下载大型视频会发出内存警告

-(void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
    if(data != nil){
       //open output stream
        NSOutputStream *stream=[[NSOutputStream alloc] initToFileAtPath:_filePath append:YES];
        [stream open];
        NSString *str=(NSString *)data;

        //write to file
        NSUInteger left = [str length];
        NSUInteger bytesWritten = 0;
        do {
            bytesWritten = [stream write:[data bytes] maxLength:left];
            downloadedData = downloadedData + bytesWritten;
            if (-1 == bytesWritten) break;
            left -= bytesWritten;

        } while (left > 0);

        if (left) {
            NSLog(@"stream error: %@", [stream streamError]);
            [self handleForCurreptedDownloading];
        }
        [stream close];
    }
    else{
        NSLog(@"data nil");
    }
}
-(void)connection:(NSURLConnection *)connection