objective-c - 为什么 NSURLSession 不使用我配置的 NSURLCache？

Question

iOS7带来了NSURLSession，借助NSURLSessionConfigure，我们可以自定义URLCache，所以我试了一下，但是没有运气，好像我的URLCache没有用到全部。

- (void)testURLCache
{
    NSURLSessionConfiguration *config = [NSURLSessionConfiguration defaultSessionConfiguration];
    config.requestCachePolicy = NSURLRequestUseProtocolCachePolicy;
    NSURLCache *cache = [[NSURLCache alloc] initWithMemoryCapacity:1024 * 100 diskCapacity:1024 * 100 diskPath:@"test.urlcache"];

    NSLog(@"usage:%lu", (unsigned long)[cache currentDiskUsage]);

    config.URLCache = cache;

    NSURLSession *session = [NSURLSession sessionWithConfiguration:config];
    for (int i = 0; i < 40; i++) {
        NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:[NSString stringWithFormat:@"http://ajax.googleapis.com/ajax/libs/angularjs/1.2.6/angular.min.js?%d", i]]];
        NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
            if (error) {
                NSLog(@"error:%@", error);
            } else {
                NSLog(@"done:%d", i);
            }
        }];
        [task resume];
        [NSThread sleepForTimeInterval:0.5];
    }
    NSLog(@"usage:%lu", (unsigned long)[cache currentDiskUsage]);
}

我观察到的东西:

1) 在Caches文件夹下创建一个test.urlcache文件夹，里面有3个文件Cache.db，Cache。 db-shm，Cache.db-wal，Cache.db大小为4KB，里面没有记录。

2)每次运行代码，第一次输出usage总是0，for循环结束后usage变为4096，下一次usage又变为0。

Answer 1

似乎有两个潜在的问题:

1. 您的缓存太小。从历史上看，如果缓存的项目超过总缓存大小的 10%，它就不会缓存。

2. 您正在检查上次请求后 0.5 秒的磁盘使用情况。将缓存写入持久存储可能为时过早。

下面我使用了一个更大的缓存并在最终检查磁盘使用情况之前等待 10 秒，缓存工作正常。我还使用 GCD 对串行任务进行排队，以消除任意 0.5 秒的延迟，这既更安全，也使它在通过网络检索数据和使用缓存时更加明显。

- (void)testURLCache
{
    NSURLSessionConfiguration *config = [NSURLSessionConfiguration defaultSessionConfiguration];
    config.requestCachePolicy = NSURLRequestUseProtocolCachePolicy;

    // this is too small and won't cache
    // NSURLCache *cache = [[NSURLCache alloc] initWithMemoryCapacity:100 * 1024 diskCapacity:100 * 1024 diskPath:@"small.urlcache"];

    // use bigger cache
    NSURLCache *cache = [[NSURLCache alloc] initWithMemoryCapacity:10 * 1024 * 1024 diskCapacity:20 * 1024 * 1024 diskPath:@"big.urlcache"];

    config.URLCache = cache;

    // I'm going to add tasks to serial queue so that (a) I don't block main queue;
    // and (b) I don't need arbitrary delay, but I can just signal when it's done.

    dispatch_queue_t queue = dispatch_queue_create("com.domain.app.datatasks", 0);

    NSURLSession *session = [NSURLSession sessionWithConfiguration:config];
    for (int i = 0; i < 40; i++) {
        dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

        dispatch_async(queue, ^{
            NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:[NSString stringWithFormat:@"http://ajax.googleapis.com/ajax/libs/angularjs/1.2.6/angular.min.js?%d", i]]];
            NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {

                if (error) {
                    NSLog(@"error:%@", error);
                } else {
                    NSLog(@"done:%d", i);
                }

                dispatch_semaphore_signal(semaphore);                   // signal dispatched block that request is done
            }];
            [task resume];

            dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);  // wait until task is complete
        });
    }

    // when the queued tasks finish, then check disk usage both immediately and 10 seconds later

    dispatch_async(queue, ^{

        // check cache size immediately after the data tasks finish

        NSLog(@"usage:%lu", (unsigned long)[cache currentDiskUsage]);

        // check again in 10 seconds

        double delayInSeconds = 10.0;
        dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
        dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
            NSLog(@"usage:%lu", (unsigned long)[cache currentDiskUsage]);
        });
    });
}

顺便说一句，对于像这样的网络任务，我通常会使用操作队列而不是调度队列，然后将我的数据任务包装在并发的 NSOperation 子类中。然后我可以使用依赖关系来指示最终的完成操作，但同时既允许并发又指示并发程度，但这似乎超出了原始问题的范围，我希望尽可能简单。