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发挥作用......

所以，我想继承NSOperation 并让它使用NSURLConnection 下载文件。正确的方法是什么？ 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

您需要正确配置您的操作以作为“并发操作”执行

Concurrency Programming Guide: Configuring Operations for Concurrent Execution

您需要返回isConcurrent = YES 并以符合KVO 的方式正确管理其他状态标志isExecuting 和isFinished。

---

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，其中包含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**

此代码还通过确保它在主线程上启动它来处理在具有活动运行循环的线程上启动 NSURLConnection 的要求。

（** 链接现在指向archive.org，我认为Pulse 已被收购并已关闭他们的旧网站）