錯誤：對象'AppDelegate *'丟失屬性

Question

自學iOS編程，從this book開始。 我碰到錯誤的「屬性'MainViewController'在'AppDelegate *'類型的對象上找不到'

我有雙重和三重檢查，我正確地跟着代碼，甚至從頭開始重新編寫。嘗試了一些解決方案，但沒有工作，很少正確地匹配我的問題。任何幫助嗎？

AppDelegate.m（其中錯誤所在）

#import "AppDelegate.h" 
#import "WeatherForecast.h" 
#import "MainViewController.h" 

@implementation AppDelegate 

@synthesize managedObjectContext = _managedObjectContext; 
@synthesize managedObjectModel = _managedObjectModel; 
@synthesize persistentStoreCoordinator = _persistentStoreCoordinator; 

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions 
{ 
    WeatherForecast *forecast = [[WeatherForecast alloc] init]; 
    self.MainViewController.forecast = forecast; 
    // Override point for customization after application launch. 
    MainViewController *controller = (MainViewController *)self.window.rootViewController; 
    controller.managedObjectContext = self.managedObjectContext; 
    return YES; 
} 

MainViewController.h

#import "FlipsideViewController.h" 
#import "WeatherForecast.h" 

#import <CoreData/CoreData.h> 

@interface MainViewController : UIViewController <FlipsideViewControllerDelegate> 

- (IBAction)showInfo; 
- (IBAction)refreshView:(id) sender; 
- (void)updateView; 

@property (strong, nonatomic) NSManagedObjectContext *managedObjectContext; 
@property (strong, nonatomic) WeatherForecast *forecast; 

@end 

Answer 1

這個問題應該在你的第二行application:didFinishLaunchingWithOptions。 self.MainViewController正在等待您的AppDelegate中的一個屬性。只需刪除此行並在return YES.之前添加controller.forecast = forecast;此時，您已獲得對MainViewController的引用，並且可以安全地設置該屬性（假設通過Storyboard或XIB將MainViewController設置爲當前rootViewController）。

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions 
{  
    WeatherForecast *forecast = [[WeatherForecast alloc] init]; 
    // Override point for customization after application launch. 
    MainViewController *controller = (MainViewController *)self.window.rootViewController; 
    controller.managedObjectContext = self.managedObjectContext; 
    controller.forecast = forecast; 
    return YES; 
}