objective-c - UIAlertController 中的选择器 presentViewController 没有已知的类方法

Question

我知道这是一个非常简单的问题，我已经研究了 3 个小时，但没能成功。我正在尝试实现 UIAlertController 以使用 Apple documentation 显示错误消息，但我在这一行上收到错误，选择器 presentViewController 没有已知的类方法AlertMessageViewController 是我自定义的类，继承自 UIViewController。

AlertMessageViewController.h

#import <UIKit/UIKit.h>
@interface AlertMessageViewController : UIViewController
+(instancetype)showAlert: (NSString *) title withMessage: (NSString*) message preferredStyle:(UIAlertControllerStyle)preferredStyle;
@end

AlertMessageViewController.m

#import "AlertMessageViewController.h"
#import <UIKit/UIKit.h>
@interface AlertMessageViewController ()
@end
@implementation AlertMessageViewController

- (void)viewDidLoad {
    [super viewDidLoad];
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
}
+(instancetype)showAlert: (NSString *) title withMessage: (NSString*) message preferredStyle:(UIAlertControllerStyle)preferredStyle
{

    UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"title" message:@"alertMessage" preferredStyle:UIAlertControllerStyleAlert];
    UIAlertAction *ok =[UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleDefault handler:^(UIAlertAction *action){NSLog(@"ok action");}];
    [alert addAction:ok];
    [self presentViewController:alert animated:YES completion:nil];
}

@end

Answer 1

为什么你的showAlert:方法的返回类型是instancetype？而且你没有返回任何东西，它应该是无效的。

编辑:此外，您的方法不应该是类方法

这应该有效:

-(void)showAlert: (NSString *) title withMessage: (NSString*) message preferredStyle:(UIAlertControllerStyle)preferredStyle
{

    UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"title" message:@"alertMessage" preferredStyle:UIAlertControllerStyleAlert];
    UIAlertAction *ok =[UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleDefault handler:^(UIAlertAction *action){NSLog(@"ok action");}];
    [alert addAction:ok];
    [self presentViewController:alert animated:YES completion:nil];
}

更新: 好的试试这个:

  +(UIAlertController*)alertWithTitle: (NSString *) title withMessage: (NSString*) message preferredStyle:(UIAlertControllerStyle)preferredStyle
{

UIAlertController *alert = [UIAlertController alertControllerWithTitle:title message:message preferredStyle: preferredStyle];
UIAlertAction *ok =[UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleDefault handler:^(UIAlertAction *action){NSLog(@"ok action");}];
[alert addAction:ok];
return alert;
}
    }

然后当你想展示它的时候，这样调用它:

[self presentViewController:[AlertMessageViewController alertWithTitle:@"Title" withMessage:@"Message" preferredStyle:UIAlertControllerStyleAlert] animated:YES completion:NULL];