Question

我有一个测试应用设置，即使用户在下载过程中切换应用，它也能成功从网络下载内容。太好了，现在我有后台下载了。现在我想添加缓存。没有必要让我多次下载图像，b / c系统设计，给定一个图像URL我可以告诉你该URL后面的内容永远不会改变。所以，现在我想使用Apple内置的内存/磁盘缓存来缓存下载的结果，我已经阅读了很多（而不是我在NSCachesDirectory中手动保存文件，然后在创建之前检查文件）请求，ick）。在尝试中使缓存在此工作代码之上工作，我添加了以下代码：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Override point for customization after application launch.

    // Set app-wide shared cache (first number is megabyte value)
    [NSURLCache setSharedURLCache:[[NSURLCache alloc] initWithMemoryCapacity:60 * 1024 * 1024
                                                                diskCapacity:200 * 1024 * 1024
                                                                    diskPath:nil]];

    return YES;
}

当我创建会话时，我添加了两行NEW（URLCache和requestCachePolicy）。

// Helper method to get a single session object
- (NSURLSession *)backgroundSession
{
    static NSURLSession *session = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration backgroundSessionConfiguration:@"com.example.apple-samplecode.SimpleBackgroundTransfer.BackgroundSession"];
        configuration.URLCache = [NSURLCache sharedURLCache]; // NEW LINE ON TOP OF OTHERWISE WORKING CODE
        configuration.requestCachePolicy = NSURLRequestReturnCacheDataElseLoad;  // NEW LINE ON TOP OF OTHERWISE WORKING CODE
        session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];
    });
    return session;
}

然后，为了看到缓存成功，我只是为了超冗余，我从

切换了我的NSURLRequest行

// NSURLRequest *request = [NSURLRequest requestWithURL:downloadURL]; // Old line, I've replaced this with...
NSURLRequest *request = [NSURLRequest requestWithURL:downloadURL cachePolicy:NSURLRequestReturnCacheDataElseLoad timeoutInterval:2*60]; // New line

现在，当我第二次下载该项目时，体验就像第一次一样！需要很长时间才能下载和进度条动画缓慢而稳定，就像原始下载一样。我想立即在缓存中的数据!!我错过了什么？

---------------------------- UPDATE ------------------ ----------

好的，感谢Thorsten的回答，我在我的didFinishDownloadingToURL委托方法中添加了以下两行代码：

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)downloadURL {

    // Added these lines...
    NSLog(@"DiskCache: %@ of %@", @([[NSURLCache sharedURLCache] currentDiskUsage]), @([[NSURLCache sharedURLCache] diskCapacity]));
    NSLog(@"MemoryCache: %@ of %@", @([[NSURLCache sharedURLCache] currentMemoryUsage]), @([[NSURLCache sharedURLCache] memoryCapacity]));
    /*
    OUTPUTS:
    DiskCache: 4096 of 209715200
    MemoryCache: 0 of 62914560
    */
}

这很棒。它证实缓存正在增长。我认为，因为我使用的是downloadTask（下载到文件而不是内存），这就是为什么DiskCache正在增长而不是内存缓存？我想一切都会进入内存缓存，直到溢出，然后将使用磁盘缓存，并且可能在操作系统在后台杀死应用程序以释放内存之前，内存缓存已写入磁盘。我误解了Apple的缓存是如何工作的吗？

这是向前迈出的一步，但是第二次下载文件它只需要第一次（可能是10秒左右）并且以下方法再次执行：

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
    // This shouldn't execute the second time around should it? Even if this is supposed to get executed a second time around then shouldn't it be lightning fast? It's not.
    // On all subsequent requests, it slowly iterates through the downloading of the content just as slow as the first time. No caching is apparent. What am I missing?
}

你对我上面的编辑有什么看法？为什么我在后续请求中没有看到文件很快返回？

如何在第二次请求时确认文件是否从缓存中提供？

Answer 1

请注意，以下SO帖子帮助我解决了我的问题：Is NSURLCache persistent across launches?

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Set app-wide shared cache (first number is megabyte value)
    NSUInteger cacheSizeMemory = 500*1024*1024; // 500 MB
    NSUInteger cacheSizeDisk = 500*1024*1024; // 500 MB
    NSURLCache *sharedCache = [[NSURLCache alloc] initWithMemoryCapacity:cacheSizeMemory diskCapacity:cacheSizeDisk diskPath:@"nsurlcache"];
    [NSURLCache setSharedURLCache:sharedCache];
    sleep(1); // Critically important line, sadly, but it's worth it!
}

除了sleep(1)行之外，还要注意我的缓存大小; 500MB。

根据docs，您需要的缓存大小比您尝试缓存的大小要多。

响应大小足够小，可以合理地放入缓存中。（例如，如果提供磁盘缓存，则响应必须为no 大于磁盘缓存大小的5％左右。）

因此，例如，如果您希望能够缓存10MB图像，那么10MB甚至20MB的缓存大小是不够的。你需要200MB。下面是Honey的评论，证明Apple正遵循这一5％的规则。对于8Mb，他必须将缓存大小设置为最小154MB。

Answer 2

解决方案 - 首先获取所有信息，你需要这样的东西

- (void)loadData
{
    if (!self.commonDataSource) {
        self.commonDataSource = [[NSArray alloc] init];
    }

    [self setSharedCacheForImages];

    NSURLSession *session = [self prepareSessionForRequest];
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:[BaseURLString stringByAppendingPathComponent:@"app.json"]]];
    [request setHTTPMethod:@"GET"];
    __weak typeof(self) weakSelf = self;
    NSURLSessionDataTask *dataTask = [session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
        if (!error) {
            NSArray *jsonResponse = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:&error];
            weakSelf.commonDataSource = jsonResponse;
            dispatch_async(dispatch_get_main_queue(), ^{
                [weakSelf updateDataSource];
            });
        }
    }];
    [dataTask resume];
}

- (void)setSharedCacheForImages
{
    NSUInteger cashSize = 250 * 1024 * 1024;
    NSUInteger cashDiskSize = 250 * 1024 * 1024;
    NSURLCache *imageCache = [[NSURLCache alloc] initWithMemoryCapacity:cashSize diskCapacity:cashDiskSize diskPath:@"someCachePath"];
    [NSURLCache setSharedURLCache:imageCache];
}

- (NSURLSession *)prepareSessionForRequest
{
    NSURLSessionConfiguration *sessionConfiguration = [NSURLSessionConfiguration defaultSessionConfiguration];
    [sessionConfiguration setHTTPAdditionalHeaders:@{@"Content-Type": @"application/json", @"Accept": @"application/json"}];
    NSURLSession *session = [NSURLSession sessionWithConfiguration:sessionConfiguration];
    return session;
}

在您需要下载每个文件后 - 在我的情况下 - 解析响应并下载图像。在提出请求之前，您还需要检查缓存是否已经响应了您的请求 - 就像这样

NSString *imageURL = [NSString stringWithFormat:@"%@%@", BaseURLString ,sourceDictionary[@"thumb_path"]];
NSURLSession *session = [self prepareSessionForRequest];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:imageURL]];
[request setHTTPMethod:@"GET"];

NSCachedURLResponse *cachedResponse = [[NSURLCache sharedURLCache] cachedResponseForRequest:request];
if (cachedResponse.data) {
    UIImage *downloadedImage = [UIImage imageWithData:cachedResponse.data];
    dispatch_async(dispatch_get_main_queue(), ^{
        cell.thumbnailImageView.image = downloadedImage;
    });
} else {
NSURLSessionDataTask *dataTask = [session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
    if (!error) {
        UIImage *downloadedImage = [UIImage imageWithData:data];
        dispatch_async(dispatch_get_main_queue(), ^{
            cell.thumbnailImageView.image = downloadedImage;
        });
    }
}];
    [dataTask resume];
}

之后，您还可以使用xCode Network Analyzer检查结果。

同样请注意@jcaron和documented by Apple

NSURLSession不会尝试缓存大于缓存5％的文件大小

结果类似

enter image description here

Answer 3

设置缓存和会话后，您应该使用会话方法下载数据：

- (IBAction)btnClicked:(id)sender {
    NSString *imageUrl = @"http://placekitten.com/1000/1000";
    NSURLSessionDataTask* loadDataTask = [session dataTaskWithRequest:[NSURLRequest requestWithURL:[NSURL URLWithString:imageUrl]] completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
        UIImage *downloadedImage = [UIImage imageWithData:data];
        NSLog(@"ImageSize: %f, %f", downloadedImage.size.width, downloadedImage.size.height);
        NSLog(@"DiskCache: %i of %i", [[NSURLCache sharedURLCache] currentDiskUsage], [[NSURLCache sharedURLCache] diskCapacity]);
        NSLog(@"MemoryCache: %i of %i", [[NSURLCache sharedURLCache] currentMemoryUsage], [[NSURLCache sharedURLCache] memoryCapacity]);
    }];
    [loadDataTask resume]; //start request
}

第一次通话后，图像被缓存。

如何使用NSURLSession和NSURLCache进行缓存。不工作

3 个答案: