NSURLConnection下载文件并显示进度

1、创建一个NSURLConnection对象：

  NSURL *url = [NSURL URLWithString: @"http://119.147.106.249/gdown_group11/M00/3F/15/d5Nq-UxBq88AAAAAAUn8QBO4rlY4306605/01_01_Android%E5%B9%B3%E5%8F%B0%E4%B8%80%E6%97%A5%E6%B8%B8.mp4?k=-4hoIXm9CXQ_Zo79EQprcQ&t=1346820047&u=3662410469-66934941-bei9apjt&s=307200&file=01_01_Android%E5%B9%B3%E5%8F%B0%E4%B8%80%E6%97%A5%E6%B8%B8.mp4"];
    NSURLRequest *request = [[NSURLRequest alloc] initWithURL:url cachePolicy:NSURLRequestReloadIgnoringCacheData timeoutInterval: 100.0];//设置缓存和超时
    NSURLConnection *connection = nil;
    connection = [[NSURLConnection alloc] initWithRequest:request delegate:self startImmediately:YES];
  //初始化connection对象并马上执行，有的初始化方法并不是马上开始执行，而需要调用start方法。

二、实现connection的回调方法：

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{//该方法在响应connection时调用
    NSLog(@"response");
    self.data = [[NSMutableData alloc] init];
    NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
    if(httpResponse && [httpResponse respondsToSelector:@selector(allHeaderFields)]){
        NSDictionary *httpResponseHeaderFields = [httpResponse allHeaderFields];
        mFileSize = [[httpResponseHeaderFields objectForKey:@"Content-Length"] longLongValue];
    }//获取文件文件的大小
}
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{//出错时调用
    NSLog(@"error");
}
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{//接受数据，在接受完成之前，该方法重复调用
    NSLog(@"receive");
    [_data appendData: data];
    _displayLabel.text = [NSString stringWithFormat:(@"%6.1fkb/%6.1fkb"),[_data length]/1024.0,mFileSize/1024.0];
    [_progressView setProgress: [_data length]/(float)mFileSize];
}
- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{//完成时调用
    NSLog(@"Finish");
    NSString *filePath = [[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0] stringByAppendingPathComponent:@"android.mp4"];
    [_data writeToFile:filePath atomically:NO];//将数据写入Documents目录。
    NSLog(@"%@",filePath);
}