This manual describes how Gnash ActionScript classes are written.
Copyright © 2007 Free Software Foundation
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. You can find a copy of the GFDL at this link or in the file COPYING-DOCS distributed with this manual.
| Revision History | |
|---|---|
| Revision ActionScript Manual V0.1 | March 2007 |
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Free Software Foundation | |
Table of Contents
In this document, the term 'ActionScript class' refers to the C++ class which is instantiated by Gnash when some ActionScript code instantiates a corresponding class. The C++ class stores instance data and implements the methods which are called on the object in the ActionScript code.
When Gnash starts, the class_init() method for each ActionScript class (listed in Global.cpp) is called. This method constructs a prototype, which is implemented as an as_object. In addition, the method registers the constructor to be used for future object creation, and attaches methods and properties to the prototype.
When a new object is needed, instance data is added to the methods and properties inherited from the prototype.
Adding a new ActionScript class is relatively simple, but the process is complicated by the fact that the interface has evolved over time and the current code base represents several different formats. This document describes the current interface. The Boolean class should be considered the authoritative example of a modern ActionScript class.
ActionScript classes contain a header file and a C++ implementation. The name is usually the name of the class as it is called in the ActionScript specifications; for instance Boolean.cpp for the Boolean class.
In the CVS source tree, there is a utility file named gen-asclass.sh which can be used to create the header file and a C++ source file stub for an ActionScript class.
In ActionScript, a prototype is a base object which contains all the methods that an instantiated object will contain. In short, it contains every part of the class except for the portions dealing with the storage of instance data.
In Gnash, the prototype of an ActionScript object is implemented as an as_object. At startup, the methods and properties of the ActionScript class are attached to the as_object. The following example demonstrates how methods can be attached:
static void
attachBooleanInterface(as_object& o)
{
o.init_member("toString", new builtin_function(boolean_tostring));
o.init_member("valueOf", new builtin_function(boolean_valueof));
}
This code was generated using gen-asclass.sh. Typically, you will need to customize the attach method to include any new methods you add to the class.
Static properties can also be added to the ActionScript prototype (dynamic properties are addressed later). They are attached in a similar way:
o.init_member("myProperty", as_value("HelloWorld"));
Properties which have been added in this manner can be directly accessed in ActionScript code without a function call, as this piece of ActionScript code compiled by Ming's makeswf compiler demonstrates:
// Get the value of the myProperty property
if (node.myProperty == "HelloWorld") {
trace("MATCHED");
}
A new class should derive from as_object, which is the base class of every ActionScript object in Gnash. The class declaration will also be generated when you use gen-asclass.sh.
The class should contain an init method; this is included in the stub when gen-asclass.sh is used.
The init method should be called in the constructor in Global.cpp, where all other ActionScript classes are similarly referenced.
Every method you implement and attach will receive an fn_call data structure as an argument when it is called.
The arguments stored in fn_call should be accessed using arg(). For instance, the first element can be popped with fn.arg(0).
The element popped off the stack is an as_value object.
The return value should be an as_value object. For example:
return as_value('Goodbye, cruel world.');
There are two other useful members of the fn_call structure, namely this_ptr and nargs. The former points to the class which is invoking this method, while the latter is a count of the number of arguments in the stack.
You may also see instances of the env pointer being used. This is being deprecated. Instances which could be replaced with arg() are already deprecated; other uses will be deprecated in the near future.
Beyond the arg() method, there is one method of note. dump_args() can be used in debugging to output the entire argument stack.
This section describes accessors to dynamic properties. Read-only properties are described in the prototype section.
Dynamic properties are not created by the gen-asclass.sh script. Accessors should be written as a single get/set method. Previously this was done by overriding get_member() and set_member(), but this practice is deprecated.
The accessor is written so that it sets the property if it is called with an argument, and puts the property in the fn_call result pointer. For instance:
void
MyClass::myProperty_getset(const fn_call& fn)
{
boost::intrusive_ptr<MyClass> ptr = ensureType<MyClass>(fn.this_ptr);
// setter
if ( fn.nargs > 0 )
{
bool h = fn.arg(0).to_bool();
ptr->MyMethod(h);
return;
}
// getter
bool h = ptr->MyMethod();
fn.result->set_bool(h);
}
It has not yet been decided whether properties should be set in the exported interface or attached to instances of the class. A property is attached in the following manner:
boost::intrusive_ptr<builtin_function> gettersetter;
gettersetter = new builtin_function(&MyClass::myProperty_getset, NULL);
o.init_property("myProperty", *gettersetter, *gettersetter);
The as_value class is used throughout the interpreter to create generic objects to hold data.
The following data types are supported: NULLTYPE, BOOLEAN, STRING, NUMBER, OBJECT, AS_FUNCTION, and MOVIECLIP (sprite). The type C_FUNCTION is being deprecated.
Several methods allow you to determine if a value stored in as_value is of a specific type. These follow the form of is_TYPE, for example is_as_function() and is_number(). In general, the type names match the data types listed above, with the exception of the type MOVIECLIP which has a method is_sprite().
Another set of methods will return a representation of the value as a particular type. They follow the to_TYPE naming convention. Examples are to_number() and to_bool(). The type names are as listed earlier, except for MOVIECLIP, which uses to_sprite().
Finally, there is the set_TYPE series of methods. They change the type to the type specified in the method name, and set the value to the one given as an argument. It is also possible to accomplish the same thing with the = operator. Again, type names match those named earlier, except in the case of MOVIECLASS. Its method is called set_sprite().
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