Question

我正在尝试从此行中删除警告：

UIViewController * appController = [[[UIApplication sharedApplication] delegate]     viewController];

我对这行代码的警告如下： “找不到实例方法'viewController'（返回类型默认为'id'）

有人可以帮助解释我如何消除这种渐弱的消失吗？

非常感谢，

Answer 1

您需要转换应用程序委托：

UIViewController * appController = [(MyAppDelegate *)[[UIApplication sharedApplication] delegate] viewController];

Answer 2

向您的委托添加演员：

UIViewController * appController = [(MyAppDelegate *)[[UIApplication sharedApplication] delegate] viewController];

Answer 3

我不知道你的...AppDelegate是如何被调用的，所以我在这里使用DemoAppDelegate。

UIViewController *appController = [(DemoAppDelegate *)[[UIApplication sharedApplication] delegate] viewController];

Answer 4

类型转换是嘈杂的，可能很危险。为了解决这个问题，我通常最终会创建一个简单的类方法。假设该类总是打算用作app委托，你可以这样写：

// MONApplicationDelegate.h
@interface MONApplicationDelegate : NSObject < UIApplicationDelegate >

// ...

+ (MONApplicationDelegate *)sharedApplicationDelegate;
- (UIViewController *)viewController;

@end

// MONApplicationDelegate.m
@implementation MONApplicationDelegate

// ...

+ (MONApplicationDelegate *)sharedApplicationDelegate
{
    id ret = [[UIApplication sharedApplication] delegate];
    if (nil == ret) {
        assert(0 && "oops - app delegate has not been created yet");
        return nil;
    }
    if (![ret isKindOfClass:[MONApplicationDelegate class]]) {
        assert(0 && "invalid class type for shared application delegate");
        return nil;
    }
    else {
        return ret;
    }
}

@end

// in action
UIViewController * appController =
  [[MONApplicationDelegate sharedApplicationDelegate] viewController];

// which is shorter and safer to use, when compared to:
UIViewController * appController =
  [(MONApplicationDelegate*)[[UIApplication sharedApplication] delegate] viewController];

IOS，删除警告？

4 个答案: