ViewController类型名称在一个类中不被识别，但它在另一个类中？

Question

当我试图构建我的项目时，我在XCode中得到了一个构建语义错误。我有三个NSOperation类设置从互联网下载信息，处理它，并将它发送到父视图控制器，一个ViewController的实例。下面是工人阶级的代码：

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

@interface ImageGetter : NSOperation

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

@end

下面是不起作用的代码：

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

@interface CurrentGetter : NSOperation

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

@end

错误：

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

我为什么要犯这个错误？我在两个文件中导入了ViewController.h，只有一个在工作。

查看控制器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

尝试在CurrentGetter头文件中对类执行前向声明，如下所示：

@class ViewController;
@interface CurrentGetter : NSOperation

这告诉编译器存在一个名为“ViewController”的类。当存在循环依赖时，例如两个类具有彼此的实例时，使用类的前向声明可以解决由此产生的混乱。