1、第一种：由于UITextfield继承自UIControl，所有可以通过addTarget来监听
[self addTarget:self action:@selector(editingDidBegin) forControlEvents:UIControlEventEditingDidBegin]; [self addTarget:self action:@selector(editingDidEnd) forControlEvents:UIControlEventEditingDidEnd];
- (void)editingDidBegin { //TODO}
- (void)editingDidEnd { //TODO}
2、第二种：通过通知实现（记得要移除通知）
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(editingDidBegin) name:UITextFieldTextDidBeginEditingNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(editingDidEnd) name:UITextFieldTextDidEndEditingNotification object:nil];
- (void)dealloc {
[[NSNotificationCenter defaultCenter] removeObserver:self];
}
- (void)editingDidBegin {
[self setValue:[UIColor whiteColor] forKeyPath:PlaceholderColorKey];
}
- (void)editingDidEnd {
[self setValue:[UIColor grayColor] forKeyPath:PlaceholderColorKey];
}
3、第三种：还可以通过UITextfieldDelegate代理来实现,不过不推荐这种写法，因为可以设置多个代理，而且代理通常是别人而非自己。
4、第四种：利用内部特性实现
- (BOOL)becomeFirstResponder {
[self setValue:[UIColor whiteColor] forKeyPath:PlaceholderColorKey];
return [super becomeFirstResponder];
}
-(BOOL)resignFirstResponder {
[self setValue:[UIColor grayColor] forKeyPath:PlaceholderColorKey];
return [super resignFirstResponder];
}
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