在iPhone应用程序中从磁盘加载图像很慢 [英] loading images from disk in iPhone app is slow

问题描述

在我的iPhone应用程序中，我正在使用iPhone的相机拍摄照片并将其保存到磁盘（应用程序的文档文件夹）。这是我如何保存它：

In my iPhone app, I am using the iPhone's camera to take a photo and save it do disk (the application's documents folder). This is how i save it:

[UIImageJPEGRepresentation(photoTaken, 0.0) writeToFile:jpegPath atomically:YES];

使用压缩程度最高，我认为从磁盘读取图像会很快。但它不是！我在我的一个视图中使用图像作为按钮的背景图像。我像这样加载它：

Using the most compression, I figured reading the image from disk would be quick. But its not! I use the image as the background image for a button in one of my views. I load it like this:

[self.frontButton setBackgroundImage:[UIImage imageWithContentsOfFile:frontPath] forState:UIControlStateNormal];

当我使用此按钮导航到视图时，它很慢且不连贯。我该如何解决这个问题？

When I navigate to the view with this button, it is slow and choppy. How do I fix this?

推荐答案

+ imageWithContentsOfFile：是同步的，所以主线程上的UI被磁盘操作中的图像加载阻塞并导致不稳定。解决方案是使用从磁盘异步加载文件的方法。您也可以在后台线程中执行此操作。这可以通过在 dispatch_async（）中包装 + imageWithContentsOfFile：，然后嵌套包装 -setBackgroundImage：的主队列上的> dispatch_async（），因为需要在主线程上运行UIKit方法。如果您希望在加载视图后立即显示图像，则需要从磁盘预先缓存图像，以便在视图出现时立即在内存中。

+imageWithContentsOfFile: is synchronous, so the UI on your main thread is being blocked by the image loading from disk operation and causing the choppiness. The solution is to use a method that loads the file asynchronously from disk. You could also do this in a background thread. This can be done easily by wrapping the +imageWithContentsOfFile: in dispatch_async(), then a nested dispatch_async() on the main queue that wraps -setBackgroundImage: since UIKit methods need to be run on the main thread. If you want the image to appear immediately after the view loads, you'll need to pre-cache the image from disk so it's in-memory immediately when the view appears.

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{

    UIImage *image = [UIImage imageWithContentsOfFile:frontPath];

    dispatch_async(dispatch_get_main_queue(), ^{
        [self.frontButton setBackgroundImage:image forState:UIControlStateNormal];
    });

});

另外，如果按钮图像出现渐变，请考虑使用以下属性来确保图像从磁盘加载的文件很小：

As an aside, if the button image happens a gradient, consider using the following properties to ensure the image file loaded from disk is tiny:

- (UIImage *)resizableImageWithCapInsets:(UIEdgeInsets)capInsets

或（不建议使用，仅在您需要支持iOS 4.x时使用）：

or (deprecated, only use if you need to support iOS 4.x):

- (UIImage *)stretchableImageWithLeftCapWidth:(NSInteger)leftCapWidth topCapHeight:(NSInteger)topCapHeight

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