顺便说一句，iOS 7（和 Mac OS 10.9）提供了URLSession 和URLSessionDownloadTask，它们可以很好地处理这个问题。如果你只想下载一堆文件，你可以这样做：
NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration];
NSString *documentsPath = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)[0];
NSFileManager *fileManager = [NSFileManager defaultManager];
for (NSString *filename in self.filenames) {
NSURL *url = [baseURL URLByAppendingPathComponent:filename];
NSURLSessionTask *downloadTask = [session downloadTaskWithURL:url completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {
NSString *finalPath = [documentsPath stringByAppendingPathComponent:filename];
BOOL success;
NSError *fileManagerError;
if ([fileManager fileExistsAtPath:finalPath]) {
success = [fileManager removeItemAtPath:finalPath error:&fileManagerError];
NSAssert(success, @"removeItemAtPath error: %@", fileManagerError);
}
success = [fileManager moveItemAtURL:location toURL:[NSURL fileURLWithPath:finalPath] error:&fileManagerError];
NSAssert(success, @"moveItemAtURL error: %@", fileManagerError);
NSLog(@"finished %@", filename);
}];
[downloadTask resume];
}
也许，鉴于您的下载需要“大量时间”，您可能希望它们在应用程序进入后台后继续下载。如果是这样，您可以使用backgroundSessionConfiguration 而不是defaultSessionConfiguration（尽管您必须实现NSURLSessionDownloadDelegate 方法，而不是使用completionHandler 块）。这些后台会话速度较慢，但是即使用户离开了您的应用程序，它们也会再次发生。因此：
- (void)startBackgroundDownloadsForBaseURL:(NSURL *)baseURL {
NSURLSession *session = [self backgroundSession];
for (NSString *filename in self.filenames) {
NSURL *url = [baseURL URLByAppendingPathComponent:filename];
NSURLSessionTask *downloadTask = [session downloadTaskWithURL:url];
[downloadTask resume];
}
}
- (NSURLSession *)backgroundSession {
static NSURLSession *session = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration backgroundSessionConfiguration:kBackgroundId];
session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:[NSOperationQueue mainQueue]];
});
return session;
}
#pragma mark - NSURLSessionDownloadDelegate
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location {
NSString *documentsPath = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)[0];
NSString *finalPath = [documentsPath stringByAppendingPathComponent:[[[downloadTask originalRequest] URL] lastPathComponent]];
NSFileManager *fileManager = [NSFileManager defaultManager];
BOOL success;
NSError *error;
if ([fileManager fileExistsAtPath:finalPath]) {
success = [fileManager removeItemAtPath:finalPath error:&error];
NSAssert(success, @"removeItemAtPath error: %@", error);
}
success = [fileManager moveItemAtURL:location toURL:[NSURL fileURLWithPath:finalPath] error:&error];
NSAssert(success, @"moveItemAtURL error: %@", error);
}
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didResumeAtOffset:(int64_t)fileOffset expectedTotalBytes:(int64_t)expectedTotalBytes {
// Update your UI if you want to
}
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite {
// Update your UI if you want to
}
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
if (error)
NSLog(@"%s: %@", __FUNCTION__, error);
}
#pragma mark - NSURLSessionDelegate
- (void)URLSession:(NSURLSession *)session didBecomeInvalidWithError:(NSError *)error {
NSLog(@"%s: %@", __FUNCTION__, error);
}
- (void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session {
AppDelegate *appDelegate = (id)[[UIApplication sharedApplication] delegate];
if (appDelegate.backgroundSessionCompletionHandler) {
dispatch_async(dispatch_get_main_queue(), ^{
appDelegate.backgroundSessionCompletionHandler();
appDelegate.backgroundSessionCompletionHandler = nil;
});
}
}
顺便说一句，这假设您的应用委托具有 backgroundSessionCompletionHandler 属性：
@property (copy) void (^backgroundSessionCompletionHandler)();
如果应用被唤醒以处理 URLSession 事件，应用委托将设置该属性：
- (void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier completionHandler:(void (^)())completionHandler {
self.backgroundSessionCompletionHandler = completionHandler;
}
有关背景NSURLSession 的Apple 演示，请参阅Simple Background Transfer 示例。