After going through few articles about performance, Not able to get this statement exactly.

"When a Drawable is attached to a view, the view is set as a callback on the drawable"
Soln: "Setting the stored drawables’ callbacks to null when the activity is destroyed."

What does that mean, e.g.

In my app , I initialize an imageButton in onCreate() like this,

imgButton= (ImageButton) findViewById(R.id.imagebtn);

At later stage, I get an image from an url, get the stream and convert that to drawable, and set image btn like this,

imgButton.setImageDrawable(drawable);

According to the above statement, when I am exiting my app, say in onDestroy() I have to set stored drawables’ callbacks to null, not able to understand this part ! In this simple case what I have to set as null ?

I am using Android 2.2 Froyo, whether this technique is required, or not necessary.