使用NSScanner和GCD时性能较低

Question

我已经创建了一个方法来解析.m3u8文件，下载引用的文件，并重新创建一个新的.m3u8文件，以便在本地提供脱机播放。一切都能正常工作，但也许我误用了NSScaner或中央调度中心，因为在我看来，这个方法应该运行得很快，并在GCD中排队下载。然而，该方法需要运行全部时间(在后台，是的，但我希望尽快创建新文件，而不是在下载完所有文件之后)。有人能看到我的瓶颈在哪里吗？提前谢谢。

- (void)beginDownloadAndCreateLocalM3U8FileForLocalPlaybackFromPlaylist:(NSString*)playlist forId:(NSString*)_id withProgressBlock:(void (^)(float))progress withCompletionBlock:(void (^)(id))success
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        @autoreleasepool {
            NSString *stringURL = playlist;
            NSURL  *url = [NSURL URLWithString:stringURL];
            NSError *error;
            NSString *stringData = [NSString stringWithContentsOfURL:url encoding:NSUTF8StringEncoding error:&error];
            NSString *foundData;
            NSScanner *scanner=[NSScanner scannerWithString:stringData];
            NSUInteger counter = 0;
            NSMutableString *m3u8 = [NSMutableString new];
            [scanner scanUpToString:@"#EXTINF" intoString:&foundData];
            [m3u8 appendString:foundData];
            NSArray   *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
            NSString  *documentsDirectory = [paths objectAtIndex:0];
            if (![self directoryExistsAtAbsolutePath:[NSString stringWithFormat:@"%@/Web", documentsDirectory]]) {
                [[NSFileManager defaultManager] createDirectoryAtPath: [NSString stringWithFormat:@"%@/Web", documentsDirectory] withIntermediateDirectories: YES attributes:nil error: &error];
            }
            if (![self directoryExistsAtAbsolutePath:[NSString stringWithFormat:@"%@/Web/%@", documentsDirectory, _id]]) {
                [[NSFileManager defaultManager] createDirectoryAtPath: [NSString stringWithFormat:@"%@/Web/%@", documentsDirectory, _id] withIntermediateDirectories: YES attributes:nil error: &error];
            } else {
                success([NSString stringWithFormat:@"http://127.0.0.1:12345/%@.m3u8", _id]);
                return;
            }
            while (![scanner isAtEnd]) {
                [scanner scanUpToString:@"\n" intoString: &foundData];
                if ([foundData hasPrefix:@"#EXTINF:"]) {
                    [m3u8 appendFormat:@"%@\n", foundData];
                } else if ([foundData hasPrefix:@"http:"]) {
                    NSData *urlData = [NSData dataWithContentsOfURL:[NSURL URLWithString:foundData]];
                    if ( urlData )
                    {
                        NSString  *filePath = [NSString stringWithFormat:@"%@/Web/%@/%i.ts", documentsDirectory, _id, counter];
                        NSString *localURL = [NSString stringWithFormat:@"http://127.0.0.1:12345/%@/%i.ts", _id, counter];
                        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
                            @autoreleasepool {
                                [urlData writeToFile:filePath atomically:YES];
                            }
                        });
                        //assemble the m3u8 file here for each entry with the original durations.  No need to recalculate.
                        //NSLog(@"Adding this: %@", filePath);
                        [m3u8 appendFormat:@"%@\n", localURL];
                        counter++;
                    }
                }
            }
            [m3u8 appendString:@"#EXT-X-ENDLIST"];
            [m3u8 writeToFile:[NSString stringWithFormat:@"%@/Web/%@.m3u8", documentsDirectory, _id] atomically:YES encoding:NSUTF8StringEncoding error:&error];
            NSLog(@"Final m3u8 is %@", m3u8);
            success([NSString stringWithFormat:@"http://127.0.0.1:12345/%@.m3u8", _id]);
        }
    });
}

Answer 1

这行在这里

NSData *urlData = [NSData dataWithContentsOfURL:[NSURL URLWithString:foundData]];

..。在与扫描仪同步运行的while循环中，扫描仪无法进入下一个令牌，直到它下载的URL已经下载完毕。尝试使用低优先级的dispatch_async()来涵盖else用例的所有逻辑，而不仅仅是writeToFile:调用。