Question

（请注意，我已经看过this other SO post。）

问题

我正在尝试将avi视频转换为mp4，以便可以使用Objective-C在iOS应用上本地播放

我尝试过的事情

我正在尝试以下代码进行转换：

- (void)convertVideoToLowQuailtyWithInputURL:(NSURL*)inputURL outputURL:(NSURL*)outputURL handler:(void (^)(AVAssetExportSession*))handler {
    [[NSFileManager defaultManager] removeItemAtURL:outputURL error:nil];
    AVURLAsset *asset = [AVURLAsset URLAssetWithURL:inputURL options:nil];
    AVAssetExportSession *exportSession = [[AVAssetExportSession alloc] initWithAsset:asset presetName:AVAssetExportPresetHighestQuality];
    exportSession.outputURL = outputURL;
    exportSession.outputFileType = AVFileTypeMPEG4;
    [exportSession exportAsynchronouslyWithCompletionHandler:^(void) {
        handler(exportSession);
    }];
}

exportSession返回的错误为Cannot Open

其他信息

当我运行要通过Mediainfo转换的视频时，我将获得以下视频：

7 332kb/s, 1920*1080 (16:9), at 25.000 FPS, AVC (Baseline@L3.1) (CABAC / 1 Ref Frames)

这是音频：

128 kb/s, 8 000 Hz, 16 bits, 1 channel, PCM (Little / Signed)

我还在exportPresetsCompatibleWithAsset:上使用了AVAssetExportSession方法，并得到了以下结果：

AVAssetExportPreset1920x1080,
AVAssetExportPresetLowQuality,
AVAssetExportPresetAppleM4A,
AVAssetExportPresetHEVCHighestQuality,
AVAssetExportPreset640x480,
AVAssetExportPreset3840x2160,
AVAssetExportPresetHEVC3840x2160,
AVAssetExportPresetHighestQuality,
AVAssetExportPreset1280x720,
AVAssetExportPresetMediumQuality,
AVAssetExportPreset960x540,
AVAssetExportPresetHEVC1920x1080

要注意的另一件事是，在播放预设和输出时，我设法获得了基本上是白噪声的纯音频文件。这是使用预设的AVAssetExportPresetAppleM4A。

我希望我记下了足够的信息。

更新

使用Ashley的评论，我创建了一个函数来返回与资产兼容的导出设置。

- (void)determineCompatibleExportForAsset:(AVURLAsset *)asset completion:(void(^)(NSArray<ExportSettings *> *exports))handler {
    NSArray<NSString *> *presets = @[
                                     AVAssetExportPresetLowQuality,
                                     AVAssetExportPresetMediumQuality,
                                     AVAssetExportPresetHighestQuality,
                                     AVAssetExportPresetHEVCHighestQuality,
                                     AVAssetExportPreset640x480,
                                     AVAssetExportPreset960x540,
                                     AVAssetExportPreset1280x720,
                                     AVAssetExportPreset1920x1080,
                                     AVAssetExportPreset3840x2160,
                                     AVAssetExportPresetHEVC1920x1080,
                                     AVAssetExportPresetHEVC3840x2160,
                                     AVAssetExportPresetAppleM4A,
                                     AVAssetExportPresetPassthrough
                                     ];

    NSArray<NSString *> *outputs = @[
                                     AVFileTypeQuickTimeMovie,
                                     AVFileTypeMPEG4,
                                     AVFileTypeAppleM4V,
                                     AVFileTypeAppleM4A,
                                     AVFileType3GPP,
                                     AVFileType3GPP2,
                                     AVFileTypeCoreAudioFormat,
                                     AVFileTypeWAVE,
                                     AVFileTypeAIFF,
                                     AVFileTypeAIFC,
                                     AVFileTypeAMR,
                                     AVFileTypeMPEGLayer3,
                                     AVFileTypeSunAU,
                                     AVFileTypeAC3,
                                     AVFileTypeEnhancedAC3,
                                     AVFileTypeJPEG,
                                     AVFileTypeDNG,
                                     AVFileTypeHEIC,
                                     AVFileTypeAVCI,
                                     AVFileTypeHEIF,
                                     AVFileTypeTIFF
                                     ];

    __block int counter = 0;
    int totalCount = (int)presets.count * (int)outputs.count;

    NSMutableArray<ExportSettings *> *exportSettingsArray = [@[] mutableCopy];

    for (NSString *preset in presets) {
        for (NSString *output in outputs) {
            [AVAssetExportSession determineCompatibilityOfExportPreset:preset withAsset:asset outputFileType:output completionHandler:^(BOOL compatible) {
                if (compatible) {
                    ExportSettings *exportSettings = [[ExportSettings alloc] initWithPreset:preset outputType:output];
                    [exportSettingsArray addObject:exportSettings];
                }
                counter++;
                if (counter == totalCount) {
                    if (handler) {
                        handler([exportSettingsArray copy]);
                    }
                }
            }];
        }
    }
}

其结果如下：

"Preset: AVAssetExportPresetAppleM4A Output: com.apple.m4a-audio",
"Preset: AVAssetExportPresetPassthrough Output: com.microsoft.waveform-audio",
"Preset: AVAssetExportPresetPassthrough Output: public.aifc-audio",
"Preset: AVAssetExportPresetPassthrough Output: public.aiff-audio",
"Preset: AVAssetExportPresetPassthrough Output: com.apple.coreaudio-format",
"Preset: AVAssetExportPresetPassthrough Output: com.apple.quicktime-movie"

据此，我推断使用预设AVAssetExportPresetPassthrough和输出类型AVFileTypeQuickTimeMovie是兼容的。

但是，当运行以下代码时：（我尝试使用.mp4，.mov和.qt作为文件类型）

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *filePath = [documentsDirectory stringByAppendingPathComponent:@"MyVideo.mov"];
    NSURL *outputURL = [NSURL fileURLWithPath:filePath];
    NSURL *localURL = [NSBundle URLForResource:@"20180626_145233-v" withExtension:@"avi" subdirectory:nil inBundleWithURL:[NSBundle mainBundle].bundleURL];

    [self convertVideoToLowQuailtyWithInputURL:localURL outputURL:outputURL handler:^(AVAssetExportSession *exportSession) {
        switch ([exportSession status]) {
            case AVAssetExportSessionStatusFailed:
                NSLog(@"Export failed: %@", [exportSession error]);
                break;
            case AVAssetExportSessionStatusCancelled:
                NSLog(@"Export canceled");
                break;
            case AVAssetExportSessionStatusCompleted:
                NSLog(@"Successfully");
                 NSLog(@"OutputURL: %@", outputURL.absoluteString);
                break;
            default:
                break;
        }

    }];

哪个电话：

- (void)convertVideoToLowQuailtyWithInputURL:(NSURL*)inputURL outputURL:(NSURL*)outputURL handler:(void (^)(AVAssetExportSession*))handler {
    [[NSFileManager defaultManager] removeItemAtURL:outputURL error:nil];
    AVURLAsset *asset = [AVURLAsset URLAssetWithURL:inputURL options:nil];       

    AVAssetExportSession *exportSession = [[AVAssetExportSession alloc] initWithAsset:asset presetName:AVAssetExportPresetPassthrough];
    exportSession.outputURL = outputURL;
    exportSession.outputFileType = AVFileTypeQuickTimeMovie;

    [exportSession exportAsynchronouslyWithCompletionHandler:^(void) {
        handler(exportSession);
    }];
}

我收到此错误：

Export failed: Error Domain=AVFoundationErrorDomain Code=-11800 "The operation could not be completed" UserInfo={NSLocalizedFailureReason=An unknown error occurred (-12842), NSLocalizedDescription=The operation could not be completed, NSUnderlyingError=0x60400024def0 {Error Domain=NSOSStatusErrorDomain Code=-12842 "(null)"}}