在后台线程中加载UIImage [英] Load UIImage in background Thread

问题描述

是否可以在后台线程中加载UIImage而不会导致线程问题？
如果不是最好的方法是什么？我正在使用iOS 8.这就是我现在这样做的方式：

Is it possible to load a UIImage in a background thread without causing threading problems? If not what is the best way of doing it? I'm using iOS 8. This is the way I do it right now:

    dispatch_queue_t backgroundQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
    dispatch_async(backgroundQueue, ^{
        UIImage *image = [UIImage imageNamed: fileName];

        // only update UI on the main thread
        dispatch_async(dispatch_get_main_queue(), ^{
            [self setImage: image];
        });

    });

推荐答案

你所做的是结构合理，但我不知道 imageNamed：是否是线程安全的 - 我没有理由相信它是。除非文档另有说明，否则您应该始终认为事情不是线程安全的。在这种情况下，文档明确说明它不：

What you're doing is structurally sound, but I don't know whether imageNamed: is thread-safe — and I have no reason to believe that it is. You should always assume that things are not thread-safe unless you are told otherwise by the documentation. In this case, the documentation specifically says that it is not:

你不能假设这个方法是线程安全。

You can not assume that this method is thread safe.

在我看来，你应该问自己是否需要这样做。 imageNamed：包含一个缓存机制，可以减轻您担心的任何问题。无论如何，过早优化会浪费你的时间和智力。这里真的有问题吗？用仪器找出;不要使用直觉或直觉。

In my view, you should ask yourself whether you need to do this at all. imageNamed: includes a caching mechanism that should relieve you of whatever you are worried about. In any case, premature optimization is a waste of your time and brainpower. Is there really an issue here? Use Instruments to find out; don't use intuition or instinct.

如果问题是您的图像太大而且格式选择不当 - 例如，您使用的是非常大的JPEG - 最好集中精力纠正 。

If the problem is that your images are too large and in a bad choice of format — for example, you are using very large JPEGs — it would be better to concentrate on correcting that.

编辑 iOS 9文档现在说：在iOS 9及更高版本，此方法是线程安全的。这表明我的答案都是正确的，问题现在已经解决了。

EDIT The iOS 9 documentation now says: "In iOS 9 and later, this method is thread safe." This suggests both that my answer was correct and that the problem is now solved.

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