objective-c - 为什么我得到 "unrecognized selector sent to instance"？

Question

我在 apple tut site 上的第二个应用教程有问题.在主类中我得到一个错误。

代码:

#import "birdwatchingAppDelegate.h"

int main(int argc, char *argv[]) {
    @autoreleasepool {
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([birdwatchingAppDelegate class]));
     } }

错误:

[birdwatchingViewController viewControllers]: unrecognized selector
sent to instance 0x6d37000'

错误位置如下:

import "birdwatchingAppDelegate.h"
        #import "BirdSightingDataController.h"
        #import "birdwatchingViewController.h"

        @implementation birdwatchingAppDelegate

        @synthesize window = _window, dataController = _dataController, firstViewController = _firstViewController;

        - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
        {
            UINavigationController *navigationController = (UINavigationController *)self.window.rootViewController;
            birdwatchingViewController *firstViewController = (birdwatchingViewController *)[[navigationController
    viewControllers] objectAtIndex:0];

            BirdSightingDataController *aDataController = [[BirdSightingDataController alloc] init];
            firstViewController.dataController = aDataController;

            return YES;
        }

导致问题的确切行是:

birdwatchingViewController *firstViewController = (birdwatchingViewController *)[[navigationController
    viewControllers] objectAtIndex:0];

我找不到问题，有人可以帮忙吗？

谢谢..

编辑:

通过添加 NSlog，我得到了以下信息:

2012-02-10 11:24:06.059 Birdwatching[3057:f803] birdwatchingViewController
2012-02-10 11:24:06.060 Birdwatching[3057:f803] -[birdwatchingViewController viewControllers]: unrecognized selector sent to instance 0x6878250

根据评论编辑:

2012-02-10 11:51:20.696 Birdwatching[3152:f803] navi : <birdwatchingViewController: 0x6a49c20>

Answer 1

这是因为 self.window.rootViewController 不是 UINavigationController 它可能很简单 UIViewcontroller

---

根据日志编辑
将您的方法更改为

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
        {
            birdwatchingViewController *firstViewController = (birdwatchingViewController *)self.window.rootViewController;

            BirdSightingDataController *aDataController = [[BirdSightingDataController alloc] init];
            firstViewController.dataController = aDataController;

            return YES;
        }