Objectivec iOS中的非延迟图像加载

我正在try 将UIImage加载到背景线程中，然后在iPad上显示它们.然而，当我将imageViews的view属性设置为图像时，会出现一个口吃.

CGImage/UIImage lazily loading on UI thread causes stutter

这实际上会强制将图像加载到线程中，但在显示图像时仍然会出现口吃.

你可以在这里找到我的示例项目:http://www.jasamer.com/files/SwapTest.zip(编辑:fixed version)，判断SwapTestViewController.try 拖动图片以查看结巴.

我创建的测试代码是这样的(forceLoad方法取self 上面发布的堆栈溢出问题):

NSArray* imagePaths = [NSArray arrayWithObjects:
                       [[NSBundle mainBundle] pathForResource: @"a.png" ofType: nil], 
                       [[NSBundle mainBundle] pathForResource: @"b.png" ofType: nil], nil];

NSOperationQueue* queue = [[NSOperationQueue alloc] init];

[queue addOperationWithBlock: ^(void) {
    int imageIndex = 0;
    while (true) {
        UIImage* image = [[UIImage alloc] initWithContentsOfFile: [imagePaths objectAtIndex: imageIndex]];
        imageIndex = (imageIndex+1)%2;
        [image forceLoad];

        //What's missing here?

        [self performSelectorOnMainThread: @selector(setImage:) withObject: image waitUntilDone: YES];
        [image release];
    }
}];

我知道口吃可以避免有两个原因:

(1) 苹果能够在照片应用程序中加载图像而不会出现口吃

(2) 在上述代码的修改版本中，占位符1和占位符2显示一次后，此代码不会导致口吃:

    UIImage* placeholder1 = [[UIImage alloc] initWithContentsOfFile:[[NSBundle mainBundle] pathForResource: @"a.png" ofType: nil]];
[placeholder1 forceLoad];
UIImage* placeholder2 = [[UIImage alloc] initWithContentsOfFile:[[NSBundle mainBundle] pathForResource: @"b.png" ofType: nil]];
[placeholder2 forceLoad];

NSArray* imagePaths = [NSArray arrayWithObjects:
                       [[NSBundle mainBundle] pathForResource: @"a.png" ofType: nil], 
                       [[NSBundle mainBundle] pathForResource: @"b.png" ofType: nil], nil];
NSOperationQueue* queue = [[NSOperationQueue alloc] init];
[queue addOperationWithBlock: ^(void) {
    int imageIndex = 0;
    while (true) {
        //The image is not actually used here - just to prove that the background thread isn't causing the stutter
        UIImage* image = [[UIImage alloc] initWithContentsOfFile: [imagePaths objectAtIndex: imageIndex]];
        imageIndex = (imageIndex+1)%2;
        [image forceLoad];

        if (self.imageView.image==placeholder1) {
            [self performSelectorOnMainThread: @selector(setImage:) withObject: placeholder2 waitUntilDone: YES];
        } else {
            [self performSelectorOnMainThread: @selector(setImage:) withObject: placeholder1 waitUntilDone: YES];
        }                

        [image release];
    }
}];

然而，我不能把所有的图片都留在记忆中.

这意味着forceLoad并不能完成全部工作——在实际显示图像之前，还有其他事情要做.

谢谢，朱利安

Update

使用了Tommys的一些技巧.我发现是CGSConvertBGRA888TorgBa8888花费了太多时间，所以看起来是 colored颜色 转换造成了延迟.

Running        Symbol Name
6609.0ms        CGSConvertBGRA8888toRGBA8888
6609.0ms         ripl_Mark
6609.0ms          ripl_BltImage
6609.0ms           RIPLayerBltImage
6609.0ms            ripc_RenderImage
6609.0ms             ripc_DrawImage
6609.0ms              CGContextDelegateDrawImage
6609.0ms               CGContextDrawImage
6609.0ms                CA::Render::create_image_by_rendering(CGImage*, CGColorSpace*, bool)
6609.0ms                 CA::Render::create_image(CGImage*, CGColorSpace*, bool)
6609.0ms                  CA::Render::copy_image(CGImage*, CGColorSpace*, bool)
6609.0ms                   CA::Render::prepare_image(CGImage*, CGColorSpace*, bool)
6609.0ms                    CALayerPrepareCommit_(CALayer*, CA::Transaction*)
6609.0ms                     CALayerPrepareCommit_(CALayer*, CA::Transaction*)
6609.0ms                      CALayerPrepareCommit_(CALayer*, CA::Transaction*)
6609.0ms                       CALayerPrepareCommit_(CALayer*, CA::Transaction*)
6609.0ms                        CALayerPrepareCommit
6609.0ms                         CA::Context::commit_transaction(CA::Transaction*)
6609.0ms                          CA::Transaction::commit()
6609.0ms                           CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned long, void*)
6609.0ms                            __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__
6609.0ms                             __CFRunLoopDoObservers
6609.0ms                              __CFRunLoopRun
6609.0ms                               CFRunLoopRunSpecific
6609.0ms                                CFRunLoopRunInMode
6609.0ms                                 GSEventRunModal
6609.0ms                                  GSEventRun
6609.0ms                                   -[UIApplication _run]
6609.0ms                                    UIApplicationMain
6609.0ms                                     main         

遗憾的是，他提出的最后一点面具改变并没有改变任何事情.