flutter_flutter/specs/modules.md

Sky Module System

This document describes the Sky module system.

Overview

The Sky module system is based on the import element. In its most basic form, you import a module as follows:

<import src="path/to/module.sky" />

As these import elements are inserted into a document, the document's list of outstanding dependencies grows. When an imported module completes, it is removed from the document's list of outstanding dependencies.

Before compiling script or inserting an element that is not already registered, the parser waits until the list of outstanding dependencies is empty. After the parser has finished parsing, the document waits until its list of outstanding dependencies is empty before the module it represents is marked complete.

The as attribute on the import element binds a name to the imported module:

<import src="path/to/chocolate.sky" as="chocolate" />

Module API

Each module consists of one or more libraries. The first library in a module is the element tree library, which imports the dart:sky module and then consists of the following code for a Sky module:

final Module module = new Module();

...and the following code for a Sky application:

final Module module = new Application();

The <script> elements found in the document create the subsequent libraries. Each one first imports the dart:mirror library, then the dart:sky module, then the first library described above, then all the modules referenced by <import> element up to that <script> element and all the libraries defined by <script> elements up to that point, interleaved so as to maintain the same relative order as those elements were first seen by the parser.

When a library imports a module, it actually imports all the libraries that were declared by that module except the aforementioned element tree library. If the as attribute is present on the import element, all the libraries are bound to the same name.

At the end of the <script> block's source, if it parsed correctly and completely, the conceptual equivalent of the following code is appended (but without affecting the library's list of declarations and without any possibility of it clashing with identifiers described in the library itself):

class _ { }
void main() {
  LibraryMirror library = reflectClass(_).owner as LibraryMirror;
  if (library.declarations.containsKey(#init) && library.declarations[#init] is MethodMirror)
    init();
  AutomaticMetadata.runLibrary(library, module);
}

Then, that main() function is called.

TODO(ianh): decide what URL and name we should give the libraries, as exposed in MirrorSystem.getName(libraryMirror.qualifiedName) etc

The Module class is defined in dart:sky as follows:

abstract class AbstractModule extends EventTarget {
  AbstractModule({this.url, this.elements});

  final String url;

  final Root elements; // O(1)
  // the Root node of the module or application's element tree

  external Future<Module> import(String url); // O(Yikes)
  // load and return the URL at the given Module
  // if it's already loaded, the future will resolve immediately
  // if loading fails, the future will have an error

  List<Module> getImports(); // O(N)
  // returns the Module objects of all the imported modules

  external registerElement(String tagname, Type elementClass); // O(1)
  // registers a tag name with the parser
  // only useful during parse time
  // verify that tagname isn't null or empty
  // verify that elementClass is the Type of a class that extends Element (directly or indirectly, but not via "implements" or "with")
  // (see the @tagname code for an example of how to verify that from dart)
  // verify that there's not already a class registered for this tag name
  // if there is, then mark this tagname is broken, so that it acts as if it's not registered in the parser,
  // and, if this is the first time it was marked broken, log a console message regarding the issue
  // (mention the tag name but not the classes, so that it's not observable that this currently happens out of order)
}

class Module extends AbstractModule {
  Module({String url, Root elements, this.application}) :
    super(url: url, elements: elements); // O(1)
  final Application application; // O(1)
}

class Application extends AbstractModule {
  Application({String url, Root elements, this.gestureManager}) :
    super(url: url, elements: elements); // O(1)
  external String get title; // O(1)
  external void set title(String newValue); // O(1)
  final GestureManager gestureManager;
}