iOS：使用UIView的“drawRect：”与其层的delagate“drawLayer：inContext：” [英] iOS: Using UIView's 'drawRect:' vs. its layer's delagate 'drawLayer:inContext:'

问题描述

我有一个类是 UIView 的子类。我可以通过实现 drawRect 方法或通过实现 drawLayer：inContext：来绘制内容是一个委托方法 CALayer 。

I have a class which is a subclass of UIView. I am able to draw stuff inside the view either by implementing the drawRect method, or by implementing drawLayer:inContext: which is a delegate method of CALayer.

我有两个问题：

1. 如何决定使用哪种方法？是否有一个用例？



2. 如果我实现 drawLayer：inContext：，它被调用（和 drawRect 不是，至少只要放置一个断点就可以告诉），即使我没有将视图分配为 CALayer 委托使用：

1. How to decide which approach to use? Is there a use case for each one?

2. If I implement drawLayer:inContext:, it is called (and drawRect isn't, at least as far as putting a breakpoint can tell), even if I don't assign my view as the CALayer delegate by using:

[[self layer] setDelegate：self];

如果我的实例没有定义为图层的委托，那么委托方法如何被调用？如果 drawLayer：inContext：被调用，什么机制阻止调用 drawRect ？

how come the delegate method is called if my instance is not defined to be the layer's delegate? and what mechanism prevents drawRect from being called if drawLayer:inContext: is called?

推荐答案

如何决定使用哪种方法？有没有一个用例？

How to decide which approach to use? Is there a use case for each one?

始终使用 drawRect：并且不要使用 UIView 作为任何 CALayer 的绘图委托。

Always use drawRect:, and never use a UIView as the drawing delegate for any CALayer.

如果我的实例没有被定义为图层的委托，那么委托方法如何被调用？如果调用了 drawLayer：inContext：，什么机制阻止drawRect被调用？

how come the delegate method is called if my instance is not defined to be the layer's delegate? and what mechanism prevents drawRect from being called if drawLayer:inContext: is called?

每个 UIView 实例是用于其支持的图纸委托 CALayer 。这就是为什么 [[self layer] setDelegate：self]; 似乎什么都不做。这是多余的 drawRect：方法实际上是视图层的绘图委托方法。在内部， UIView 实现 drawLayer：inContext：它在哪里做一些自己的东西，然后调用 drawRect：。您可以在调试器中看到它：

Every UIView instance is the drawing delegate for its backing CALayer. That's why [[self layer] setDelegate:self]; seemed to do nothing. It's redundant. The drawRect: method is effectively the drawing delegate method for the view's layer. Internally, UIView implements drawLayer:inContext: where it does some of its own stuff and then calls drawRect:. You can see it in the debugger:

这就是为什么 drawRect：在实现 drawLayer时没有被调用：inContext ：。这也是为什么你不应该在自定义的 UIView 子类中实现任何 CALayer 绘制委托方法。你也应该永远不要让任何人看到另一层的图纸委托。如果您正在实现 drawLayer：inContext：，因为您需要访问这些文件，那么这将导致各种糟糕的事情。

This is why drawRect: was never called when you implemented drawLayer:inContext:. It's also why you should never implement any of the CALayer drawing delegate methods in a custom UIView subclass. You should also never make any view the drawing delegate for another layer. That will cause all sorts of wackiness.

CGContextRef ，您可以通过调用 UIGraphicsGetCurrentContext（）从 drawRect： / code>。

If you are implementing drawLayer:inContext: because you need to access the CGContextRef, you can get that from inside of your drawRect: by calling UIGraphicsGetCurrentContext().

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