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109 lines
8.0 KiB
Plaintext
109 lines
8.0 KiB
Plaintext
title: TiddlyWikiArchitecture
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modifier: JeremyRuston
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!! Overview
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The heart of TiddlyWiki can be seen as an extensible representation transformation engine. Given the text of a tiddler and its associated MIME type, the engine can produce a rendering of the tiddler in a new MIME type. Furthermore, it can efficiently selectively update the rendering to track any changes in the tiddler or its dependents.
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The most important transformations are from `text/x-tiddlywiki` wikitext into `text/html` or `text/plain` but the engine is used throughout the system for other transformations, such as converting images for display in HTML, sanitising fragments of JavaScript, and processing CSS.
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The key feature of wikitext is the ability to include one tiddler within another (usually referred to as //transclusion//). For example, one could have a tiddler called //Disclaimer// that contains the boilerplate of a legal disclaimer, and then include it within lots of different tiddlers with the macro call `<<tiddler Disclaimer>>`. This simple feature brings great power in terms of encapsulating and reusing content, and evolving a clean, usable implementation architecture to support it efficiently is a key objective of the TiddlyWiki5 design.
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It turns out that the transclusion capability combined with the selective refreshing mechanism provides a good foundation for building TiddlyWiki's user interface itself. Consider, for example, the StoryMacro in its simplest form:
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{{{
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<<story story:MyStoryTiddler>>
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}}}
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The story macro looks for a list of tiddler titles in the tiddler `MyStoryTiddler`, and displays them in sequence. The subtle part is that subsequently, if `MyStoryTiddler` changes, the `<<story>>` macro is selectively re-rendered. So, to navigate to a new tiddler, code merely needs to add the name of the tiddler and a line break to the top of `MyStoryTiddler`:
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{{{
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var storyTiddler = store.getTiddler("MyStoryTiddler");
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store.addTiddler(new Tiddler(storyTiddler,{text: navigateTo + "\n" + storyTiddler.text}));
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}}}
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The mechanisms that allow all of this to work are fairly intricate. The sections below progressively build the key architectural concepts of TiddlyWiki5 in a way that should provide a good basis for exploring the code directly.
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!! Tiddlers
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Tiddlers are an immutable dictionary of name:value pairs called fields.
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The only field that is required is the {{{title}}} field, but useful tiddlers also have a {{{text}}} field, and some or all of the standard fields {{{modified}}}, {{{modifier}}}, {{{created}}}, {{{creator}}}, {{{tags}}} and {{{type}}}.
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Hardcoded in the system is the knowledge that the `tags` field is a string array, and that the `modified` and `created` fields are JavaScript `Date` objects. All other fields are strings.
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The {{{type}}} field identifies the representation of the tiddler text with a MIME type.
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!! ~WikiStore
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Groups of uniquely titled tiddlers are contained in WikiStore objects.
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The WikiStore also manages the plugin modules used for macros, and operations like serializing, deserializing, parsing and rendering tiddlers.
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Each WikiStore is connected to another shadow store that is used to provide default content. Under usual circumstances, when an attempt is made to retrieve a tiddler that doesn't exist in the store, the search continues into its shadow store (and so on, if the shadow store itself has a shadow store).
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!! ~WikiStore Events
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Clients can register event handlers with the WikiStore object. Event handlers can be registered to be triggered for modifications to any tiddler in the store, or with a filter to only be invoked when a particular tiddler or set of tiddlers changes.
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Whenever a change is made to a tiddler, the wikistore registers a `nexttick` handler (if it hasn't already done so). The `nexttick` handler looks back at all the tiddler changes, and dispatches any matching event handlers.
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!! Parsing and Rendering
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TiddlyWiki parses the content of tiddlers to build an internal tree representation that is used for several purposes:
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* Rendering a tiddler to other formats (e.g. converting wikitext to HTML)
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* Detecting outgoing links from a tiddler, and from them...
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* ...computing incoming links to a tiddler
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* Detecting tiddlers that are orphans with no incoming links
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* Detecting tiddlers that are referred to but missing
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The parse tree is built when needed, and then cached by the WikiStore until the tiddler changes.
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TiddlyWiki5 uses multiple parsers:
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* Wikitext ({{{text/x-tiddlywiki}}}) in `js/WikiTextParser.js`
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* JavaScript ({{{text/javascript}}}) in `js/JavaScriptParser.js`
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* Images ({{{image/png}}} and {{{image/jpg}}}) in `js/ImageParser.js`
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* JSON ({{{application/json}}}) in `js/JSONParser.js`
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Additional parsers are planned:
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* CSS ({{{text/css}}})
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* Recipe ({{{text/x-tiddlywiki-recipe}}})
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One global instance of each parser is instantiated in `js/App.js` and registered with the main WikiStore object.
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The parsers are all used the same way:
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$$$.js
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var parseTree = parser.parse(type,text) // Parses the text and returns a parse tree object
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$$$
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The parse tree object exposes the following fields:
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$$$.js
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var renderer = parseTree.compile(type); // Compiles the parse tree into a renderer for the specified MIME type
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console.log(parseTree.toString(type)); // Returns a readable string representation of the parse tree (either `text/html` or `text/plain`)
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var dependencies = parseTree.dependencies; // Gets the dependencies of the parse tree (see below)
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$$$
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The dependencies are returned as an object like this:
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{{{
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{
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tiddlers: {"tiddlertitle1": true, "tiddlertitle2": false},
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dependentAll: false
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}
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}}}
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The `tiddlers` field is a hashmap of the title of each tiddler that is linked or included in the current one. The value is `true` if the tiddler is a //'fat'// dependency (ie the text is included in some way) or `false` if the tiddler is a //`skinny`// dependency.
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The `dependentAll` field is used to indicate that the tiddler contains a macro that scans the entire pool of tiddlers (for example the `<<list>>` macro), and is potentially dependent on any of them. The effect is that the tiddler should be rerendered whenever any other tiddler changes.
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!! Rendering
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The `parseTree.compile(type)` method returns a renderer object that contains a JavaScript function that generates the new representation of the original parsed text.
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The renderer is invoked as follows:
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$$$.js
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var renderer = parseTree.compile("text/html");
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var html = renderer.render(tiddler,store);
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$$$
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The `tiddler` parameter to the `render` method identifies the tiddler that is acting as the context for this rendering -- for example, it provides the fields displayed by the `<<view>>` macro. The `store` parameter is used to resolve any references to other tiddlers.
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!! Rerendering
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When rendering to the HTML/SVG DOM in the browser, TiddlyWiki5 also allows a previous rendering to be selectively updated in response to changes in dependent tiddlers. At the moment, only the WikiTextRenderer supports rerendering.
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The rerender method on the renderer is called as follows:
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{{{
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var node = document.getElementById("myNode");
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var renderer = parseTree.compile("text/html");
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myNode.innerHTML = renderer.render(tiddler,store);
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// And then, later:
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renderer.rerender(node,changes,tiddler,store,renderStep);
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}}}
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The parameters to `rerender()` are:
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|!Name |!Description |
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|node |A reference to the DOM node containing the rendering to be rerendered |
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|changes |A hashmap of `{title: "created|modified|deleted"}` indicating which tiddlers have changed since the original rendering |
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|tiddler |The tiddler providing the rendering context |
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|store |The store to use for resolving references to other tiddlers |
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|renderStep |See below |
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Currently, the only macro that supports rerendering is the `<<story>>` macro; all other macros are rerendered by calling the ordinary `render()` method again. The reason that the `<<story>>` macro goes to the trouble of having a `rerender()` method is so that it can be carefully selective about not disturbing tiddlers in the DOM that aren't affected by the change. If there were, for instance, a video playing in one of the open tiddlers it would be reset to the beginning if the tiddler were rerendered.
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