在iOS中使用线程返回图像 [英] Returning image with threading in iOS

问题描述

您好，我正在学习Stanford iOS开发课程.我有一个关于线程的问题.我了解UIKit调用应由主线程处理.我想知道这样的事情是否合法?

Hi I'm working through the Stanford iOS development class. I have a question regarding threading. I understand UIKit calls should be handled by the main thread. I was wondering if something like this is legal?

- (UIImage *)mapViewController:(MapViewController *)sender imageForAnnotation:(id<MKAnnotation>)annotation {
    FlickrPhotoAnnotation *fpa = (FlickrPhotoAnnotation *) annotation;
    NSURL *url = [FlickrFetcher urlForPhoto:fpa.photo format:FlickrPhotoFormatSquare];

    __block UIImage *image;
    dispatch_queue_t downloadQ = dispatch_queue_create("download queue", NULL);
    dispatch_async(downloadQ, ^{
        NSData *data = [NSData dataWithContentsOfURL:url];
        if (data) {
            dispatch_async(dispatch_get_main_queue(), ^{
                image = [UIImage imageWithData:data];
            });
        }
    });
    dispatch_release(downloadQ);
    return image;
}

还是我应该只返回NSData并处理调用方法中的所有线程?

or I should just return NSData and handle all the threading in the calling method?

谢谢

推荐答案

您的代码不会执行您想要的操作.

Your code won't do what you want.

您正在将异步块放入队列中，然后立即从方法中返回.您不能保证该块实际上会在返回之前运行-实际上，很有可能它不会运行.

You are putting an asynchronous block into a queue, and then immediately returning from the method. You are not guaranteed that the block will actually run before you return -- in fact, the odds are that it won't.

因此，您将返回image的当前值.由于您没有初始化它，所以可能是垃圾.调用此方法的任何人都将尝试使用指向图像的垃圾指针，并且(如果幸运的话)崩溃.如果您将其初始化为nil:

So you'll return the current value of image. Since you didn't initialize it, it's probably garbage. Whoever calls this method will try to use a garbage pointer to an image, and (if you're lucky) crash. If you had initialized it to nil:

__block UIImage *image = nil;

礼貌些.

这里的问题是:您的方法必须返回UIImage，因此在返回之前，必须等待制作完全构造的UIImage所花费的时间.在其他线程上执行此操作的好处为零-您仍在等待相同的时间，而切换线程只会增加开销.

The problem here is: your method must return a UIImage, so you must wait for the time it takes to make a fully constructed UIImage before you return. There is zero benefit to doing this on some other thread -- you're still waiting the same amount of time, and switching threads just adds overhead.

为了以一种有用的异步方式加载图像，您需要某种方式，通过对委托方法或块的回调，异步地告诉调用者何时图像加载完成.例如，查看Foundation中的NSURLConnection.它具有一个通过委托进行回调的较旧的方法，以及通过一个块进行回调的一个较新的方法(+sendAsynchronousRequest:queue:completionHandler:).

In order to load the image in a usefully asynchronous way, you need some way to asynchronously tell the caller when the image is done loading, via a callback to a delegate method or block. For example, look at NSURLConnection in Foundation. It has an older method that calls back via a delegate, and a newer method (+sendAsynchronousRequest:queue:completionHandler:) that calls back via a block.

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