最近需要开发一个音乐下载的模块，考虑到方便封装和对象化，使用了NSOpreation。

考虑到复用，我这里使用了继承NSOpreation的方法

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@interface YOYMusicDownloadOperation : NSOperation
@property (copy, nonatomic) void(^processBlock)(double process);
@property (copy, nonatomic) void(^completeBlock)(BOOL r, NSURL *localUrl);

+ (NSString *)localMusicFilesPath;

- (instancetype)initWithURL:(NSURL *)url processCallback:(void(^)(double process))process complete:(void(^)(BOOL finised, NSURL *localUrl))complete;

@end

这个类很简单，两个block属性，一个用于进度监控，一个用于结束回调。

然后开始重写NSOpreation的两个方法：start 和 main

因为是下载需要异步回调，所以在函数main里使用了死循环，等下载完成后再结束线程。（如果线程结束，这个opreation会被释放）。

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- (void)start
{
    if ([self isCancelled]){

        [self willChangeValueForKey:@"isFinished"];
        finished = YES;
        [self didChangeValueForKey:@"isFinished"];
        
        return;
    } else {

        [self willChangeValueForKey:@"isExecuting"];
        executing = YES;
        [self main];
        [self didChangeValueForKey:@"isExecuting"];
    }
}

- (void)main
{
    _session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration] delegate:self delegateQueue:[[NSOperationQueue alloc] init]];
    NSURLRequest *req = [NSURLRequest requestWithURL:self.downloadUrl];
    _task = [_session downloadTaskWithRequest:req];
    
    [_task resume];
    
    while(_task != nil) {
        
        [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
        
    }
    
    [self willChangeValueForKey:@"isFinished"];
    [self willChangeValueForKey:@"isExecuting"];
    finished = YES;
    executing = NO;
    [self didChangeValueForKey:@"isFinished"];
    [self didChangeValueForKey:@"isExecuting"];
}
- (BOOL) isFinished{
    return(finished);
}

- (BOOL) isExecuting{
    return(executing);
}

注意： 在重写NSOpreation时，注意也要重写 isFinished和isExecuting这两个属性的getter，同时还需要手动触发相关的KVO。如果没重写这两个方法，isFinished和isExecuting这两个属性会一直是NO，导致在线程完成后还留在NSOpreationQueue里，不会移除。

然后实现NSURLSession的几个代理方法

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-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location{

    _musicLocation = location;
}
-(void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error{

    if(!error){
        dispatch_sync_on_main_queue(^{
            if(self.completeBlock)
            {
                if(_musicLocation)
                {
                    self.completeBlock(YES, _musicLocation);
                }
                else
                {
                    self.completeBlock(NO, nil);
                }
            }
        });
    }
    
    _task = nil;
}
-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite{
    double process = totalBytesWritten/(double)totalBytesExpectedToWrite;
    
    if(self.processBlock)
    {
        self.processBlock(process);
    }
}

特别说明下，因为下载完成后需要刷新UI，我选择了在主线程调用completeBlock