如何监听一个控件内部的事件

• 如果继承自UIControl

- (void)addTarget:(nullable id)target action:(SEL)action forControlEvents:(UIControlEvents)controlEvents;

• 代理

• 通知

• 利用内部的某些机制

  • 比如重写UITextField的becomeFirstResponder和resignFirstResponder来监听UITextField的获得焦点和失去焦点事件

监听UITextField的获得焦点和失去焦点事件

• addTarget

[textField addTarget:target action:@selector(editingDidBegin) forControlEvents:UIControlEventEditingDidBegin];
[textField addTarget:target action:@selector(editingDidEnd) forControlEvents:UIControlEventEditingDidEnd];

UIControlEventEditingDidBegin
1.开始编辑
2.获得焦点
3.弹出键盘

UIControlEventEditingDidEnd
1.结束编辑
2.失去焦点
3.退下键盘

• delegate

textField.delegate = self;

#pragma mark - <UITextFieldDelegate>
- (void)textFieldDidBeginEditing:(UITextField *)textField
{

}

- (void)textFieldDidEndEditing:(UITextField *)textField
{

}

• 重写UITextField的becomeFirstResponder和resignFirstResponder方法

/**
 *  调用时刻 : 成为第一响应者(开始编辑\弹出键盘\获得焦点)
 */
- (BOOL)becomeFirstResponder
{

    return [super becomeFirstResponder];
}

/**
 *  调用时刻 : 不做第一响应者(结束编辑\退出键盘\失去焦点)
 */
- (BOOL)resignFirstResponder
{

    return [super resignFirstResponder];
}

• 通知

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(beginEditing) name:UITextFieldTextDidBeginEditingNotification object:textField];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(endEditing) name:UITextFieldTextDidEndEditingNotification object:textField];

- (void)dealloc
{
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

- (void)beginEditing
{

}

- (void)endEditing
{

}

通知相关的补充

使用block监听通知

// object对象发出了名字为name的通知, 就在queue队列中执行block
self.observer = [[NSNotificationCenter defaultCenter] addObserverForName:UITextFieldTextDidBeginEditingNotification object:self queue:[[NSOperationQueue alloc] init] usingBlock:^(NSNotification * _Nonnull note) {
    // 一旦监听到通知, 就会执行这个block中的代码
}];

// 最后需要移除监听
[[NSNotificationCenter defaultCenter] removeObserver:self.observer];

一次性通知(监听1次后就不再监听)

id observer = [[NSNotificationCenter defaultCenter] addObserverForName:UITextFieldTextDidBeginEditingNotification object:self queue:[[NSOperationQueue alloc] init] usingBlock:^(NSNotification * _Nonnull note) {


    // 移除通知
    [[NSNotificationCenter defaultCenter] removeObserver:observer];
}];

其他

• 如果在线程A发出通知，那么就会在线程A中接收通知

dispatch_async(dispatch_get_global_queue(0, 0), ^{
    // 系统会在子线程中处理test这个通知
    [[NSNotificationCenter defaultCenter] postNotificationName:@"test" object:nil];
});