未实现的委托方法导致崩溃 [英] not implemented delegate method leads to crash

问题描述

我创建了一个协议并将其分配给委托对象

I created a protocol and assigned it to a delegate object

@protocol AppBrainDelegate <NSObject>
@optional
- (void)didLocateUser;
- (void)didFinishLoadingDataWithData:(NSDictionary *)fetchedData;
@end

@interface Brain : NSObject
@property (strong, nonatomic) id <AppBrainDelegate> delegate;

我认为协议声明中这个@optional的含义意味着控制器不必如果不想，请听代理方法。

I thought the meaning of this @optional in the protocol declaration means, that controllers don't have to listen to the delegate method if they don't want to.

如果不在控制器中实现第一个委托方法，那么这是崩溃日志。如果我这样做，我不会崩溃。似乎我不明白将委托方法声明为可选的概念。你可以向我解释我的错误吗？

Here's the crash log if do not implement the first of the delegate methods in the controller. If I do, I don't crash. Seems like I did not understand the concept of declaring delegate methods as optional. Can you explain to me where my mistake is?

* 由于未捕获的异常终止应用程序NSInvalidArgumentException原因：' - [EventViewController didLocateUser]：无法识别的选择器发送到实例0x1fb300'

* Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[EventViewController didLocateUser]: unrecognized selector sent to instance 0x1fb300'

推荐答案

@optional 只要在符合协议的类中没有实现该方法，就会禁止编译器警告。在调用委托方法之前，您仍然需要检查代理是否实现它：

The @optional simply suppresses a compiler warning if the method is not implemented in a class that conforms to the protocol. Before calling the delegate method, you still need to check that the delegate implements it:

if ([delegate respondsToSelector:@selector(didLocateUser)]) {
    [delegate didLocateUser];
}

顺便提一句，你已经使用 strong创建了你的委托属性语义。除非你有一个特别好的理由使用 strong ，否则代表应该是 weak ，因为你的大脑类不会自己的它的委托（如果你考虑对象图）。

Incidentally, you have created your delegate property using strong semantics. Unless you have a particularly good reason to use strong, delegates should be weak, since your Brain class doesn't own its delegate (if you think about the object graph).

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