Question

如何从OS X应用程序中的URL下载文件？我是一名Unix C和Perl程序员，他是Objective C的新成员。我一直在阅读URL Loading System Programming Guide，它建议NSURLSession用于新的OS X应用程序。所以我更喜欢使用NSURLSession或NSURLSeesionDownloadTask的解决方案，但我愿意接受NSURLDownload或其他解决方案。

我的第一次尝试基于找到here

的示例

        NSURLSessionDownloadTask *downloadTask =
        [[NSURLSession sharedSession]
         downloadTaskWithURL:[NSURL URLWithString:pdfFile]
         completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {

             // move tmp file to permanent location
             NSLog(@"Done...");
             NSLog(@"Location:   %@", location);
             NSLog(@"Response:   %@", response);
             NSLog(@"Error       %@", error);

             NSFileManager *fileManager = [NSFileManager defaultManager];
             BOOL fileCopied = [fileManager moveItemAtPath:[location path] toPath:[appDir stringByAppendingString:@"/demo.pdf"] error:&error];
             NSLog(fileCopied ? @"File Copied OK" : @"ERROR Copying file.");
            }];
    [downloadTask resume];
    NSLog(@"Now we are here");

示例代码（和我的版本）没有单独的委托。 completionHandler是“在线”。（不确定这是否是正确的术语）。我假设在下载任务完成后将执行此代码。这是对的吗？

我的第二次尝试是基于URL编程指南的“下载文件”部分：

    // PDF file Download, try #2
    NSError *error = nil;
    NSString *appDir = NSHomeDirectory();
    NSString *pdfFile = @"https://developer.apple.com/library/mac/documentation/IDEs/Conceptual/AppDistributionGuide/AppDistributionGuide.pdf";

    // Configure Cache behavior for default session
    NSURLSessionConfiguration
    *defaultConfigObject = [NSURLSessionConfiguration defaultSessionConfiguration];
    NSString *cachePath = [NSTemporaryDirectory()
                           stringByAppendingPathComponent:@"/pdfDownload.cache"];
    NSLog(@"Cache path: %@\n", cachePath);
    NSURLCache *myCache = [[NSURLCache alloc] initWithMemoryCapacity: 16384
                                                        diskCapacity: 268435456 diskPath: cachePath];
    defaultConfigObject.URLCache = myCache;
    defaultConfigObject.requestCachePolicy = NSURLRequestUseProtocolCachePolicy;
    defaultConfigObject.timeoutIntervalForRequest = 100;
    defaultConfigObject.timeoutIntervalForResource = 100;

    // Create a delegate-Free Session
    NSURLSession *delegateFreeSession =
        [NSURLSession sessionWithConfiguration: defaultConfigObject
                                      delegate: nil
                                 delegateQueue: [NSOperationQueue mainQueue]];

    [[delegateFreeSession downloadTaskWithURL:[NSURL URLWithString:pdfFile] completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {

        // move tmp file to permanent location
        NSLog(@"Done...");
        NSLog(@"Location:   %@", location);
        NSLog(@"Response:   %@", response);
        NSLog(@"Error       %@", error);

        NSFileManager *fileManager = [NSFileManager defaultManager];
        BOOL fileCopied = [fileManager moveItemAtPath:[location path] toPath:[appDir stringByAppendingString:@"/demo.pdf"] error:&error];
        NSLog(fileCopied ? @"File Copied OK" : @"ERROR Copying file.");
    }] resume];

    sleep(10);
    NSLog(@"After sleep...");

在这两种情况下，代码执行永远不会到达作为completionHandler一部分的NSLog语句。我在模拟器中设置断点，永远不会到达那里。如何创建简单的“委托免费”文件下载方法？

由于我正在编写一个“命令行”OS X应用程序，因此我没有UIViewController或其他现有的委托结构。

有关如何进行的任何建议？我不需要下载进度条等，只需要“通过/失败”状态。

另外，我的例子是单个PDF文件。我的应用程序有一系列我想下载的网址。我不需要并行下载它们。我可以在循环中重用单个NSURLSession吗？

任何帮助都将不胜感激!!

Answer 1

一旦我得到解决方案，忘了关闭它。正如Zev指出的那样，我选择的PDF文件很大，需要更多时间下载。 10秒钟的睡眠（10）＆＃39;没有足够的时间下载39 MB的文件。

应该使用通知来指示completionHandler何时完成。但就目前而言，我正在使用一个简单的轮询循环。我改变了睡眠（10）;到

int  timeoutCounter = 60;
while ((timeoutCounter > 0) && !downloadDone) {
    sleep(1);
    NSLog(@"Timout: %d", timeoutCounter--);
}

if (!downloadDone) {
    NSLog(@"ERROR: Timeout occurred before file completed downloading");
}

这需要声明downloadDone以允许completionHandler块设置它：

    __block BOOL downloadDone = FALSE;

NSFileManger移动文件后，必须设置此信号量：

     downloadDone = TRUE;

再次感谢您帮助Zev！

我还在处理错误检查/处理，但这是完整的更正方法：

+ (NSError *)           downloadFileAtURL:(NSString *)fileURL
                                toDir:(NSString *)toDir
                              timeout:(NSNumber *)timeout {

// completion handler semaphore to indicate download is done
__block BOOL downloadDone = FALSE;

// same PDF file name for later
NSString *downloadFileName = [fileURL lastPathComponent];
NSString *finalFilePath = [toDir stringByAppendingPathComponent:downloadFileName];
NSLog(@"PDF file name: '%@'", downloadFileName);
NSLog(@"Final file path: '%@'", finalFilePath);

// Configure Cache behavior for default session
NSURLSessionConfiguration
*defaultConfigObject = [NSURLSessionConfiguration defaultSessionConfiguration];
NSString *cachePath = [NSTemporaryDirectory()
                       stringByAppendingPathComponent:@"/downloadFileCache"];
NSLog(@"Cache path: %@\n", cachePath);
NSURLCache *myCache = [[NSURLCache alloc] initWithMemoryCapacity: 16384
                                                    diskCapacity: 268435456 diskPath: cachePath];
defaultConfigObject.URLCache = myCache;
defaultConfigObject.requestCachePolicy = NSURLRequestUseProtocolCachePolicy;
defaultConfigObject.timeoutIntervalForRequest = 100;
defaultConfigObject.timeoutIntervalForResource = 100;

// Create a delegate-Free Session
//      delegateQueue: [NSOperationQueue mainQueue]];
NSURLSession *delegateFreeSession =
[NSURLSession sessionWithConfiguration: defaultConfigObject
                              delegate: nil
                         delegateQueue: nil];

[[delegateFreeSession downloadTaskWithURL:[NSURL URLWithString:fileURL] completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {

    // move tmp file to permanent location
    NSLog(@"In the completion handler...");
    NSLog(@"Location:   %@", location);
    NSLog(@"Response:   %@", response);
    NSLog(@"Error       %@", error);
    NSLog(@"Location path: %@", [location path]);

    // Verify temp file exists in cache
    NSFileManager *fileManager = [[NSFileManager alloc] init];
    if ( [fileManager fileExistsAtPath:[location path]]) {
        NSLog(@"File exists.  Now move it!");

        error = nil;
        BOOL fileCopied = [fileManager moveItemAtPath:[location path]
                                               toPath:finalFilePath
                                                error:&error];
        NSLog(@"Error: %@", error);
        NSLog(fileCopied ? @"File Copied OK" : @"ERROR Copying file.");
    }
    downloadDone = TRUE;

}] resume];

// use timeout argument
int  timeoutCounter = [timeout intValue];
while ((timeoutCounter > 0) && !downloadDone) {
    sleep(1);
    NSLog(@"Timout: %d", timeoutCounter--);
}

if (!downloadDone) {
    NSLog(@"ERROR: Timeout occurred before file completed downloading");
}

NSError *error = nil;
return error;

}

如何从OS X app中的URL下载文件

1 个答案: