.AS File Format – Compiler

Compilers

Publish settings

Compile errors

Flash reports errors in the Output panel, from Flash CS3 in a Compiler Errors panel, with the file or the scene, layer and frame, and the line. Typical errors, in paraphrase:

• a type mismatch, naming the type found and the type required;
• no property or method of that name in the class;
• a class or interface that could not be loaded, because no file on the classpath matches;
• a class that must be defined in a file whose path matches its name;
• a member used against its private or static declaration;
• a class that does not implement every method of an interface it names;
• syntax errors, as in ActionScript 1.0.

The checks end there. Anything the annotations do not cover, untyped values, bracket access and timeline paths, is left to fail or succeed when the movie runs.

What a class becomes

For each class it compiles, Flash adds a movie clip symbol to the SWF named __Packages. and the class’s full name, such as __Packages.com.example.Volume, exported but never placed. The class code goes in a DoInitAction tag for that symbol, in the export frame, so it runs once before that frame’s scripts, superclasses before subclasses. In outline, as a decompiler shows it, the code is:

if (!_global.com) { _global.com = new Object(); }
if (!_global.com.example) { _global.com.example = new Object(); }
if (!_global.com.example.Volume) {
    var v1 = function () { … };            // the constructor
    com.example.Volume = v1;
    com.example.Volume extends MovieClip;    // only if the class extends one
    var v2 = v1.prototype;
    v2.setLevel = function (n) { … };       // each method
    v2.__get__percent = function () { … };  // get percent
    v2.__set__percent = function (value) { … };
    v2.addProperty("percent", v2.__get__percent, v2.__set__percent);
    ASSetPropFlags(com.example.Volume.prototype, null, 1);
}

• The guard if (!_global…) means a class already defined, for example by a movie loaded earlier, is not replaced.
• Methods and starting values of variables go on the prototype; static members on the constructor.
• A getter without a setter is given an empty function as its setter. Where the compiler knows an object’s type, it may call __get__name() directly instead of reading the property.
• The extends line is the Extends action of SWF 7, which links the prototypes and records the superclass constructor for super.
• A symbol whose linkage names an ActionScript 2.0 class gets its own DoInitAction with Object.registerClass("linkage name", com.example.Ball).
• Types, public and private are gone; names of members and classes remain as strings.

Flash Player 7 actions

Publishing for Flash Player 7 or later lets the compiler use the actions SWF 7 added for this language:

For Flash Player 6 these actions do not exist: functions use the older DefineFunction, and exceptions cannot be used. The actions and the tags are described in ActionScript in SWF, and where scripts are kept in the document on the FLA pages.