Question

我正在使用最新的SDK XCode 4.2 和 ARC 开发iOS 4应用程序。

我已向appDelegate.h

添加了一种方法

#import <UIKit/UIKit.h>

@class ViewController;
@class SecondViewController;

@interface AppDelegate : UIResponder <UIApplicationDelegate>
{
    UINavigationController* navController;
    ViewController* viewController;
    SecondViewController* secondViewController;
}

@property (strong, nonatomic) UIWindow *window;

- (void) showSecondViewController;

@end

它已在appDelegate.m

中实施

#import "AppDelegate.h"

#import "ViewController.h"
#import "SecondViewController.h"

@implementation AppDelegate

@synthesize window = _window;

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    viewController = [[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];
    viewController.title = @"First";
    navController = [[UINavigationController alloc] initWithRootViewController:viewController];
    self.window.rootViewController = navController;

    [self.window makeKeyAndVisible];
    return YES;
}

- (void)applicationWillResignActive:(UIApplication *)application
{
    ...
}

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    ...
}

- (void)applicationWillEnterForeground:(UIApplication *)application
{
    ...
}

- (void)applicationDidBecomeActive:(UIApplication *)application
{
    ...
}

- (void)applicationWillTerminate:(UIApplication *)application
{
    ...
}

- (void) showSecondViewController
{
    secondViewController = [[SecondViewController alloc] initWithNibName:@"SecondViewController" bundle:nil];
    secondViewController.title = @"Second";
    [navController pushViewController:secondViewController animated:YES];
}

@end

但是，当我在ViewController.m

中向该方法发送消息时

- (IBAction)goSecondClicked:(id)sender
{
    [[[UIApplication sharedApplication] delegate] showSecondViewController];
}

我收到以下编译错误：

自动引用计数问题没有已知的选择器'showSecondViewController'的实例方法

有任何线索吗？

Answer 1

您需要将您获得的委托对象强制转换为：

AppDelegate *appDelegate = (AppDelegate*)[[UIApplication sharedApplication] delegate];

然后在appDelegate

上调用该方法

Answer 2

将您的goSecondClicked操作方法更改为：

- (IBAction)goSecondClicked:(id)sender 
{
    [[[UIApplication sharedApplication] delegate] performSelector:@selector(showSecondViewController)];
}

编辑：虽然这个替代方案适用于给定的情况，但应该注意的是，如果更改代理中的方法名称而忘记更改选择器调用上的名称，编译器将无法帮助您。因此，应谨慎使用。

Answer 3

您也可以在AppDelegate.h上定义此宏

#define APP_DELEGATE (AppDelegate *)[[UIApplication sharedApplication] delegate]

在此之后，您可以使用以下命令调用您的选择器：

[APP_DELEGATE showSecondViewController];

没有已知的选择器实例方法

3 个答案: