以下是addTarget：

import "ViewController.h"

@interface ViewController ()
@property (weak, nonatomic) IBOutlet UITextField *nameView;
@property (weak, nonatomic) IBOutlet UITextField *pwdView;
@property (weak, nonatomic) IBOutlet UIButton *loginBtn;

@end

@implementation ViewController

• (void)viewDidLoad {
  [super viewDidLoad];

  [self.nameView addTarget:self action:@selector(textValueChanged) forControlEvents:UIControlEventEditingChanged];
  [self.pwdView addTarget:self action:@selector(textValueChanged) forControlEvents:UIControlEventEditingChanged];
  }

• (void)textValueChanged
  {
  self.loginBtn.enabled = (self.nameView.text.length != 0 && self.pwdView.text.length != 0);
  }

@end
以下是通知：
通知

• (void)viewDidLoad {
  [super viewDidLoad];
  //创建通知
  NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
  //注册通知
  [center addObserver:self selector:@selector(textValueChanged:) name:UITextFieldTextDidChangeNotification object:self.nameView];
  [center addObserver:self selector:@selector(textValueChanged:) name:UITextFieldTextDidChangeNotification object:self.pwdView];

}
//监听文本框的值的改变

• (void)textValueChanged:(NSNotification *)notice
  {
  self.loginBtn.enabled = (self.nameView.text.length != 0 && self.pwdView.text.length != 0);
  }
  //移除通知

• (void)dealloc
  {
  [[NSNotificationCenter defaultCenter] removeObserver:self];
  }
  以下是KVO：

• (void)viewDidLoad {
  [super viewDidLoad];
  //KVO

  [self.nameView addObserver:self forKeyPath:@"text" options:NSKeyValueObservingOptionNew context:nil];
  [self.pwdView addObserver:self forKeyPath:@"text" options:NSKeyValueObservingOptionNew context:nil];

}
/**

• kvo回调

*/

• (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context
  {
  self.loginBtn.enabled = (self.pwdView.text.length != 0 && self.nameView.text.length != 0);
  }

//移除KVO

• (void)dealloc
  {
  [self.nameView removeObserver:self forKeyPath:@"text"];
  [self.pwdView removeObserver:self forKeyPath:@"text"];
  }
  以下是EditingChanged：
  /**

• 监听登录文本框的输入
  */

• (IBAction)nameViewChange
  {
  self.loginBtn.enabled = (self.nameView.text.length != 0 && self.pwdView.text.length != 0);
  }
  /**

• 登录密码输入框的输入
  */

• (IBAction)pwdViewChange
  {
  self.loginBtn.enabled = (self.nameView.text.length != 0 && self.pwdView.text.length != 0);
  }
  这个在连线的时候，选择的是EditingChanged 这个方法