CALayer 与 UIView

1、关系

On iOS, every UIView is backed by a Core Animation CALayer. Simply speaking,UIView inherit from NSResponder,handle events from users, contains CALayer,which inherit from NSObject,mainly focus on rendering,animation etc.

One thing UIViews provide over CALayers is built-in support for user interaction. They handle hit-testing on touches and other related actions that you would need to build yourself if managing a hierarchy of CALayers. It's not that hard to implement this yourself, but it is extra code you'd need to write when building a CALayer-only interface.

You will often need to access the underlying layers for a UIView when performing more complex animations than the base UIView class allows. UIView's animation capabilities have grown as the iOS SDK has matured, but there are still a few things that are best done by interacting with the underlying CALayer.

2、功能

UIView 主要是对显示内容的管理，CALayer 主要侧重显示内容的绘制。

3、动画（可参考 objc.io - 动画部分）

在对UIView 的属性修改时不会产生默认动画，而对单个 layer 属性直接修改会产生默认动画。因为UIView 默认情况下禁止了 layer 动画，但是在 animation block 中又重新启用了它们。

4、CALayer

The neat thing about the CALayer class is that it contains a lot of properties that you can set on it to affect the visual appearance, such as:

The size and position of the layer
The layer’s background color
The contents of the layer (an image, or something drawn with Core Graphics)
Whether the corners of the layers should be rounded
Settings for applying a drop shadow to the layer
Applying a stroke around the edges of the layer
And much more!

The other neat thing about the properties on CALayer is that most of them are animatable.

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