In my last post, we saw how to use MXML (and Flex Builder) to instantiate one top-level object. However, to actually be useful, let’s learn how to add some children. There are two main ways to do this, through the IMXMLObject and through default properties. Both are pretty easy to understand.
Looking at the IMXMLObject interface, you’ll see one method:
package mx.core { /** * The IMXMLObject interface defines the APIs that a non-visual component * must implement in order to work properly with the MXML compiler. * Currently, the only supported method is the <code>initialized()</code> * method. */ public interface IMXMLObject { //-------------------------------------------------------------------------- // Methods //-------------------------------------------------------------------------- /** * Called after the implementing object has been created and all * component properties specified on the MXML tag have been initialized. * * @param document The MXML document that created this object. * * @param id The identifier used by <code>document</code> to refer * to this object. * If the object is a deep property on <code>document</code>, * <code>id</code> is null. */ function initialized(document:Object, id:String):void; } } |
If your child object implements this interface, the MXML compiler will generate code to automatically call this method. It’s a pretty limited way to add children because it only has two properties, document and id. It actually doesn’t tell you who your parent is–just your document, which is your top-level root node object (usually Application).
Sticking with stupid-simple examples, I’m just going to draw some ellipses and rectangles. The code for drawing shapes has been ripped off of my co-workers, Chet Haase, and modified to make it as simple as possible and fit the example.
<?xml version="1.0" encoding="utf-8"?> <components:MyContainer xmlns:mx="http://www.adobe.com/2006/mxml" xmlns:components="components.*" xmlns:shapes="components.shapes.*"> <shapes:Rect color="0xFF0000" startX="20" startY="20" endX="100" endY="30" /> <shapes:Ellipse color="0x00FF00" startX="20" startY="120" endX="100" endY="130" /> </components:MyContainer> |
Rather than post all the code here, I’ll just post some snippets (full code is in two flex builder projects: IMXMLObject example and the Default property example). The interesting part is ArtShape, which is the base class for all shapes. That class is the one that implements IMXMLObject, and all it does is:
public function initialized(document:Object, id:String):void { document.addChild(this); renderShape(); } |
So basically this initialized method just adds ourselves to the top-level application and calls a method that Chet had called renderShape(). The ArtShape version of that method is empty, but here’s what Rectangle looks like:
/** * Draws a rectangle into this shape's display list */ override protected function renderShape():void { if (filled) { graphics.beginFill(color); } else { graphics.lineStyle(strokeWidth, color); } graphics.drawRect(startX, startY, endX - startX, endY - startY); } |
Looking at the generated code, you’ll see what’s going on behind the scenes with our MXML.
package { // begin class def public class FrameworklessChildren extends components.MyContainer { // instance variables /** * @private **/ public var _FrameworklessChildren_Ellipse1 : components.shapes.Ellipse; /** * @private **/ public var _FrameworklessChildren_Rect1 : components.shapes.Rect; // type-import dummies // constructor (non-Flex display object) /** * @private **/ public function FrameworklessChildren() { super(); // our style settings // properties _FrameworklessChildren_Ellipse1_i(); _FrameworklessChildren_Rect1_i(); // events } // scripts // end scripts // supporting function definitions for properties, events, styles, effects private function _FrameworklessChildren_Ellipse1_i() : components.shapes.Ellipse { var temp : components.shapes.Ellipse = new components.shapes.Ellipse(); _FrameworklessChildren_Ellipse1 = temp; temp.color = 65280; temp.startX = 20; temp.startY = 120; temp.endX = 100; temp.endY = 130; temp.initialized(this, "_FrameworklessChildren_Ellipse1") return temp; } private function _FrameworklessChildren_Rect1_i() : components.shapes.Rect { var temp : components.shapes.Rect = new components.shapes.Rect(); _FrameworklessChildren_Rect1 = temp; temp.color = 16711680; temp.startX = 20; temp.startY = 20; temp.endX = 100; temp.endY = 30; temp.initialized(this, "_FrameworklessChildren_Rect1") return temp; } // embed carrier vars // end embed carrier vars // end class def } // end package def } |
So basically to create the children it calls the methods in the constructor for the top-level root class. In those methods, the child object gets created, its properties get set, and the initialized method is called. The full code for the above example can be found here.
However, as I said before, this doesn’t really support nesting–these objects are essentially just direct children of the root object. So to support this, we need to use default properties. This is actually how the majority of objects will work in Flex 4 (Flex 3 essentially uses something else entirely which is built around UIComponents and UIComponentDescriptors).
Anyways, here’s our sample MXML file that we want to support:
<?xml version="1.0" encoding="utf-8"?> <components:MyContainer xmlns="http://ns.adobe.com/mxml/2009" xmlns:components="components.*" xmlns:shapes="components.shapes.*"> <shapes:Rect color="0xFF0000" startX="20" startY="20" endX="100" endY="30" /> <shapes:Ellipse color="0x00FF00" startX="20" startY="120" endX="100" endY="130" /> <components:MyContainer x="100" y="100"> <shapes:Rect color="0xFF0000" startX="20" startY="20" endX="100" endY="30" /> <shapes:Ellipse color="0x00FF00" startX="20" startY="120" endX="100" endY="130" /> </components:MyContainer> </components:MyContainer> |
You see I’ve added another container that has children. Another important thing to note is that I changed the compiler namespace to use the new 2009 namespace (so you’ll need the new compiler to take advantage of this). This isn’t actually needed, and I’ll show you what it looks like with just the 2006 namespace later on.
The basic idea of default properties is that every MXML object has a “default property.” In our case, MyContainer will have a default property of content. This is basically the property where the children go. Instead of the MXML code above, I could’ve used:
<?xml version="1.0" encoding="utf-8"?> <components:MyContainer xmlns="http://www.adobe.com/2006/mxml" xmlns:components="components.*" xmlns:shapes="components.shapes.*"> <components:content> <shapes:Rect color="0xFF0000" startX="20" startY="20" endX="100" endY="30" /> <shapes:Ellipse color="0x00FF00" startX="20" startY="120" endX="100" endY="130" /> <components:MyContainer x="100" y="100"> <components:content> <shapes:Rect color="0xFF0000" startX="20" startY="20" endX="100" endY="30" /> <shapes:Ellipse color="0x00FF00" startX="20" startY="120" endX="100" endY="130" /> </components:content> </components:MyContainer> </components:content> </components:MyContainer> |
The difference here is that when I instantiate MyContainer, I’m explicitly saying “fill the content property with these objects.” However, rather than having to explicitly say that the objects should be stuffed into the content property, I can tell MyContainer to have a default property of content. There was a bug in the MXML compiler in how it handled top-level default properties (only top-level ones), and that’s why I had to use the 2009 namespace to use it; however, if you explicitly specify the property, you can use the old compiler and the 2006 namespace.
So let’s see how we modified MyContainer to make this happen:
package components { import flash.display.DisplayObjectContainer; import flash.display.Sprite; [DefaultProperty("content")] public class MyContainer extends Sprite { public function MyContainer() { } private var _content:*; public function get content():* { return _content; } public function set content(value:*):void { _content = value; if (_content is Array) { for (var i:int = 0; i < _content.length; i++) { _content[i].initializeMe(this); } } else { _content.initializeMe(this); } } public function initializeMe(parent:DisplayObjectContainer):void { parent.addChild(this); } } } |
With the metadata, we defined MyContainer’s default property to be content. Then I just filled in some basic getters/setters for the property. Because content could be a single item or an Array of items, I special cased some logic in there. Basically all I did was loop through all the content and call initializeMe(this) on all the children. That’s just a method I made up, and I probably should create an interface for it. You can see I implemented initializeMe(parent:DisplayObjectContainer) in here as well, and all it does is call addChild. In the case of ArtBoard, it’s pretty similar:
public function initializeMe(parent:DisplayObjectContainer):void { parent.addChild(this); renderShape(); } |
So to complete the picture of how all this works, let’s take a look at the generated code for our MXML class (the first one which uses default properties):
package { // begin class def public class FrameworklessChildren extends components.MyContainer { // instance variables // type-import dummies // constructor (non-Flex display object) /** * @private **/ public function FrameworklessChildren() { super(); // our style settings // properties this.content = [_FrameworklessChildren_Rect1_c(), _FrameworklessChildren_Ellipse1_c(), _FrameworklessChildren_MyContainer2_c()]; // events } // scripts // end scripts // supporting function definitions for properties, events, styles, effects private function _FrameworklessChildren_Rect1_c() : components.shapes.Rect { var temp : components.shapes.Rect = new components.shapes.Rect(); temp.color = 16711680; temp.startX = 20; temp.startY = 20; temp.endX = 100; temp.endY = 30; return temp; } private function _FrameworklessChildren_Ellipse1_c() : components.shapes.Ellipse { var temp : components.shapes.Ellipse = new components.shapes.Ellipse(); temp.color = 65280; temp.startX = 20; temp.startY = 120; temp.endX = 100; temp.endY = 130; return temp; } private function _FrameworklessChildren_MyContainer2_c() : components.MyContainer { var temp : components.MyContainer = new components.MyContainer(); temp.x = 100; temp.y = 100; temp.content = [_FrameworklessChildren_Rect2_c(), _FrameworklessChildren_Ellipse2_c()]; return temp; } private function _FrameworklessChildren_Rect2_c() : components.shapes.Rect { var temp : components.shapes.Rect = new components.shapes.Rect(); temp.color = 16711680; temp.startX = 20; temp.startY = 20; temp.endX = 100; temp.endY = 30; return temp; } private function _FrameworklessChildren_Ellipse2_c() : components.shapes.Ellipse { var temp : components.shapes.Ellipse = new components.shapes.Ellipse(); temp.color = 65280; temp.startX = 20; temp.startY = 120; temp.endX = 100; temp.endY = 130; return temp; } // embed carrier vars // end embed carrier vars // end class def } // end package def } |
You can see it’s basically the same as the one using IMXMLObject; however, rather than calling the initialized() method for us, it just sets the content property. You can download the full code for this example here.
One thing that might be worth checking out is how this class gets generated in the first place. In the compiler, we have a few velocity templates that we use (for style sheets, MXML classes, binding, etc…). The one for the MXML->ActionScript is called ClassDef.vm (a lot of the methods used in that class exist in ClassDefLib.vm). If you’re interested in a lot of this stuff, definitely download the compiler code and take a look at these files. They’re pretty simple to follow and easy to modify if you want to start mucking around with it.
So that’s basically it for my short trip into compiler-land. If you’ve got any questions, let me know!
good work … great goin… visit mine…link1
Thank you! This is exactly what I was looking for on a project I’m doing.
One question though, I noticed when you add height and width in the mxml that the component completely disappears. Not going to affect me on this project but any ideas why this happens?
Great work, thanks again for sharing.
Hey Sean,
I’m pretty sure it’s a bug. I filed SDK-17012. Thanks for finding it and reporting it.
-Ryan
Interesting to know.
This information is so useful, thanks for taking the time to share this. I know I'm in the minority, but I would have preferred this as my intro to flex. Seriously. I hate magic that I don't understand.
Thanks again!
Cheers,
Dru
Hey. I don't normally leave comments, but I just wanted to say thanks for the great information. I have a blog too, though
I don't write as good as you do, but if you want to check it out here it is. Thanks again and have a great day!
Magic The Gathering Software