iOS - NSOperation 中的异步 NSURLConnection

Question

我知道这个问题在 SO 上被问过很多次，但我没有设法让它在我的项目中工作......

所以，我想子类化 NSOperation 并让它使用 NSURLConnection 下载文件。正确的做法是什么？ 这是我的代码不起作用: 首先，我将所有操作添加到一个循环中:

DownloadFileOperation *operation;
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
for (int i=0; i<10; i++) {
operation = [[DownloadFileOperation alloc] init];
operation.urlString = pdfUrlString;
[queue addOperation:operation];
operation = nil; }

这是我的子类:

@interface DownloadHandbookOperation : NSOperation <NSURLConnectionDelegate>
{

}

@property (strong, nonatomic) NSString *urlString;

@end


@implementation DownloadHandbookOperation
{
    NSString *filePath;
    NSFileHandle *file;
    NSURLConnection * connection;
}

- (void)start
{
    if (![NSThread isMainThread])
    {
        [self performSelectorOnMainThread:@selector(start) withObject:nil waitUntilDone:NO];
        return;
    }

    NSURL *url = [[NSURL alloc] initWithString:[self.urlString stringByAddingPercentEscapesUsingEncoding: NSUTF8StringEncoding]];

    NSMutableURLRequest *req = [NSMutableURLRequest requestWithURL:url];
    [req addValue:@"Basic ***=" forHTTPHeaderField:@"Authorization"];
    connection = [[NSURLConnection alloc] initWithRequest:req delegate:self startImmediately:YES];

}

- (void)connection:(NSURLConnection *)conn didReceiveResponse:(NSURLResponse *)response
{
    NSString *filename = [[conn.originalRequest.URL absoluteString] lastPathComponent];
    filename = [filename stringByReplacingPercentEscapesUsingEncoding:NSUTF8StringEncoding];

    filePath = [[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0] stringByAppendingPathComponent:filename];
    [[NSFileManager defaultManager] createFileAtPath:filePath contents:nil attributes:nil];

    file = [NSFileHandle fileHandleForUpdatingAtPath:filePath] ;
    if (file)
    {
        [file seekToEndOfFile];
    }
    else
        [self finish];
}

- (void)connection:(NSURLConnection *)conn didReceiveData:(NSData *)data
{
    if (file) {
        [file seekToEndOfFile];
    }
    [file writeData:data];
}

- (void)connectionDidFinishLoading:(NSURLConnection *)conn
{
    [file closeFile];
    [self finish];
}

- (void)connection:(NSURLConnection *)conn didFailWithError:(NSError *)error
{
    connection = nil;

    [self finish];
}

- (void)cancel
{
    [super cancel];
    [connection cancel];
}


- (void)finish
{
    NSLog(@"operationfinished.");
}


@end

我做错了什么？

Answer 1

您需要正确配置操作以作为“并发操作”执行

Concurrency Programming Guide: Configuring Operations for Concurrent Execution

您需要返回isConcurrent = YES并妥善管理其他州旗帜，isExecuting和 isFinished以符合 KVO 的方式。

---

为了说明这里的总体思路，Pulse 的工程师发表了一篇文章，其中描述了他们的解决方案以及一些易于理解的演示代码，您可以下载和查看。

Pulse Engineering Blog: Concurrent Downloads using NSOperationQueues **

此代码还处理了 NSURLConnection 在具有事件运行循环的线程上启动的要求，方法是确保它在主线程上启动。

(** 链接现在指向 archive.org，我认为 pulse 已被收购并已关闭其旧站点)