从后台线程中的委托方法更改 UILabel 的文本不起作用

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

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基本上，我创建一个标签并将其添加到屏幕上。此时，您可以清楚地看到一个蓝色按钮，上面有白色文字“Bob”。当您旋转屏幕时，按钮会很好地更改文本和位置。但是，如果我单击该按钮，它会创建一个 dataParser 对象，将委托设置为 self，并调用“loadSomething”。在 dataParser 对象中，“loadSomething”只是调用了委托方法“loadMessage”。

问题是，当调用“loadMessage”时，NSLog 语句会正确打印（“Received message: ASDF”），但是，theLabel 不会移动，也不会更改其文本。为什么是这样？我该如何解决这个问题？

Answer 1

试试这个代码：

-(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!";
    });
}

它应该在主线程上运行它。