更改文本在後臺線程不工作

Question

HomeViewController.h

#import "DataParser.h" 
@interface HomeViewController : UIViewController <DataParserDelegate> 
{ 
    UILabel *theLabel; 
} 

HomeViewController.m

#import "HomeViewController.h" 
@implementation HomeViewController 
-(void)viewDidLoad 
{ 
    theLabel = [[UILabel alloc] initWithFrame:CGRectMake(200, 200, 100, 30)]; 
    theLabel.text = @"Bob"; 
    theLabel.font = [UIFont italicSystemFontOfSize:20.0]; 
    theLabel.textColor = [UIColor whiteColor]; 
    theLabel.backgroundColor = [UIColor blueColor]; 
    [self.view addSubview:theLabel]; 

    UIButton *theButton = [UIButton buttonWithType:UIButtonTypeRoundedRect]; 
    [theButton addTarget:self action:@selector(clicked:) forControlEvents:UIControlEventTouchUpInside]; 
    [theButton setTitle:@"Big Button" forState:UIControlStateNormal]; 
    theButton.frame = CGRectMake(100,100,200,50); 
    [self.view addSubview:theButton]; 
} 
-(void)clicked:(id)sender 
{ 
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND,0), 
    ^{ 
     DataParser *data = [[DataParser alloc] init]; 
     data.delegate = self; 
     [data loadSomething]; 

     dispatch_async(dispatch_get_main_queue(), 
     ^{ 
      NSLog(@"Thread done"); 
     }); 
    }); 
} 
-(void)willRotateToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation duration:(NSTimeInterval)duration 
{ 
    theLabel.frame = CGRectMake(250,250,100,30); 
    theLabel.text = @"This message displays fine"; 
} 
-(void)loadMessage:(NSString *)message 
{ 
    NSLog(@"Received message: %@", message); 
    theLabel.frame = CGRectMake(300,300,100,30); 
    theLabel.text = @"This message won't display"; 
} 
@end 

DataParser.h

@protocol DataParserDelegate; 

@interface DataParser : NSObject 
@property (weak, nonatomic) id <DataParserDelegate> delegate; 
-(void)loadSomething; 
@end 

@protocol DataParserDelegate <NSObject> 
-(void)loadMessage:(NSString *)message; 
@end 

DataParser.m

#import "DataParser.h" 
@implementation DataParser 
@synthesize delegate=_delegate; 
-(void)loadSomething 
{ 
    [[self delegate] loadMessage:@"ASDF"]; 
} 
@end 

- = - = - = - = - = - = - = - = - = - = - = - = - = -

基本上，我創建了一個標籤，並將其添加到屏幕上。在這一點上，你可以清楚地看到一個白色文字的藍色按鈕，上面寫着「Bob」。當你旋轉屏幕時，按鈕改變文本和位置就好了。然而，如果我點擊按鈕，它會創建一個dataParser對象，將委託設置爲self，並調用「loadSomething」。在dataParser對象中，「loadSomething」僅調用委託方法「loadMessage」。

問題是，當調用「loadMessage」時，NSLog語句正確打印（「Received message：ASDF」），但是，標籤不會移動並且不會更改其文本。爲什麼是這樣？我該如何解決這個問題？

Answer 1

試試這個代碼：

-(void)loadMessage:(NSString *)message 
{ 
    dispatch_async(dispatch_get_main_queue(), ^{ 
     NSLog(@"Received message: %@", message); 
     theLabel.frame = CGRectMake(300,300,100,30); 
     theLabel.text = @"This message displays great!"; 
    }); 
} 

應該在主線程上運行它。