LoginButton

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;
    };