UIAlertController

1.创建
UIAlertController *alert =[UIAlertController alertControllerWithTitle: @“警告"  message:@"出错了!"  preferredStyle:

UIAlertControllerStyleAlert];
2.添加textField
[alert  addTextFieldWithConfigurationHandler:^(UITextField * _Nonnull textField){

// 设置TextField的属性
textField.placeholder = @"倪灏";
// 实时监控textField的内容
[[NSNotificationCenter defaultCenter]  addObserver:self  selector:  @selector(changeAlertValue:)  name:
UITextFieldTextDidChangeNotificationobject:textField];
}];

3. 添加按钮
// 创建按钮
UIAlertAction *action = [UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
// 点击获取textField里的内容
UITextField *textField = [alert.textFields firstObject];

NSLog(@"textField :%@",textField.text);
}];
// 添加按钮
[alert addAction:action];
// 设置按钮不可点击
action.enabled = NO;

// 显示AlertController
[self presentViewController:alert animated:YES completion:^{}];

4.实现实时监控TextField内容的方法
- (void)changeAlertValue:(NSNotification *)notification{
// 找到AlertController
UIAlertController *alert = (UIAlertController *) self.presentedViewController;
// 找到TextField
UITextField *textField = [alert.textFields firstObject];
// 找到按钮
UIAlertAction *action = [alert.actions firstObject];
// 长度大于4时设置按钮可点击
action.enabled = textField.text.length > 4;
}