how to hook UIView's animation?
<p>I want to send my own message during UIView's animation process. 
For example:</p>

<pre><code>myView.alpha = 1.0;
[UIView animateWithDuration:2.0f animations:^{myView.alpha = 0.0;} completion:^(BOOL isFinished){}];
</code></pre>

<p>I want to send the message when iOS render the specific alpha's animation frame.</p>

<pre><code>- (void) alphaChangedTo:(float) val
{
    [sendMyOwnMessage:val];
}
</code></pre>

<p>as the animation is carried out by the iOS system, can I hook the function to finish my task? 
Thanks!</p>
