TiddlyWiki5/tiddlywiki5/tiddlers/TiddlyWikiArchitecture.tid

title: TiddlyWikiArchitecture
modifier: JeremyRuston

!! Overview

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.

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.

!! Tiddlers

Tiddlers are a dictionary of name:value pairs called fields.

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}}}.

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.

The {{{type}}} field identifies the representation of the tiddler text with a MIME type.

!! ~WikiStore

Groups of uniquely titled tiddlers are contained in WikiStore objects.

The WikiStore also manages the plugin modules used for macros, and operations like serializing, deserializing, parsing and rendering tiddlers.

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).

!! ~WikiStore Events

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.

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. 

!! Parsing

TiddlyWiki parses the content of tiddlers to build an internal tree representation that is used for several purposes:
* Rendering a tiddler to other formats (e.g. converting wikitext to HTML)
* Detecting outgoing links from a tiddler, and from them...
* ...computing incoming links to a tiddler
* Detecting tiddlers that are orphans with no incoming links
* Detecting tiddlers that are referred to but missing

The parse tree is built when needed, and then cached by the WikiStore until the tiddler changes.

TiddlyWiki5 uses multiple parsers:
* Wikitext ({{{text/x-tiddlywiki}}})
* JavaScript ({{{text/javascript}}})
* CSS ({{{text/css}}})
* JSON ({{{application/json}}})
* Recipe ({{{text/x-tiddlywiki-recipe}}})

!! Compiling and Rendering

When the text of a tiddler is requested in a different format than its native type, TiddlyWiki5 compiles a JavaScript function that generates the new format from the text of the tiddler.

So, a simple tiddler in {{{application/x-tiddlywiki}}} format might read:
{{{
Hello World
}}}
The function to render it to {{{text/html}}} might look like this:
{{{
function() {
	return "<p>Hello World</p>";
}
}}}

The function can also include calls to the store to incorporate the values of other tiddlers. Consider this tiddler, called {{{HelloThere}}}:
{{{
Hello <<tiddler Who>>
}}}
And this one called {{{Who}}}:
{{{
World
}}}
The function to generate {{{HelloThere}}} in {{{text/html}}} might be:
{{{
function() {
	return ["<p>","Hello ", getTiddlerText("Who","text/html"), "</p>"].join(""); 
}
}}}

Now, the return value of this function can be cached until a tiddler in the dependency chain changes. The function itself can be cached until the tiddler itself changes, or a macro that it uses changes.

The dependency chain is calculated when a tiddler is parsed. Every tiddler that is directly referenced is accumulated (until the point at which it is concluded that it is simpler to mark the tiddler as being dependent on any other tiddler changing).

Evaluated macro parameters are parsed and can be checked for safeness, and then included in the compiled code. For example,
{{{
Hello <<echo {{2+2}}>>
}}}
Compiles to:
{{{
function() {
	return ["Hello ",(function(){
		return 2+2;
	})().toString()].join("");
}
}}}

The compilation process has several steps:
* First, the parse tree is used to generate a JavaScript tree
* The JavaScript tree is scanned to determine the tiddlers on which this one depends
* Finally, executable JavaScript text is generated by walking the JavaScript tree