How to Create a Flash MX Component - Page 12

Step 10. Programmatically Introducing a Triangle into a Movie

The most straightforward way to instantiate a triangle component is to drag it onto the stage from the Component Panel or, if you already have an instance on the stage, from the Library. However, you may want to dynamically create triangles through the course of the movie, and you would prefer not to have to create all the possible instances in the beginning. This is easily accomplished using either the attachMovie or duplicateMovieClip methods.

You simply specify the Linkage ID and Flash takes care of the rest, for example:

attachMovie("FTriangle", "tri0", 10);

The first argument is the Linkage ID, the second is the name of the new component on the current timeline, and the third is the depth (order). This statement pulls out the triangle component "FTriangle" from the Library, names the clip "tri0", places it on the stage at coordinates _x = 0, _y = 0, and sets its stacking order depth to 10, and calls the TriangleClass constructor. Clips placed on higher-value depths appear on top of clips placed on lower depths.

Before you rush off to try this out, we have to solve the problem of how to get component parameter values into the component at the time the constructor is called. If we did something like this

attachMovie("FTriangle", "tri0", 10);
tri0.tbase = 5;

We would be too late for the tbase variable to be used in the constructor. In our case, it would not make a difference, but in most cases it could have an impact. Flash MX does not leave you hanging. There is an optional final parameter (an Object) to attachMovie and duplicateMovieClip, which you can use as in the following example:

attachMovie("FTriangle", "tri0", 10, { tbase : 5, theight : 10, tcolor : 0x000000, applyTint : false);

This code creates an Object (using { }) and populates it with tbase, theight, tcolor, and applyTint properties. Flash copies all the properties from that Object into the component instantiation it is creating, before invoking the component constructor. In that way, the properties are available to the constructor at the right time.