ASSetPropFlags and Enumeration
In the addProperty example concerning a "collapsible" menu, a for..in loop was used to cycle through the elements of a movieclip on the screen. The process of going through all the elements of an object, be it movieclip or otherwise, is called enumeration.

All properties and methods defined for an object are enumerable through the use of a for..in loop in Flash. This includes properties created with addProperty as well as prototype properties and methods. Flash properties, even inherited ones, are all public and available like that, even in for loops and enumeration. Such a for..in loop, however, does NOT include hidden properties like constructor, __proto__ or __constructor__ (the latter two are discussed later). With your run of the mill prototype properties, though, you may find a for..in loop to be giving you more than you bargained for. See the following example.

// UsersGroup class takes an unknown number of user

// arguments and converts them into properties of the

// instance created – each property being a generic object

UsersGroup = function( /* multiple user names */){

// add a new property based on the arguments passed

for (var i=0; i<arguments.length; i++){

this[ arguments[i] ] = new Object();

}

};

// reportUsers uses a for.. in to enumerate through the

// the properties in a UsersGroup object and trace each

// by their property name

UsersGroup.prototype.reportUsers = function(){

for (var user in this) trace(user);

};

 

groupA = new UsersGroup("Carl", "Harriet", "Daisy");

groupA.reportUsers();

/* traces:

reportUsers

Daisy

Harriet

Carl

*/

Notice how not only were the names defined for the object as properties given, but also the reportUsers method itself. The for..in caught that as well, recognizing it as a property of the UsersGroup instance. Of course with every problem there is a solution (atleast that’s something good to keep telling yourself). The solution to this problem is ASSetPropFlags.

ASSetPropFlags is a function in Actionscript that allows you to define for an object how its properties are handled; whether they can be changed, deleted or enumerated. To read more about ASSetPropFlags, see flashcoders explanation. Using it on a prototype object, or really any property you desire, will allow you to control how your object instances are enumerated.

// UsersGroup class takes an unknown number of user

// arguments and converts them into properties of the

// instance created – each property being a generic object

UsersGroup = function( /* multiple user names */){

// add a new property based on the arguments passed

for (var i=0; i<arguments.length; i++){

this[ arguments[i] ] = new Object();

}

};

// reportUsers uses a for.. in to enumerate through the

// the properties in a UsersGroup object and trace each

// by their property name

UsersGroup.prototype.reportUsers = function(){

for (var user in this) trace(user);

};

// prevent enumeration of all prototype properties

ASSetPropFlags(UsersGroup.prototype, null, 1);

 

groupA = new UsersGroup("Carl", "Harriet", "Daisy");

groupA.reportUsers();

/* traces:

Daisy

Harriet

Carl

*/

If using for..in loops with your objects, this can be an important function to keep handy. Unfortunately, it’s undocumented by Macromedia so don’t be surprised if finding information on ASSetPropFlags seems harder than it should be. On that note, there’s yet another hidden method in the same vein of ASSetPropFlags, isPropertyEnumerable. The isPropertyEnumerable will tell you whether or not a property will be found in a for..in loop. For example, if used on reportUsers above, false would be returned. On Daisy, however, true would be returned.

trace(groupA.isPropertyEnumerable("reportUsers")); // traces false (hidden with ASSetPropFlags)

trace(groupA.isPropertyEnumerable("Daisy")); // traces true

The idea behind sharing properties and methods like this is consistent throughout other concepts in OOP. It does not exist solely in or end with prototypes alone. Inheritance among classes is also based upon such sharing.

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