, you will use a the SheetViewer subclass, which will show the contents in arbitrary views specified by the user. The special TOCSheetViewer subclass is restricted to showing "toc" views for the sake of compactness.

1.6 Viewer

The Viewer class is probably the most complex class in the Sheets system, since it serves as the primary interface between the user and the Fragments which make up his program. It serves as the "view" portion of the MVC paradigm, and as such interacts closely with the Fragment class (which it is viewing) and the ViewPanel class (which provides high-level control). It also handles some control functions which are delegated by the ViewPanel.

Viewers closely mimic AWT components, but do not inherit from the Component class. This is because we needed to have full double-buffering support and found that early versions of AWT could not perform proper buffering on nested Components. It is unclear whether the combination of lightweight components and Swing would now suffice, but we are in no rush to retro-fit the code, especially in light of Swing's potential performance problems.

Viewers serve many different functions: layout, display, view customization, editing, mouse-event handling, attribute display, and context-dependent help.

1.6.1 Layout and Display

Most Viewers can support window layout by simply extending the baseHeight method so that enclosing ViewContainers know how much vertical space to allocate for them. When any operations change this height, they should call invalidate so that the layout can be recomputed. Viewers which wish to display non-standard information in the sidebar should consider extending controlBlockHeight, while viewers which contain sub-views may need to extend contentHeight.

In order to display themselves in the window, most Viewers need to extend the paintContents method, which should paint the "base" value and recursively paint any "contents", but need not concern itself with attributes or highlighting. Some special Viewers may wish to implement non-standard behavior by overriding highlightSelection, paintBackground, or drawControlBlock. You should only override paint if you wish to drastically change the way a fragment is displayed or if you need to do some special preparation before the fragment is painted.

1.6.2 View Customization

Viewers adjust their appearances according to several different states. They have a view which controls the gross appearance or verbosity of the display, and any number of view modifiers which permit minor customizations. Many viewers tend to support the standard views "header" and "full", while other standard views such as "toc" and "summary" are typically handled by special-purpose Viewers.

Each viewer should extend getView to return the name of the current view, and extend setView to change to a potentially different view. Note that setView is allowed to simply call the superclass method which will create a new Viewer to handle the specified view.

Modifiers are handled by a suite of functions which specify possible modifiers and their status and which allow the modifiers to be changed. The former include supportedModifiers and showingWith, while the latter include showWith and showWithout.

1.6.3 Editing

For the most part, both display and editing is handled automatically be inheriting from TextualViewer or RichTextViewer. If you inherit from the latter, you may wish to extend RichTextViewer.restyleLines so that the styling can be updated whenever the text changes. Some viewers might also wish to extend TextualViewer.desiredIndentation to handle automatic indentation or TextualViewer.fragmentComplete to support auto-commit.

1.6.4 Mouse Event Handling

Although ViewPanel is responsible for handling all windowing events, mouse clicks are typically passed on to the containing Viewer for interpretation. The methods leftClick and doubleClick tend to perform selection actions, and can usually simply be inherited from Viewer. rightClick and clickControlBlock are used to pop up context-sensitive and view-modification menus. You probably needn't override them, but you will likely want to extend fillContextMenu to include any commands specific to this Viewer or the Fragments it views. middleClick typically implements a smart "goto-word" capability, and is optional.

1.6.5 Attribute Display

Attribute display, editing, and modification are all handled automatically by the Viewer class. You only need extend any methods if you wish to prevent attribute display or editing for a certain Viewer class, in which case you should override hasAttributes or canBeEdited.

1.6.6 Context help

Context help, like the middle-click "goto-word" capability is optional, but may be worth supporting for Viewers with strong semantic knowledge. You do this by overridding showContext with a function that examines the selection and calls newContextInfo with information about that selection.

1.7 ListViewer

The ListViewer class is another major workhorse class. It is the primary class used for viewing and manipulating FragmentLists (whether they occur as Sheet contents or list-valued Attributes). It is both a Viewer and a ViewContainer -- it holds the subviews for all of the Fragments contained within the FragmentList and thus handles layout and delegation for each of the contained Viewers.

ListViewers always lay out their subviews in the default vertical tiling scheme favored by Sheets. More exotic layouts for Graphs or Tables are handled by other ViewContainer classes.

1.8 Query

The Query class implements a framework for performing queries -- i.e. finding a complete list of Fragments which match a certain arbitrary criterion. Queries are typically added to the QueryDialog by an appropriate LanguageExpert. Queries may operate by searching through the entire set of Fragments known to Sheets, or they may accelerate their searches by using information gathered by associated Sentinels. They may also supply ListConstraints for any Sheet created from the query results.

Each subclass should implement one or both of the methods fitsQuery and quickSearch. The Query class will then perform a quick search to find all potential matches, possibly weed out any fragments that don't appear on a particular target Sheet, and finally call fitsQuery on any fragments that remain.

Subclasses should override the toString method to provide a plain English description of the query. They can also suggest a string for search highlighting by overriding whatToHighlight, return the results in some preferred order by overridding sortResults, or provide a ListConstraint for any associated Sheet by overriding createConstraint. (If createConstraint returns a GraphableConstraint, you should also override graphIsUsable method to return true.)

After creating a constraint, callers can use getQueryResults to retrieve a Vector of results.

2 Support Classes

2.1 ViewContainer

The ViewContainer interface describes a set of operations supplied by any class which can contain nested Viewers. This includes facilities for redrawing, recentering, navigating through, or updating the layout for the subviews, replacing Viewers with alternate versions, and specifying the default display characteristics for subviews. Note that, because of the potential for displaying attributes, all Viewers are also ViewContainers.

2.2 ViewPanel

The ViewPanel class is an AWT Component which displays the contents of a Sheet or other ContainerFragment. As such, it anchors an arbitrary hierarchy of Viewers, and serves as an intermediary between the true AWT Components comprising windows, scrollbars, etc. and the lightweight imitations represented by fragment Viewers. ViewPanels may be accessed from any Viewer via the Viewer.getViewPanel method.

ViewPanels handle several sorts of responsibilities: double-buffered display, scrolling, event handling, and selection management.

2.2.1 Display and Scrolling

The most common display-management operation is update, which takes responsibility for making sure that all contained Viewers are properly formatted and then redraws the screen. Other display functions are handled by an inner class named Painter, whose single instance is stored in the painter field. This class mostly operates behind the scenes, but is reponsible for maintaining the double-buffer canvas, tracking the size of the panel, and calling appropriate paint routines on the contained Viewers. Most external callers will only be interested in forceUpdate, which forces an immediate repaint of the window rather than queuing it for eventual painting, and getMetrics which grabs the font metrics used for character painting. (We recommend using this routine rather than calling getFontMetrics directly, since the latter can sometimes produce unreliable results under JDK1.2.)

Scrolling is handled by the inner class Position, whose single instance is stored in the position field. It is responsible for updating and responding to the scroll bar and through the getFirstLine method, tells the painting routines which portion of the contained Viewer should be painted. Other classes can affect the positioning directly by calling doLineUp, doLineDown, doPageUp, and doPageDown, or can make sure that desired locations are visible by calling recenter or recenterCursor.

2.2.2 Event Handling

ViewPanels intercept all AWT events, and can thus be considered to be the "Controller" portion of the MVC paradigm. However, some events are simply passed on to Viewers for specialized interpretation. Event handling is handled by the inner class Controller, whose single instance is stored in the controller field. For the most part, this class can be considered a black-box, but you may find yourself wishing to set the goingToContext, middleClick, or openingSheet fields. Setting these fields trigger certain actions when the mouse-button is released, and thus work around a bug in some JavaVMs that causes very strange results when windows are created while the mouse-button is still pressed.

Most events handled by the Controller end up either setting the selection (described below) or executing EditCommands. Keystrokes are generally converted directly into editing actions, while mouse clicks are passed on to the selected Viewers which will determine and execute an appropriate action. Viewers can customize behavior by specializing the methods leftClick, middleClick, rightClick, doubleClick, and clickControlBlock.

2.2.3 Selection Management

Each ViewPanel maintains a Selection which denotes either multiple contained Viewers or a portion of a single contained Viewer. This selection is queried by calling getSelection and hasMultiSelection, and updated by calling setSelection or by calling methods on the Selection object returned by getSelection. Further information can be obtained from selectedSheet, selectedList, and selectedContainer, which seek to find an appropriate object which contains the current selection. There are also a variety of operations which perform some operation upon the current selection and which were placed in ViewPanel for lack of a better place. These include removeSelectedFragment, destroySelectedFragment, cutSelectedFragment, pasteFragment, copySelectedFragment, copySelection, cutSelection, and deleteSelection.

2.3 LanguageExpert

The LanguageExpert class attempts to act as the sole point of contact between a family of Fragment classes and the rest of Sheets. It serves as a delegate for many different subsystems, including fragment creation, persistence, viewing, queries, commands, sentinels, importation, exportation. While this seems like a great deal, it should have the happy effect of allowing a new family of Fragments to be added to the system by simply adding a single statement to LanguageExpert.initializeAllExperts.

2.3.1 Persistence

Each of the two persistence mechanisms supported by Sheets require an arbiter to translate an arbitrary object/type token into a set of actions for loading an actual object. The "dump/undump" mechanism relies upon persistenceTypes to specify a set of tag strings which are understood by the expert and loadFragment to call appropriate loading routines for any dumped fragment which is labelled by one of those tags. The "load/store" mechanism relies upon loadableClasses to provide a complete list of PObject classes associated with the family of Fragments.

2.3.2 Viewing

The defaultViewModifiers method specifies a list of view modifier strings which are applicable for fragments controlled by the expert. It needn't list all such modifiers -- just the ones which should be turned on by default.

2.3.3 Queries

The makeSomeQueryPanels method creates a set of QueryPanels which can be plugged in to the general QueryDialog menu. Each of these panels collects data with which to create a Query object which can the be executed to return a set of Fragments.

2.3.4 Commands

The editCommands method returns a list of EditCommand objects listing all specialized commands which operate on Fragments controlled by the expert.

2.3.5 Sentinels

The sentinels method returns a list of Sentinel objects which can perform arbitrary semantic analysis and processing for Fragments controlled by the expert.

2.3.6 Importation and Exportation

File importation is controlled by two methods: filenameExtensions returns a list of strings which are typical file extensions for Fragments controlled by the expert. importFile takes a filename with one of these extensions and converts it into a list of Fragments.

File exportation is supported by toObjectFilename and matchingExports. The first function maps from source-file names to object-file names to facilitate our Makefile support. The second one converts an arbitrary keyword to a set of "relevant" exported files in order to support keyword substitution in the "compile" dialog.

2.4 QueryDialog & QueryPanel

The QueryDialog class is used to create a single dialog which allows the user to access all generally available Querys. It is extended through LanguageExperts, which can each supply an arbitrary number of QueryPanels.

The QueryPanel class is closely linked to Query. Any user-visible Query will have an associated QueryPanel which collects a set of parameters and then (upon request) creates an instance of the corresponding Query. Any QueryPanel which is registered by a LanguageExpert will automatically be included in the QueryDialog.

2.5 EditCommand

The EditCommand class provides a universal framework for performing some single operation which may affect the Sheets database. EditCommands may easily be bound to keys in the user Profile, or placed in menus. Most commands are typically registered by an appropriate LanguageExpert.

When EditCommands are invoked, they are passed a target ViewPanel and a selected Viewer. The command may provide facilities for checking its applicability to the targets and executing an appropriate action. The framework automatically records any changes for later reveral via the UndoDialog.

Commands may inherit special behaviors by extending the classes ContainerCommand, ChangeContainerCommand, LethalChangeContainerCommand, CursorMotionCommand, or ChangeContentsCommand or by inheriting from the interfaces MultiCommand, IterativeMultiCommand, SpecialMultiCommand, or NoArgCommand.

2.6 Profile

The Profile class provides a placeholder for any static variables which can modify the general behavior of Sheets. In addition, it encapsulates facilities for setting the values of these variables and mapping key-bindings based upon the contents of the user's .sheetsrc.

Whenever you add a new static field is added to Profile, you should edit initVariables to provide a default value and trySet to read the value from .sheetsrc. You may wish to make use of the utility functions getBoolean, getString, getName, or getInteger.

2.7 UndoDialog (and support classes)

The UndoDialog class provides users with a way to undo the results of one or more ContainerCommands which changed the Sheets database. It uses the Checkpoints gathered by the HistoryManager and HistorySentinel classes to determine what the state of the world was before the command was executed, restores the old state, and then attempts to update Viewers to reflect the difference. (The former operations are currently fairly solid and robust, but the latter is known to have some problems.)

2.8 Console

The Console class provides several ways to convey extra information to the user. Most "messages" of any variety are transmitted through static methods on this class.

The most import methods are probably status, which updates the status line within the "context dependent help" window; display, which pops up a message dialog and (usually) waits for confirmation; beep, which lets the user know that he's made a boo-boo; and debug, which just prints to console to assist programmers in finding problems.

3 Semantic Analysis

3.1 ListConstraint

The ListConstraint class can be used to control precisely what Fragments are allowed in a FragmentList. It acts directly to prevent addition of forbidden fragments or removal of required fragments, but relies upon the assistence of some Sentinel to ensure that required fragments will be added as soon as they are created. Constraints can be created as a result of executing a Query or at the request of an Attribute object.

We actually have three categories of fragments: required, permitted, and forbidden. The routines isRequired and isForbidden directly define two of these categories. Whatever is left over is permitted.

Sometimes it is appropriate for a user to be able to get rid of a constraint that seem to be getting in the way. This can be accomplished by overridding the canBeRemoved method. (This is done automatically if you inherit from GenericQueryConstraint.)

The trickiest part of developing a constraint comes in ensuring that any required fragments which are newly created end up in the lists. We cannot automate this in the general case (especially for list attributes), so the creator of a constraint is simply held responsible. Typically, he'll assign a Sentinel to do the job.

For most constraints, it is best to use the AffectedBySentinel. This sentinel collects pretty much all the info which might be relevant and passes it on to the handleChanges routine. All query constraints are automatically added to it's list of constraints. Other constraints may be added via AffectedBySentinel.addConstraint. (We recommend against using it for constrained attributes for efficiency reasons.)


3.1.1 Query constraints:

There will typically be a ListContraint class for each Query class. We have not conflated them because they require different inheritance hierarchies, and because we could conceivable create several constrained lists from a single query.

Extend Query.createConstraint to associate a constraint with a query. Remember that the constraint is persistent, so be careful what you store in it.

Most queries will return a subclass of GenericQueryConstraint, which already does the work of registering with an AffectedBySentinel.

3.2 Sentinel

The Sentinel class provides a convenient extension framework for supporting incremental semantic analysis or maintenance of ListConstraints. Sentinels must be registered via a LanguageExpert, and will then recieve event notifications whenever something happens to any Fragment. Typically it will be extended to detect creation and deletion of some variety of Fragments, and to update its internal records to reflect the new state of the world. (Note that this notification will occur even if the fragment dies a "natural death" through no longer being referenced anywhere.)

Most Sentinels simply perform semantic analysis for Fragments which are added to or removed from the overall program. They can do this by simply responding to AscensionEvents and DescentEvents for which affectsInTheProgram() is true. However, they can receive and can process ContainerEvents and FragmentEvents.

3.3 ContainerEvent

The ContainerEvent class is used to notify Observers and Sentinels when something happens to a FragmentList which will change the way it should be displayed. In practice, a ContainerEvent is how FragmentLists communicate with the Viewers for the lists.

ContainerEvents are created when Fragments are added to, removed from, or moved within a FragmentList. They are also created when the list is totally changed via a global undo operation, or if a contained fragment is replaced. (Note that there is a distinct FragmentEvent which is also triggered for replacements. Both events are always generated, but they are typically delivered to different observers.)

3.4 FragmentEvent

The FragmentEvent class is used to notify Observers and Sentinels when something interesting happens to a Fragment. This is primarily for keeping Viewers in sync with Fragments, although the mechanism is also used by the clipboard and a few Sentinels.

FragmentEvents when Fragments change status by being replaced, modified, changing export state, or being put into edit mode. (Note that there is a distinct ContainerEvent which is also triggered for replacements. Both events are always generated, but they are typically delivered to different observers.)

3.5 AscensionEvent & DescentEvent

The AscensionEvent and DescentEvent classes are used to notify Sentinels of changes to the reachability (plane of existence) of a Fragment. The plane of existence can have on of four values: IN_THE_PROGRAM indicates that the fragment is reachable from the root (Projects) sheet, and should be considered in global semantic analysis. HANGING_OUT indicates a fragment which is contained in some (presumably temporary) container, but is not reachable from the root sheet. GONE_BUT_NOT_FORGOTTEN represents fragments which must be retained in the database, but do not appear in any containers. This mostly represents Fragments retained in the global undo history. WAY_TOTALLY_DEAD refers to fragments which should be deleted from the database and to which you should retain no pointers.

AscensionEvents are created when Fragments become more reachable, typically by being adding to a container during creation, pasting, or global undo. DescentEvents are created when Fragments become less reachable, typically by being replaced or removed from a container. The events contain the Fragment which is affected, and a bit-mask indicating what planes-of-existence are changing. Typically, you will simply call one of: affectsInTheProgram or affectsHangingOut to determine how the reachability changed.

4 Persistence

4.1 DB

The DB class provides a simple object-oriented database which holds PObjects. The interface is encapsulated in a bunch of methods which are static because there can only be one database open at any single time. Allowing multiple open databases might be useful, but then you get into the whole problem of what database is an id relative to and how do you represent cross-database references.

The general idea is that you open a database, use State.getGlobalStateVar and State.setGlobalStateVar to get and set named roots, call commitChanges whenever you want to checkpoint your state, and then finally call close when you are done.

Any PObject that gets stored in the database is assigned an integer ID via assignID. Anyone who wants to reference some database object records the ID for that object. When they need the actual object, they use lookupID to map the ID to an actual object. Object references are indirected through an explicit ID lookup so that having one object in memory does not require that all of the objects it references also be loaded into memory.

There are two magical IDs which you may wish to make use of -- nullID represents the "null" object, while deletedID can be used to warn the system that a reference is dead.

When a PObject is no longer used, it should be explicitly removed via freeID. There is no automatic garbage collection, so items which are not freed will simply accumulate and eat up disk space. Typically PObjects will create a "free" method which not only calls freeID for its own ID, but also cleans up any other PObjects which it owns.

When some detail about a object stored in the database changes, noteIsDirty must be called on the object so that the database knows to schedule a writeback of the object. The typical way this is handled is to have all fields be private to the class and offer getMumble and setMumble methods. The setMumble method just calls DB.noteIsDirty if the field actually changes. The get/setMumble methods can also automatically convert object IDs into objects making it look like the field holds the actual object when in reality it just holds the ID.

DB provides a few extra utility routines: readInt and writeInt provide compact and fast storage of any length of integer. validate and printUsageSummary are used primarily for debugging detecting leaks. The former checks the consistency of the database, while the latter determines how much space is being used up by each variety of PObject.

4.2 PObject

All persistent objects must implement the PObject interface and either implement the Serializable interface or be registered by a LangageExpert. The interface is described in detail below. For information on the Serializable and Externalizable interfaces, see the Java Serialization documentation.

4.2.1 The Interface

The PObject interface consists of four methods. loadData and storeData must do explicit reads and writes for all of the persistent data, calling super as necessary. (If the object is Serializable and not registered, these won't be called, but must still have trivial definitions.) getID returns the ID for the object, calling DB.assignID if necessary. free recursivly calls free on any sub-objects if necessary and then calls DB.freeID to release the DB storage for the object. If Java allowed interfaces to supply default implementations, your life would be a lot simpler, but well, it doesn't.

Something along the lines of:

 private int id = DB.nullID;

 public int getID ()
   if (id == DB.nullID)
     id = DB.assignID(this);
   return id;

 public void free ()
   // recursivly call free if necessary.
   if (id != DB.nullID)
   id = DB.deletedID;

should do the trick.

4.2.2 Fields in Persistent Objects

Most PObjects implement readObject and writeObject routines which explicitly dump each of the simple fields. If they are arrays or Vectors, you'll have to explicitly read and write elements. This is tedious, but immensely faster and more compact than the serialization protocol. You must make each of these classes public and implement a null constructor (i.e. one with zero arguments which doesn't bother initializing the fields which will be loaded).

If you must use the serialization protocol, you should make the class Serializable, and should not register it with any LanguageExpert. You must make sure that all non-transient fields are themselves Serializable. All of the primitive types and many others, like Vector and String, are serializable, so this isn't a big restriction. If you flag a field as transient, the serialization stuff won't save or restore it. You can use this to store cached values in memory without forcing that value to be stored to disk. When the object is read back in, the transient field will end up as the default value for that type (NOT the initial value, if one was supplied). You can also use transient fields along with readObject and writeObject methods to save the field in a different format than the serialization stuff would itself.

Absolutely never store one PObject directly inside another PObject. This won't work because when the output PObject gets written, the inside PObject will be written right along with it. Then when the outside PObject gets loaded back in, a duplicate of the inside PObject will be created. This would result in multiple distinct objects existing in memory for the same ID. Instead, store the ID of the PObject in an int field, and call DB.lookupID whenever you need the object. If you absolutly don't want to call DB.lookupID, then use a transient field to cache the results of the lookup. But DB.lookupID is fast enough that you shouldn't have to bother.

4.2.3 Changing fields

If a field ever changes, you need to call DB.noteIsDirty on the PObject. This queues the object to be written back to the database. You don't need to immediately call noteIsDirty, you just have to guarantee that it will be called. So you can make multiple changes and then call noteIsDirty once, if you want. But calling noteIsDirty should be sufficiently fast that you shouldn't have to. The simplest solution is to just offer setMumble methods for all your mutable fields and have them call noteIsDirty.

The one exception is that you don't need to call noteIsDirty if getID has never been called. This is because objects are only stored into the database if someone asks for the ID for the object. In fact, calling noteIsDirty will cause getID to be called, so calling noteIsDirty will force the object to be stored into the database even if nobody else is actually referencing it.

So if you can guarantee that nobody will have asked for the object ID yet, you don't need to call noteIsDirty. One place this routinely happens is in constructors. Until the constructor finishes, nobody outside the constructor can have their hands on the object to be able to call getID. So unless some super constructor method calls getID (or causes getID to be called) you can skip the call to noteIsDirty.

A common idiom is to offer a protected stateChanged method that checks to see if an id has been assigned, and only calls noteIsDirty if one has. For example:

 protected void stateChanged () {
   if (id != DB.nullID)

Then your setMumble methods can call stateChanged without worring about whether or not doing so will cause garbage to accumulate in the database.

4.2.4 Identity

The database stuff carefully preserves identity for PObjects. What this means is that two distinct PObjects will never have the same ID and two different lookups of the same ID will always result in the same (==) PObject. Well, at least as far as you can tell, that is. If you lookup some ID and then drop all references to the resultant PObject, the database is free to let the system garbage collect that PObject and return you a new one the next time that ID is looked up. But if you keep that original PObject live, the database will always return it for successive lookups.

So you can just use == to check to see if two PObjects are in fact the same PObject.

This also means that you can meaningfully make Hashtables that use PObjects as keys. These hashtables can't be stored in the database because they directly reference PObjects, but they can be used ephemerally during a single session.

4.2.5 The ProtoPObject class.

The ProtoPObject class implements the PObject interfaces, supplies implementations for all the interface methods, and also offers a stateChanged method as described above. (You will, of course still have to implement readObject and writeObject methods to handle all directly defined fields.)

If your persistent object doesn't need to extend something else (Dictionary, for example), you can just extend ProtoPObject and pick up those methods for free. Then all you have to do is make sure you follow the conventions for fields as described above.

The SerializablePObject class implements the above interfaces and also implements Serializable. If you extend this, you won't have to implement readObject and writeObject methods and shouldn't register the class with any LanguageExpert. On the other hand, this will be apallingly slow and wasteful of space.

4.3 PVector

The PVector class is similar to Vector, except that it stores PObjects and is a PObject itself. All of the operations available on Vectors (except the capacity stuff) are available on PVectors. In addition, there is a toVector method for converting a PVector into a Vector, and a free operation to remove it from the DB when the PVector is no longer needed.

4.4 PTable

A PTable is a persistent Dictionary that can map PObject or Serializable keys to PObject or Serializable elements. It uses a linear hashing mechanism to keep the cost of rehashing down and uses large buckets to keep the number of disk accesses per lookup low. The operations are exactly those of Dictionary, with the addition of a free method to clean up the DB when the PTable is no longer needed.

Note: PObject keys will use "getID" instead of "hashCode". The reasons are obscure -- primarily because we need to use an ephemeral hashcode in order to compute the persistent ID.

4.5 PObservable & PObserver

The PObservable class is similar to Observable, except that it correctly handles persistent (i.e. PObject) observers. At present, all the bogosities of the Observable interface are faithfully reproduced in the PObservable interface, but that might change.

Note that PObservable is not a PObject but does implement the loadData and storeData methods. This means that by default, subclasses of PObservable can call super.loadData and super.storeData to ensure that the state of the PObservable will be saved.

PObservers do not have to be PObjects either. If a PObserver is a PObject, it will correctly be handled and the observing relationship will be saved in the database. But if the PObserver is not a PObject, the observing relationship will last only as long as the PObservable is kept in memory. So any non-PObject PObservers should keep ahold of the PObservable itself (not just its ID) to make sure it doesn't get collected and read back in later.

PObservable implements a few utility methods that might be useful for various users. The setChangedAndNotify routine allows any class to make sure notification happen (unlike notifyObservers which relies upon the protected setChanged method). The readStrings, writeStrings, and writeStringsOrNull methods are simple routines for reading and writing arrays of strings. (They don't really belong in this class, but they don't fit anywhere else either.)

4.6 ArchivalObject

The ArchivalObject class represents objects which are persistent and can be read and written to a dump file using a FragmentReader and FragmentWriter. (At this point, this just consists of Fragments, but there could be others.) The ArchivalObject holds the state to be saved, in is subclassed by various kinds of fragments in the database. The split between ArchivalObject and FragmentReader/Writer allows fragment classes to be added without awareness of all the ways they might be archived, and similarly new kinds of archiving to be added without awareness of what all the fragment classes are.

We inherit from PObservable because Java is single-inheritence and Fragment must inherit PObservable.

Subclasses must implement saveCoreInfo and add to the typesTable a LanguageExpert which can load the fragment again. They must also implement an isDumped method which computes whether this particular object should be written out at all.

Subclasses can also optionally override the default do-nothing methods saveAuxilaryInfo, loadAuxilaryInfo, and finalizeLoad. These methods must do their saving and loading by using the interfaces offered by fragment reader/writer.

All archival fragments must either have an objectId or be a LightWeightFragment. You can assign an archival fragment an id by calling useIdFrom or useUniqueId.

4.7 FragmentReader & FragmentWriter

The FragmentReader and FragmentWriter classes provide an abstract interface for reading and writing the non-derived state of an ArchivalObject, assisted by an appropriate LanguageExpert. (This process is sometimes referred to as dumping and undumping.) This is used to provide a compact encoding suitable for data interchange and database evolution.

The only readers and writers currently supported are AsciiFragmentReader and AsciiFragmentWriter, which provide a textual representation suitable for human eyes and typical revision-control systems.

Sheets automatically uses the readers and writers to read and write interchange or version databases. There should be little need for others to call these routines. However, each subclass of ArchivalObject must be extended to support the process. For writing, they should extend SaveCoreInfo and possibly saveAuxilaryInfo. For output, they depend upon a LanguageExpert to provide an appropriate loadFragment routine, but may also wish to extend loadAuxilaryInfo or finalizeLoad.

4.8 LightWeightFragment

The LightWeightFragment class represents ArchivalObjects which are considered equivalent to all other ArchivalObjects of the same class. For instance, all separators are basically the same, so a loadSeparator method would just cons up a new one rather than try to recreate the old one. For that reason, we don't really need to save light weight fragments to disk.

This interface has no real semantics associated with it; it's just useful for doing instanceof checks. This interface is not a true subclass of ArchivalObject because Java doesn't let interfaces extend classes.

The only thing you have to do is write a getObjectId method that returns "lightWeight:whatever" and modify understand it.

5 Java Support

5.1 JavaFragment

Instances of the JavaFragment class represent the various pieces of code which make up a Java program. Various subclasses represent the various different types of code: JavaFragment, JavaFileHeaderFragment, JavaClassFragment, JavaInitializerFragment, JavaConstructorFragment, JavaMethodFragment, JavaFieldFragment, JavaCommentFragment, UnknownJavaFragment.

New instances of JavaFragment are created by JavaExpert, aided by JavaFragmentParser. They are viewed through the JavaViewer class, and inter-fragment semantic relationships are tracked through the JavaSentinel and JavaPackageSentinel. The meaning of the fragments text is interpreted through the JavaTokens class, which breaks it up into tokens and then uses ad-hoc techniques to determine the meanings of those tokens.

5.2 JavaViewer

The JavaViewer class is reponsible for displaying all varieties of JavaFragments. Most of its work is done by simply deferring to JavaTokens for syntax highlighting, indentation, identifier completion, and context-sensitive help.

5.3 JavaTokens

The JavaTokens class accepts a piece of text which is presumed to correspond to Java code and maintains a corresponding set of Tokens. These tokens are used by JavaViewers to perform syntax highlighting. They are also analyzed in an ad-hoc manner to attempt to determine the meaning of the code from these tokens. This information is used to assist with context-sensitive help, identifier completion, and Java-related queries.

It attempts to be as robust as possible, since the code is expected to be applied to code which is in the process of being edited. The modification is facilitated by the two methods replaceLines and replaceMultiLines.

5.4 JavaSentinel & JavaPackageSentinel

The JavaSentinel and JavaPackageSentinel classes work together to gather and store information about all of the JavaFragments in the system. This is used to track the inter-relationships between fragments, and to aid in Java related Queries.