委托方法如何知道何时被调用 [英] How does a delegate method know when to be called

问题描述

我只是想知道一个委托方法怎么知道什么时候被调用？例如在 UITextFieldDelegate 协议中，当在文本字段中开始编辑时调用 textFieldDidBeginEditing：方法（假设我实现了此方法）。

I'm just wondering how exactly does a delegate method know when to be called? For example in the UITextFieldDelegate protocol the textFieldDidBeginEditing: method is called when editing begins in the textfield (provided I implemented this method).

那么代码如何检测什么时候调用 textFieldDidBeginEditing：？系统是否正在检查 textFieldDidBeginEditing：是否已经实现，如果是运行该方法？有没有我看不到的东西？

So how exactly does the code to detect when to call textFieldDidBeginEditing:? Does the system just check if textFieldDidBeginEditing: is already implemented and if it is it runs that method? Is there something under the hood that I'm not seeing?

推荐答案

正确。

我不能保证Apple的框架代码如何实现，但非常普遍的缺点是：

I can't vouch for how Apple's framework code is implemented under the hood, but an exceedingly common refrain is:

if ([[self delegate] respondsToSelector:@selector(someInstance:didDoSomethingWith:)]) {
    [[self delegate] someInstance:self didDoSomethingWith:foo];
}

这允许你有可选的委托方法，这似乎是你的问题。

This allows you to have optional delegate methods, which appears to be your question.

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