plugin dependencies

[FND]

When processing plugins, TiddlyWiki should evaluate the Requires* slice to check for dependencies.

If a dependency does not exist, an error should be raised.

* this slice name is an implicit convention in the TiddlyWiki community

[EricShulman]

The core currently *does* use the "Requires" slice value to check for dependencies.

in loadPlugins(),

the code first gathers the slice information for all tiddlers:

for(var i=0; i<nPlugins; i++) {
	var p = getPluginInfo(tiddlers[i]);
	installedPlugins[i] = p;
	...

then, it recursively 'visits' each tiddler to build the toLoad[] array:

var visit = function(p) {
	if(!p || p.done)
		return;
	p.done = 1;
	var reqs = p.Requires;
	if(reqs) {
		reqs = reqs.readBracketedList();
		for(var i=0; i<reqs.length; i++)
			visit(map[reqs[i]]);
	}
	toLoad.push(p);
};
for(i=0; i<nPlugins; i++)
	visit(installedPlugins[i]);

and finally, it processes the toLoad[] array to actually load the tiddler in dependent-specific order:

for(i=0; i<toLoad.length; i++) {
	p = toLoad[i];
	pluginInfo = p;
	tiddler = p.tiddler;
	if(isPluginExecutable(p)) {
		if(isPluginEnabled(p)) {
		...
			window.eval(tiddler.text);
		...

Note: the Requires slice is supposed to contain a space-separated list of tiddler titles. However, in some cases, this slice has been used to enter free-form or comma-separated information.

Currently, TiddlyTools uses comma-separated values. I will correct this shortly.

[FND]

Replying to EricShulman:

The core currently *does* use the "Requires" slice value to check for dependencies.

True, but that only affects loading order - if a dependency does not exist, that's simply being ignored (at least that's my understanding).

I've clarified the ticket description accordingly.

[FND]

The Requires mechanism exists primarily to resolve loading-order issues.

Elaborate dependency handling should be delegated to plugins, which could specifically check for the required objects (functions, variables etc.) rather than using plugin names. (By analogy, this is similar to using object detection vs. user-agent sniffing for browsers.)

[FND]

documented on the community wiki: http://www.tiddlywiki.org/wiki/Dev:Best_Practices#Dependency_Handling