iOS 使用 NSURLSessionDataTask 的后台任务

Question

我的应用程序中有航班搜索功能，但获取数据的时间太长（超过 25 秒）。如果应用程序进入后台或进入睡眠模式，互联网连接会断开。

我使用 apple 示例编写了以下逻辑，使 api 请求继续运行，即使应用程序进入后台但它不工作。

self.session = [self backgroundSession];
self.mutableData = [NSMutableData data];

NSURL *downloadURL = [NSURL URLWithString:@"http://jsonplaceholder.typicode.com/photos"];
NSURLRequest *request = [NSURLRequest requestWithURL:downloadURL];
self.dataTask = [self.session dataTaskWithRequest:request];
[self.dataTask resume];

- (NSURLSession *)backgroundSession
{
    static NSURLSession *session = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:@"com.example.apple-samplecode.SimpleBackgroundTransfer.BackgroundSession"];
        session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];
    });
    return session;
}

下面是委托方法

- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask
didReceiveResponse:(NSURLResponse *)response
completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler {
 NSLog(@"response: %@", response.debugDescription);
 NSURLSessionResponseDisposition disposition = NSURLSessionResponseAllow;
    if (completionHandler) {
        completionHandler(disposition);
    }
}

- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask
    didReceiveData:(NSData *)data {
    [self.mutableData appendData:data];
}


- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error
{
    BLog();

    if (error == nil)
    {
        NSData *data = nil;
        if (self.mutableData) {
            data = [self.mutableData copy];
            self.mutableData = nil;
        }

        NSError* error;
        NSArray* json = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:&error];
        if (!json) {
            NSLog(@"Error parsing JSON: %@", error);
        } else {
            NSLog(@"Data: %@", json);
        }
    }
    else
    {
        NSLog(@"Task: %@ completed with error: %@", task, [error localizedDescription]);
    }

    double progress = (double)task.countOfBytesReceived / (double)task.countOfBytesExpectedToReceive;
    dispatch_async(dispatch_get_main_queue(), ^{
        self.progressView.progress = progress;
    });

    self.dataTask =nil;
}

应用程序在前台运行时一切正常，但一旦我将应用程序置于后台运行，就会出现以下错误消息。

Completed with error: Lost connection to background transfer service

Answer 1

您不能使用数据任务进行后台传输。这些必须使用下载任务:

完成

Download tasks retrieve data in the form of a file, and support background downloads while the app is not running.

这在 Apple 的 documentation 中有解释。

另外一定要查看他们的 background transfer considerations:

With background sessions, because the actual transfer is performed by a separate process and because restarting your app’s process is relatively expensive, a few features are unavailable, resulting in the following limitations...

这里的关键是它运行在一个单独的进程中，无法访问您保存在内存中的数据。它必须通过文件路由。

我在iOS上收集了很多关于后台传输的资料（很长）blog post。

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