Re: [flexcoders] Flex2: Bizarre binding problem

[Ralf Bokelberg]

This is really bizarre. For some reason the bindings get updated three
times when you use Object, but only two times, when you use Class as
the return type. Strange ... 

Cheers,
Ralf. On 7/18/06, Tobias Patton <[EMAIL PROTECTED]> wrote:















Hello Flexcoders;

 

I'm hoping someone can help me understand what's
going on here.

 

I have a custom component that subclasses Canvas. It works
much list a List in that it uses a dataProvider to pass a data object to child
object (item renderer). The child objects render the data object in some
appropriate manner. One of the item renderers is very simple – an image
with some text below it. The image displayed is chosen from a number of
embedded images based on a property of the data object.

 

The image object looks like:

 

<mx:Image source="{GetImage( MyObject( data ).kind}"/>

 

And there is a function in the Script section:

 

private function GetImage( kind : uint ) : Class

{

     if( kind == 0 )

     {

          
return image1;
     }

     else …
}

 

This does not work. The source parameter of the Image object
is never populated. In fact, the GetImage() function is never called with a non-null
value! 

 

There is a workaround, but I'm truly at a loss to
explain why it works. If the GetImage() function is changed to return Object
instead of Class, everything works as expected. Since Class is a subclass of
Object, the up-cast is automatic and no other changes are required.  With
this change in place, the GetImage() function is called initially with null,
but later with a real value. 

 

How could the return value of a function change the way
binding operates?

 

I've attached some simple code at the end of this
email that reproduces the problem with the minimal work. If someone has an
explanation for what's going on here, I'd be very grateful.

 

Thanks.

Tobias.

 

Kodak Graphic Communications Canada Company


Tobias Patton | Software Developer | Tel: +1.604.451.2700
ext: 5148 | 
mailto:[EMAIL PROTECTED] 
| 
http://www.creo.com 

--------- ImageBinding.mxml ---------

 

<?xml version="1.0"
encoding="utf-8"?>

<mx:Application xmlns:mx="
http://www.adobe.com/2006/mxml" 

            layout="vertical"


            xmlns:local="*">

            

            <local:MyData
id="myData" num="3"/>

            

            <mx:Canvas
backgroundColor="0xffffff" width="300"
height="100">

                       
<local:ImageBox
data="">

            </mx:Canvas>

</mx:Application>

 

--------- ImageBox.mxml ---------

 

<?xml version="1.0" encoding="utf-8"?>

<mx:VBox xmlns:mx="
http://www.adobe.com/2006/mxml"
width="75" height="75">

            <mx:Script>

                       
<![CDATA[

                       


                                   
//private
function GetImageForData( val : MyData ) :
Object                             

                                    private
function GetImageForData( val : MyData ) : Class

                                    {

                                                trace(
"GetImageForData", val ? val.num : "null" );

                                                return
( val.num % 2 ) ? m_image : m_image2;

                                    }

                                    

                                    [
Embed( "image.png" ) ]

                                    private
static const m_image : Class;

                                    

                                    [
Embed( "image2.png" ) ]

                                    private
static const m_image2 : Class;

                       
]]>

            </mx:Script>

            

            <mx:Image
source="{GetImageForData( MyData( data ) )}"/>

</mx:VBox>

 

--------- MyData.as ---------

 

package

{

public class MyData

{

            [
Bindable ]

            public var num : Number;

}

}

 









-- Ralf Bokelberg <[EMAIL PROTECTED]>Flex & Flash Consultant based in Cologne/Germany

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