登陆状态的按钮继承UIButton
接口：
传入要观察的textfield,可传入多个。
- (instancetype)initAndNotifyTextFields:(NSArray <UITextField *> *)textFields;
+ (instancetype)buttonWithType:(UIButtonType)buttonType andNotifyTextFields:(NSArray <UITextField *> *)textFields;
使用： 调用backCurrentTextfieldStatuts可以接收当前textfield的状态
self.loginButton = [LoginEnableButton buttonWithType:UIButtonTypeCustom andNotifyTextFields:@[self.textfield]];
self.loginButton.backCurrentTextfieldStatuts = ^(BOOL enable) {
weakSelf.loginButton.enabled = enable;
};