Functions as Objects (ActionScript: The Definitive Guide)

9.11.1. Passing Functions to Functions

Perhaps surprisingly, we can use any function as an argument to another function like this:

function1(function2);

Note that if there are no parentheses following function2, the interpreter doesn't execute function2( ) but instead just passes its "object reference" as an argument to function1( ). That is, function1( ) receives function2 itself, not the return value of function2( ). Because objects are passed by reference, we can pass a function identifier to another function and it will arrive unscathed. The passed function can be executed like this:

function doCommand(command) {
  command( );      // Executes the passed function
}

// Some examples:
doCommand(stop);  // Pass the internal stop( ) function (stops the current movie)
doCommand(play);  // Pass the internal play( ) function (plays the current movie)

Because functions are a type of object, we may treat them like any other data. In the following example, we assign the internal gotoAndPlay function to the variable gp, which gives us a shorter way to refer to the function:

gp = gotoAndPlay;  // Create a shortcut reference to gotoAndPlay( )
gp(25);            // Invoke gotoAndPlay( ) using our reference

In addition to passing and storing functions as objects, we can exploit the "objectness" of functions by attaching properties to them, like this:

// Create a function
function myFunction ( ) {
  trace(myFunction.x);
}

// Attach a property to it
myFunction.x = 15;

// Check the property value by invoking the function
myFunction( );  // Displays: 15

TIP

By attaching properties to functions, we can maintain the state of a piece of information between function executions without cluttering up a timeline with variables.

Function properties offer the benefits of local variables without expiring between function invocations. This is useful when a function needs to be invoked with a unique identifier. Here, for example, is a generic function that duplicates a movie clip and gives the duplicated clip a unique name and level:

makeClip.count = 0;    // Define a property of makeClip( ) (remember that 
                       // makeClip( ) already exists because functions are
                       // defined before code runs)

// Duplicate a passed clip and assign the new clip an automatic name
function makeClip (theClip) {
  // Add one to our clip counter
  makeClip.count++

  // Now duplicate the clip, assigning it a unique name and depth
  theClip.duplicateMovieClip(theClip._name + makeClip.count, makeClip.count);
}

makeClip(square);   // Make a duplicate of square using makeClip( )

square1._x += 100;  // Now move the duplicated square to the right

9.11. Functions as Objects

9.11.1. Passing Functions to Functions

TIP