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：
- 如何决定使用哪种方法？是否有一个用例？
-
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：
- How to decide which approach to use? Is there a use case for each one?
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
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.
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().
这篇关于iOS：使用UIView的“drawRect：”与其层的delagate“drawLayer：inContext：”的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！