UITextField增加textDidChange回调功能

在使用UITextField来判断登陆按钮状态时只有

shouldChangeCharactersInRange函数,是在文件还没有改变前就调用了,而不是在改变后调用,要想实现改变后调用的功能,导致登陆按钮显示状态不准确，我们可以增加事件监听的方式

先来看看objective-c提供的接口:
// add target/action for particular event. you can call this multiple times and you can specify multiple target/actions for a particular event.
// passing in nil as the target goes up the responder chain. The action may optionally include the sender and the event in that order
// the action cannot be NULL.
- (void)addTarget:(id)target action:(SEL)action forControlEvents:(UIControlEvents)controlEvents;

使用方法：

//第一步,对组件增加监听器
[textField addTarget:self action:@selector(textFieldDidChange:) forControlEvents:UIControlEventEditingChanged];
...
//第二步,实现回调函数
- (void) textFieldDidChange:(id) sender {
UITextField *_field = (UITextField *)sender;
NSLog(@"%@",[_field text]);
}