Recipe 6.3. Moving Objects Forward and Backward


Problem

You want to change the order in which objects are drawn on-screen, moving them either in front of or behind other display objects.

Solution

Use the setChildIndex( ) method of the DisplayObectContainer class to change the position of a particular item. Use the getChildIndex( ) and getChildAt( ) methods to query siblings of the item so the item can be positioned properly relative to them.

Discussion

Recipes 6.1 and 6.2 introduced how the display list model deals with the visual stacking order (depth). Essentially, every DisplayObjectContainer instance has a list of children, and the order of the children in this list determines the order in which child display objects are drawn inside of the container. The children are each given a position index, ranging from 0 to numChildren 1, much like an array. The child at position 0 is drawn on the bottom, underneath the child at position 1, etc. There are no empty position values in the list; if there are three children, the children will always have index values of 0, 1, and 2 (and not, say, 0, 1, and 6).

The setChildIndex( ) method is provided by DisplayObjectContainer to reorder the children inside the container. It takes two parameters: a reference to the child to be moved and the child's new position in the container. The index position specified must be a valid value. Negative values or values too large will generate a RangeError and the function won't execute properly.

The following example creates three colored circles, with the blue one being drawn on top. The setChildIndex( ) method is used to move the blue circle underneath the two other circles, changing its position from 2 to 0 in the container. The positions of the other children are adjusted accordingly; red is moved to 1 and green is moved to 2:

package {   import flash.display.*;   public class SetChildIndexExample extends Sprite {     public function SetChildIndexExample(  ) {       // Create three different colored circles and       // change their coordinates so they are staggered       // and aren't all located at (0,0).       var red:Shape = createCircle( 0xFF0000, 10 );       red.x = 10;       red.y = 20;       var green:Shape = createCircle( 0x00FF00, 10 );       green.x = 15;       green.y = 25;       var blue:Shape = createCircle( 0x0000FF, 10 );       blue.x = 20;       blue.y = 20;              // Add the circles, red has index 0, green 1, and blue 2       addChild( red );       addChild( green );       addChild( blue );              // Move the blue circle underneath the others by placing       // it at the very bottom of the list, at index 0       setChildIndex( blue, 0 );     }          // Helper function to create a circle shape with a given color     // and radius     public function createCircle( color:uint, radius:Number ):Shape {       var shape:Shape = new Shape(  );       shape.graphics.beginFill( color );       shape.graphics.drawCircle( 0, 0, radius );       shape.graphics.endFill(  );       return shape;     }   } }

One of the requirements for setChildIndex( ) is that you know the index value you want to give to a specific child. When you're sending a child to the back, you use 0 as the index. When you want to bring a child to the very front, you specify numChildren 1 as the index. But what if you want to move a child underneath another child?

For example, suppose you have two circlesone green and one blueand you don't know their positions ahead of time. You want to move the blue circle behind the green one, but setChildIndex( ) requires an integer value for the new position. There are no setChildAbove or setChildBelow methods, so the solution is to use the getChildIndex( ) method to retrieve the index of a child, and then use that index to change the position of the other child. The getChildIndex( ) method takes a display object as a parameter and returns the index of the display object in the container. If the display object passed in is not a child of the container, an ArgumentError is thrown.

The following example creates two circlesone green and one blueand uses getChildIndex( ) on the green circle so the blue circle can be moved beneath it. By setting the blue circle to the index that the green circle has, the blue circle takes over the position and the green circle moves to the next higher position because blue had a higher position initially:

package {   import flash.display.*;   public class GetChildIndexExample extends Sprite {     public function GetChildIndexExample(  ) {       // Create two different sized circles       var green:Shape = createCircle( 0x00FF00, 10 );       green.x = 25;       green.y = 25;       var blue:Shape = createCircle( 0x0000FF, 20 );       blue.x = 25;       blue.y = 25;              // Add the circles to this container       addChild( green );       addChild( blue );              // Move the blue circle underneath the green circle. First       // the index of the green circle is retrieved, and then the       // blue circle is set to that index.       setChildIndex( blue, getChildIndex( green ) );     }          // Helper function to create a circle shape with a given color     // and radius     public function createCircle( color:uint, radius:Number ):Shape {       var shape:Shape = new Shape(  );       shape.graphics.beginFill( color );       shape.graphics.drawCircle( 0, 0, radius );       shape.graphics.endFill(  );       return shape;     }   } }

When a child is moved to an index lower than the one it currently has, all children from the target index up to the one just before the child index will have their indexes increased by 1 and the child is assigned to the target index. When a child is moved to a higher index, all children from the one just above the child index up to and including the target index are moved down by 1, and the child is assigned the target index value.

In general, if object a is above object b, the following code to moves a directly below b:

setChildIndex( a, getChildIndex( b ) );

Conversely, if object a is below object b, the preceding code moves a directly above b.

So far, we've always been moving around children that we've had a reference to. For example, the blue variable referenced the display object for the blue circle, and we were able to use this variable to change the index of the blue circle. What happens when you don't have a reference to the object you want to move, and the blue variable doesn't exist? The setChildIndex( ) method requires a reference to the object as its first parameter, so you'll need to get the reference somehow if it isn't available with a regular variable. The solution is to use the getChildAt( ) method.

The getChildAt( ) method takes a single argument, an index in the container's children list, and returns a reference to the display object located at that index. If the specified index isn't a valid index in the list, a RangeError is thrown.

The following example creates several circles of various colors and sizes and places them at various locations on the screen. Every time the mouse is pressed, the child at the very bottom is placed on top of all of the others:

package {   import flash.display.*;   import flash.events.*;   public class GetChildAtExample extends Sprite {     public function GetChildAtExample(  ) {       // Define a list of colors to use       var color:Array = [ 0xFF0000, 0x990000, 0x660000, 0x00FF00,                           0x009900, 0x006600, 0x0000FF, 0x000099,                           0x000066, 0xCCCCCC ];       // Create 10 circles and line them up diagonally       for ( var i:int = 0; i < 10; i++ ) {         var circle:Shape = createCircle( color[i], 10 );         circle.x = i;         circle.y = i + 10; // the + 10 adds padding from the top                  addChild( circle );       }                    stage.addEventListener( MouseEvent.CLICK, updateDisplay );     }          // Move the circle at the bottom to the very top     public function updateDisplay( event:MouseEvent ):void {       // getChildAt(0) returns the display object on the       // very bottom, which then gets moved to the top       // by specifying index numChildren - 1 in setChildIndex       setChildIndex( getChildAt(0), numChildren - 1 );     }          // Helper function to create a circle shape with a given color     // and radius     public function createCircle( color:uint, radius:Number ):Shape {       var shape:Shape = new Shape(  );       shape.graphics.beginFill( color );       shape.graphics.drawCircle( 0, 0, radius );       shape.graphics.endFill(  );       return shape;     }   } }

See Also

Recipes 6.1 and 6.2




ActionScript 3. 0 Cookbook
ActionScript 3.0 Cookbook: Solutions for Flash Platform and Flex Application Developers
ISBN: 0596526954
EAN: 2147483647
Year: 2007
Pages: 351

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