You are trying to write a DisplayObject into the file directly, this is prevented by Flash engine due to the way Flash handles default serialization of any object. In order to save a DisplayObject into the external resource, you need to employ IExternalizable on that object’s class and any class of objects you will plan to store as well. The implementation of writeExternal should save all data required to rebuild the said object from scratch, and readExternal should also employ methods to restore the integrity of said DisplayObject by performing addChild() on nested display objects, or adding them into other internal structures that object might contain.
Note, other answers contain valid points for doing a custom serialization with XML or JSON, and also contain links to requires import, in particular, flash.utils.registerClassAlias and flash.utils.getDefinitionByName are gravely needed to recreate the structure from a serialized data chunk.
An example: Let’s say you have a drawing board in a Board class, and a set of rectangles that you can drag by using mouse, that differ by size and color. Rectangles are custom made MovieClips and don’t have a class of their own, but each MovieClip is also assigned a color property to simplify their distinction. This means you need to implement IExternalizable on Board class only. Let’s also assume Board class has a pieces array that contains all links to nested rectangles, and a method to create a new properly sized rectangle based on width, height and color supplied as parameters. (There might be more requirements to the data structure of Board to meet in your case, so watch closely) So, the process of serializing Board will be to collect all the data from nested MCs and stuff it in order into IDataOutput supplied, and the process of restoring an instance of Board should retrieve stored data, parse it to find what is where, create the nested MCs to be the same like they’ve been stored, position them properly, addChild() to self and rebuild thepieces` array.
public class Board extends Sprite implements IExternalizable {
private var pieces:Array;
public function createRectangle(_width:Number,_height:Number,color:uint):MovieClip {
var mc:MovieClip=new MovieClip();
mc.graphics.beginFill(color);
mc.graphics.drawRect(0,0,_width,_height);
mc.graphics.endFill();
mc.color=color;
pieces.push(mc);
return mc;
}
A refinement to data structure is already visible – you need to store the passed _width and _height in the MC somewhere, because the actual width of that MC will differ from what’s passed by the default line thickness (1, 0.5 on either side). x and y are properly retrieved from MC’s properties, though. So, adding both lines into createRectangle is necessary.
mc._width=_width;
mc._height=_height;
With this, serializing the Board becomes more easy.
public function writeExternal(output:IDataOutput):void {
var pl:int=pieces.length; // cache
output.writeInt(pl); // assuming we keep this array in integral state
for (var i:int=0;i<pl;i++) {
var _mc:MovieClip=pieces[i];
output.writeDouble(_mc.x); // this is usually not rounded when dragging, so saving as double
output.writeDouble(_mc.y);
output.writeDouble(_mc._width);
output.writeDouble(_mc._height);
output.writeInt(_mc._color);
}
// if anything is left about the "Board" itself, write it here
// I'm assuming nothing is required to save
}
To restore, you need to read the data out of IDataInput in the very same order as it was written in writeExternal and then process to rebuilding the display list we’ve stored.
public function readExternal(input:IDataInput):void {
// by the time this is called, the constructor has been processed
// so "pieces" should already be an instantiated variable (empty array)
var l:int;
var _x:Number;
var _y:Number;
var _width:Number;
var _height:Number;
var _color:uint;
// ^ these are buffers to read data to. We don't yet have objects to read these into
input.readInt(l); // get pieces length
for (var i:int=0;i<l;i++) {
input.readDouble(_x);
input.readDouble(_y);
input.readDouble(_width);
input.readDouble(_height);
input.readInt(_color);
// okay we got all the data representing the rectangle, now make one
var mc:MovieClip=createRectangle(_width,_height,_color);
mc.x=_x;
mc.y=_y;
addChild(mc); // createRectangle does NOT have addchild call
// probably because there are layers for the parts to be added to
// I'm assuming there are no layers here, but you might have some!
// pieces array is populated inside createRectangle, so we leave it alone
}
// read all the data you have stored after storing pieces
}
In case your nested MCs have a class that also implements IExternalizable, you can save the entire array in a single instruction, writeObject(pieces), this will make Flash walk through the array, find all data it contains and call writeObject on any nested object, essentially calling that class’s writeExternal function for each of the instance in the array. Restoring such an array should include rebuilding the display list by walking the array and calling addChild() on each of the restored instances.
And last but not the least, registerClassAlias() should be called prior to doing any serialization or deserialization of custom objects. Best place to call these is probably your main object’s constructor, as this will surely be called before any other code your application contains.