视频裁剪，长度裁剪

//首先拿到要裁剪视频的url

AVAsset* asset = [AVAssetassetWithURL:self.outputUrl];

AVMutableComposition *composition = [AVMutableComposition composition];

[composition addMutableTrackWithMediaType:AVMediaTypeVideopreferredTrackID:kCMPersistentTrackID_Invalid];

AVAssetTrack *videoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];

//判断视频的方向，看是否需要旋转

UIInterfaceOrientation orientation = UIInterfaceOrientationPortrait;

NSArray *tracks = [asset tracksWithMediaType:AVMediaTypeVideo];

    if([tracks count] >0) {

        AVAssetTrack *videoTrack = [tracks objectAtIndex:0];

        CGAffineTransform t = videoTrack.preferredTransform;

        // Portrait

        if(t.a ==0 && t.b ==1.0 && t.c == -1.0 && t.d ==0) {

            orientation = UIInterfaceOrientationPortrait;

        }

        // PortraitUpsideDown

        if(t.a ==0 && t.b == -1.0 && t.c ==1.0 && t.d ==0) {

            orientation = UIInterfaceOrientationPortraitUpsideDown;

        }

        // LandscapeRight

        if(t.a ==1.0 && t.b ==0 && t.c ==0 && t.d ==1.0) {

            orientation = UIInterfaceOrientationLandscapeRight;

        }

        // LandscapeLeft

        if(t.a == -1.0 && t.b ==0 && t.c ==0 && t.d == -1.0) {

            orientation = UIInterfaceOrientationLandscapeLeft;

        }

    }

 BOOL isPortrait = (orientation == UIInterfaceOrientationPortrait || orientation == UIInterfaceOrientationPortraitUpsideDown) ? YES : NO;

//视频渲染大小，这里以4：3为例

    CGFloat complimentSize = videoTrack.naturalSize.height *4 /3;

    CGSize videoSize;

    if(isPortrait) {

        videoSize = CGSizeMake(videoTrack.naturalSize.height, complimentSize);

    } else {

        videoSize = CGSizeMake(complimentSize, videoTrack.naturalSize.height);

    }

    AVMutableVideoComposition *videoComposition = [AVMutableVideoComposition videoComposition];

    videoComposition.renderSize = videoSize;

    videoComposition.frameDuration = CMTimeMake(1, 30);

    AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];

    instruction.timeRange = CMTimeRangeMake(kCMTimeZero,asset.duration);

    // rotate and position video

    AVMutableVideoCompositionLayerInstruction *transformer = [AVMutableVideoCompositionLayerInstructionvideoCompositionLayerInstructionWithAssetTrack:videoTrack];

    CGFloat tx = (videoTrack.naturalSize.width-complimentSize)/2;

    if (orientation ==UIInterfaceOrientationPortrait || orientation ==UIInterfaceOrientationLandscapeRight) {

        // invert translation

        tx *= -1;

    }

    CGAffineTransform t1 =CGAffineTransformTranslate(videoTrack.preferredTransform, tx,0);

    CGAffineTransform t2 = CGAffineTransformTranslate(t1, isPortrait?0:videoTrack.naturalSize.width, isPortrait?videoTrack.naturalSize.height:0);

    CGAffineTransform t3 = CGAffineTransformScale(t2, isPortrait?1:-1, isPortrait?-1:1);

    [transformer setTransform:t1 atTime:kCMTimeZero];

    instruction.layerInstructions = [NSArray arrayWithObject: transformer];

    videoComposition.instructions = [NSArray arrayWithObject: instruction];

    // 设置要输出文件的位置

    NSString* outputPath = [self.tempVideoPath stringByAppendingPathComponent:[NSString stringWithFormat:@"%@.mp4",[TRCommonTool getCurrentDate]]];

    NSURL *outPutUrl = [NSURL fileURLWithPath:outputPath];

    if ([[NSFileManager defaultManager] fileExistsAtPath:outputPath]){

        [[NSFileManager defaultManager] removeItemAtPath:outputPatherror:nil];

    }

    // export

    AVAssetExportSession *exporter = [[AVAssetExportSessionalloc]initWithAsset:asset presetName:AVAssetExportPreset640x480];

    exporter.videoComposition = videoComposition;

    exporter.outputURL = outPutUrl;

    exporter.outputFileType =AVFileTypeMPEG4;

    exporter.shouldOptimizeForNetworkUse =true;

//开始时间，要裁剪的视频长度

CMTime beforeTime = CMTimeMakeWithSeconds(beginSecond, 30);

CMTime rangeTime = CMTimeMakeWithSeconds(timeRange, 30);

exporter.timeRange = CMTimeRangeMake(beforeTime, rangeTime);

    [exporter exportAsynchronouslyWithCompletionHandler:^{

        AVAssetExportSessionStatus status = exporter.status;

//注意这里一定要回到主线程，因为该任务是在子线程

        dispatch_async(dispatch_get_main_queue(), ^{

            switch (status) {

                caseAVAssetExportSessionStatusCompleted:{

          //将视频存入系统相册          [[[ALAssetsLibraryalloc]init] writeVideoAtPathToSavedPhotosAlbum:outPutUrl completionBlock:^(NSURL *assetURL,NSError *error) {

                    break;

                }

                caseAVAssetExportSessionStatusFailed:{

                    break;

                }

                default:{

                    HHLog(@"exporter%@",exporter.error);

                    break;

                }

            }

            });

    }];