我正在尝试在后台线程中加载UIImages,然后在iPad上显示它们.但是,当我将imageViews的视图属性设置为图像时,会出现断断续续的情况.我很快发现iOS上的图像加载是懒惰的,并且在这个问题中找到了部分解决方案:
在UI线程上懒洋洋地加载CGImage/UIImage会导致口吃
这实际上会强制图像加载到线程中,但在显示图像时仍然存在断断续续的情况.
你可以在这里找到我的示例项目:http://www.jasamer.com/files/SwapTest.zip(编辑:修复版),检查SwapTestViewController.尝试拖动图片以查看口吃.
我创建的测试代码是断断续续的(forceLoad方法是我从上面发布的堆栈溢出问题中获取的方法):
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];
}
}];
我知道可以避免口吃的原因有两个:
(1)Apple可以在照片应用中加载图像而不会出现断断续续的情况
(2)在上述代码的此修改版本中,placeholder1和placeholder2已显示一次后,此代码不会导致断断续续:
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];
}
}];
但是,我无法将所有图像保存在内存中.
这意味着forceLoad不能完成整个工作 - 在实际显示图像之前还会发生其他事情.有谁知道那是什么,我怎么能把它放到后台线程？
谢谢,朱利安
更新
使用了一些Tommys技巧.我想到的是它的CGSConvertBGRA8888到RGBA8888需要花费很多时间,因此它似乎是导致延迟的颜色转换.这是该方法的(反向)调用堆栈.
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,他提出的最后一次掩码改变没有改变任何东西.
Tom*_*mmy 39
UIKit只能在主线程上使用.因此,您的代码在技术上无效,因为您从主线程以外的线程使用UIImage.您应该单独使用CoreGraphics在后台线程上加载(并且非延迟解码)图形,将CGImageRef发布到主线程并将其转换为UIImage.在您当前的实现中,它似乎可以工作(尽管有你不想要的口吃),但不能保证.围绕这个领域似乎有很多迷信和不良做法,所以你设法找到一些不好的建议就不足为奇了......
建议在后台线程上运行:
// get a data provider referencing the relevant file
CGDataProviderRef dataProvider = CGDataProviderCreateWithFilename(filename);
// use the data provider to get a CGImage; release the data provider
CGImageRef image = CGImageCreateWithPNGDataProvider(dataProvider, NULL, NO,
kCGRenderingIntentDefault);
CGDataProviderRelease(dataProvider);
// make a bitmap context of a suitable size to draw to, forcing decode
size_t width = CGImageGetWidth(image);
size_t height = CGImageGetHeight(image);
unsigned char *imageBuffer = (unsigned char *)malloc(width*height*4);
CGColorSpaceRef colourSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef imageContext =
CGBitmapContextCreate(imageBuffer, width, height, 8, width*4, colourSpace,
kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little);
CGColorSpaceRelease(colourSpace);
// draw the image to the context, release it
CGContextDrawImage(imageContext, CGRectMake(0, 0, width, height), image);
CGImageRelease(image);
// now get an image ref from the context
CGImageRef outputImage = CGBitmapContextCreateImage(imageContext);
// post that off to the main thread, where you might do something like
// [UIImage imageWithCGImage:outputImage]
[self performSelectorOnMainThread:@selector(haveThisImage:)
withObject:[NSValue valueWithPointer:outputImage] waitUntilDone:YES];
// clean up
CGImageRelease(outputImage);
CGContextRelease(imageContext);
free(imageBuffer);
如果您使用的是iOS 4或更高版本,则无需执行malloc/free,您只需将NULL作为CGBitmapContextCreate的相关参数传递,并让CoreGraphics整理出自己的存储.
这与您发布的解决方案不同,因为它:
所以加载的东西和显示的东西之间没有对象的连续性; 像素数据通过一个C数组(因此,没有隐藏的恶作剧的机会),只有当它正确地放入数组中才有可能制作出最终的图像.
jas*_*mer 14
好吧,想通了 - 在汤米的帮助下.谢谢!
如果您使用创建上下文
CGContextRef imageContext =
CGBitmapContextCreate(imageBuffer, width, height, 8, width*4, colourSpace,
kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little);
主运行循环不会再导致主线程上的任何转换.显示现在是黄油顺利.(标志有点违反直觉,有谁知道为什么你必须选择kCGImageAlphaPremultipliedFirst？)
编辑:
上传了固定的示例项目:http://www.jasamer.com/files/SwapTest-Fixed.zip.如果您的图像性能有问题,这是一个很好的起点!
Kaz*_*oto 10
怎么样的UIImage + ImmediateLoad.m？
它立即解码图像.除了UIImage -initWithContentsOfFile:, - imageWithCGImage:和CG*在iOS4之后是线程安全的.