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POOn iOS, why does setting a layer's sublayerTransform turn itself to act like CATranformLayer?
 

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  1. POOn iOS, why does setting a layer's sublayerTransform turn itself to act like CATranformLayer?
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    <p>It is known that the <code>zPosition</code> of the layers only determines which layer cover up which layer. Whether it is a <code>zPosition</code> of 10 or 1000 won't affect its position.</p> <p>That is, unless if we <a href="https://developer.apple.com/library/mac/#documentation/GraphicsImaging/Reference/CATransformLayer_class/Reference/Reference.html" rel="noreferrer">use <code>CATransformLayer</code> to contain those layers</a>, then the <code>zPosition</code> of those layers will affect the layers' position.</p> <p>However, the following code running in iOS 5.1.1 does make the <code>zPosition</code> alter the position of the layers... you can try it in a new Single View App, and add the following code to <code>ViewController.m</code>. If the <code>zPosition</code> of <code>layer2</code> is changed from <code>88</code> to <code>188</code>, we can see that the layer moves accordingly. So no <code>CATransformLayer</code> is in the code; why will it behave like that? (Please quote Apple docs or any reference).</p> <p>Also related is, if the line <code>self.view.layer.sublayerTransform = transform3D;</code> is changed to <code>self.view.layer.transform = transform3D;</code> then the <code>zPosition</code> will have no effect on the position. But <a href="https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CoreAnimation_guide/Articles/Layers.html#//apple_ref/doc/uid/TP40006082-SW1" rel="noreferrer">according to the Apple docs, <code>transform</code> and <code>sublayerTransform</code> only differ in whether self is transformed or not</a>:</p> <blockquote> <p>Two layer properties specify transform matrices: <code>transform</code> and <code>sublayerTransform</code>. The matrix specified by the <code>transform property</code> is applied to the layer and its sublayers relative to the layer's <code>anchorPoint</code>. [...] The matrix specified by the <code>sublayerTransform</code> property is applied only to the layer’s sublayers, rather than to the layer itself.</p> </blockquote> <p>So it is strange that why changing that will cause <code>self.view.layer</code> to act like a <code>CATransformLayer</code>.</p> <pre><code>-(void) viewDidAppear:(BOOL)animated { CATransform3D transform3D = CATransform3DIdentity; transform3D.m34 = -1.0 / 1000; transform3D = CATransform3DRotate(transform3D, M_PI / 4, 0, 1, 0); self.view.layer.sublayerTransform = transform3D; CALayer *layer1 = [[CALayer alloc] init]; layer1.zPosition = 33; layer1.frame = CGRectMake(100, 100, 100, 100); layer1.backgroundColor = [[UIColor orangeColor] CGColor]; [self.view.layer addSublayer:layer1]; CALayer *layer2 = [[CALayer alloc] init]; layer2.zPosition = 88; layer2.frame = CGRectMake(100, 120, 100, 100); layer2.backgroundColor = [[UIColor yellowColor] CGColor]; [self.view.layer addSublayer:layer2]; } </code></pre>
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