ViewController類型名稱在一個類中不被識別，但它在另一個類中？

Question

當我嘗試構建我的項目時，XCode中出現構建語義錯誤。我有3 NSOperation類設置從互聯網下載信息，處理它，並將其發送到父視圖控制器，ViewController的實例。下面是一個工人階級的代碼：

#import <Foundation/Foundation.h> 
#import "ViewController.h" 

@interface ImageGetter : NSOperation 

@property (nonatomic) int sid; 
@property (nonatomic, retain) ViewController* pvc; 

@end 

下面是一個是不工作代碼：

#import <Foundation/Foundation.h> 
#import "ViewController.h" 

@interface CurrentGetter : NSOperation 

@property (nonatomic) int sid; 
@property (nonatomic, retain) ViewController* pvc; 

@end 

錯誤：

ERROR: Unknown type name 'ViewController'; did you mean 'UIViewController'? 

爲什麼我收到這個錯誤？我在兩個文件中導入了ViewController.h，只有一個正在工作。

ViewController.h：

#import <UIKit/UIKit.h> 
#import <AVFoundation/AVFoundation.h> 
#import <MediaPlayer/MediaPlayer.h> 

#import "DataGetter.h" 
#import "CurrentGetter.h" 
#import "AppDelegate.h" 

@interface ViewController : UIViewController <UITabBarDelegate> 
{ 
    NSTimer* tmr; 
} 

@property (nonatomic) float max; 
@property (nonatomic, retain) NSDictionary* plist; 
@property (nonatomic, retain) NSArray* processingobj; 

@property (nonatomic, retain) AVAudioPlayer* intro; 
@property (nonatomic, retain) AVAudioPlayer* woop; 

@property (nonatomic) float dataDone; 
@property (nonatomic) BOOL introDone; 
@property (nonatomic) BOOL currentDone; 

@property (nonatomic) int newSid; 
@property (nonatomic) int newPart; 

@property (nonatomic, retain) NSMutableArray* views; 

@property (nonatomic) int audioTab; 
@property (nonatomic) int currentSid; 
@property (nonatomic) BOOL isPlaying; 

@property (nonatomic, retain) UIScrollView* partTab; 
@property (nonatomic, retain) NSMutableArray* nowPlayingObj; 
@property (nonatomic, retain) UINavigationBar* audioNav; 

@property (nonatomic, retain) MPMoviePlayerController* player; 
@property (nonatomic, retain) UILabel* elapsed; 
@property (nonatomic, retain) UILabel* remaining; 
@property (nonatomic, retain) UISlider* slider; 
@property (nonatomic) BOOL sliderEditing; 
@property (nonatomic, retain) UIButton* playBtn; 

@property (nonatomic, retain) UITabBar* tabBar; 

//Images 

@property (nonatomic, retain) UIImage* announcement; 
@property (nonatomic, retain) NSMutableArray* seriesBanners; 
@property (nonatomic, retain) UIProgressView* prog; 

- (void)plistDone; 

@end 

Answer 1

嘗試做之類的前向聲明在CurrentGetter頭文件中，就像這樣：

@class ViewController; 
@interface CurrentGetter : NSOperation 

這告訴編譯器存在一個所謂的「類視圖控制器「。當存在循環依賴關係時（例如兩個類彼此具有實例時），使用類的前向聲明可解決導致的混淆問題。