在后台线程上绘制上下文

Question

我有一个 UIView 必须显示一些信息。由于此视图非常复杂并且绘制需要大量时间，因此我已将其移至后台线程以避免阻塞主线程。绘制完成后，我分派回主线程来设置 ImageView。

下面的代码为我绘制了图形，但我偶尔会收到一些崩溃消息。通常 xcode 指向 [view release]; 并说 Thread 6: Program receive signal: EXC_BAD_ACCESS

NSManagedObjectID *objectID = [self.managedObject objectID];
        dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0);
        dispatch_async(queue,^{
            NSManagedObjectContext *backgroundContext = [[NSManagedObjectContext alloc] init];
            [backgroundContext setPersistentStoreCoordinator:[[self.managedObject managedObjectContext] persistentStoreCoordinator]];
            NSManagedObject *mo = [backgroundContext objectWithID:objectID];
            CGImageRef resultImage;
            CGContextRef context;
            void *bitmapData;

    CGRect frame = self.frame;
    frame.origin = CGPointZero;

    CGColorSpaceRef colorSpace;
    CGSize canvasSize;
    int bitmapByteCount;
    int bitmapBytesPerRow;

    canvasSize = frame.size;
    CGFloat scale = [UIScreen instancesRespondToSelector:@selector(scale)] ? [[UIScreen mainScreen] scale] : 1.0f;
    canvasSize.width *= scale;
    canvasSize.height *= scale;

    bitmapBytesPerRow = (canvasSize.width * 4);
    bitmapByteCount = (bitmapBytesPerRow * canvasSize.height);

    //Create the color space
    colorSpace = CGColorSpaceCreateDeviceRGB();

    bitmapData = malloc( bitmapByteCount );

    //Check the the buffer is alloc'd
    if( bitmapData == NULL ){
        DLog(@"Buffer could not be alloc'd");
    }

    //Create the context
    context = CGBitmapContextCreate(bitmapData, canvasSize.width, canvasSize.height, 8, bitmapBytesPerRow, colorSpace, kCGImageAlphaPremultipliedLast);
    CGContextClearRect(context, CGRectMake(0, 0, canvasSize.width, canvasSize.height));

    // Setup transformation
    CGContextSetInterpolationQuality(context, kCGInterpolationNone);
    CGContextTranslateCTM(context, 0.0f, canvasSize.height); 
    CGContextScaleCTM(context, scale, -scale);

    UIView *view = [[UIView alloc] initWithFrame:frame];
    [view setBackgroundColor:[UIColor clearColor]];

    UILabel *nameLabel = [[UILabel alloc] initWithFrame:CGRectMake(137, 5, 255, 30)];
    [nameLabel setFont:[UIFont fontWithName:@"Arial" size:22.0f]];
    [nameLabel setText:[mo valueForKey:@"name"]];
    [nameLabel setTextAlignment:UITextAlignmentRight];
    [view addSubview:nameLabel];
    [nameLabel release];
    nameLabel = nil;
    ... Creating 20 or so label/imageviews all use 'mo' for data access to the NSManagedObject.

    [view.layer renderInContext:context];
    [view release];
    view = nil;
    //Get the result image
    resultImage = CGBitmapContextCreateImage(context);
    UIImage *viewImage = [UIImage imageWithCGImage:resultImage scale:0.0 orientation:UIImageOrientationUp];
    dispatch_async(dispatch_get_main_queue(),^ {
        [dataImageView setImage:viewImage];
    });

    //Cleanup
    free(bitmapData);
    CGColorSpaceRelease(colorSpace);
    CGImageRelease(resultImage);  
    CGContextRelease(context);
}); 

希望你们能帮助我，因为我对此一无所知。我尝试使用 NSZombieEnabled YES 运行它，有时会导致控制台中出现以下错误 -[UILabel hash]: message sent to deallocated instance 0x22841c00

Answer 1

尝试调查问题的可能根源：
当您创建 UILabel 时，您将它们添加到 UIView 中，然后释放它们（没关系），但您也“nil”它们。所以此时你有了 UIView 保留的 UILabel，但同时它指向 nil！
因此，最后当您释放“视图”时，它将尝试释放其子视图，并且一旦遇到保留计数 = 1 但已为零的 UILabel，它就会收到“已释放实例”异常。
这可以演示为什么调试器在 [view release] 处停止，以及为什么 NSZombieEnabled 会抱怨尝试释放 UILabel：当指针指向 nil 时，它会将其视为已释放。
所以可能的解决方案：删除“nameLabel = nil”语句。