AVAssetExportSession是系统自带的压缩
首先导入头文件

#import <AVFoundation/AVFoundation.h>
#import <MobileCoreServices/MobileCoreServices.h>
#import <AssetsLibrary/AssetsLibrary.h>

设置代理

<UIImagePickerControllerDelegate,UINavigationControllerDelegate>

申请打开视频库权限

AVAuthorizationStatus authStatus = [AVCaptureDevice authorizationStatusForMediaType:AVMediaTypeVideo];
    if (authStatus == AVAuthorizationStatusRestricted
        || authStatus == AVAuthorizationStatusDenied) {
        NSLog(@"摄像头已被禁用，您可在设置应用程序中进行开启");
        return;
    }

打开视频库

    if ([UIImagePickerController isSourceTypeAvailable:UIImagePickerControllerSourceTypeSavedPhotosAlbum]) {
        UIImagePickerController *picker = [[UIImagePickerController alloc] init];
        picker.delegate = self;
        picker.allowsEditing = YES;
        picker.sourceType = UIImagePickerControllerSourceTypeSavedPhotosAlbum;
        picker.mediaTypes = @[(NSString *)kUTTypeMovie];
        [self presentViewController:picker animated:YES completion:NULL];
    } else {
        NSLog(@"手机不支持摄像");
    }

视频选择完成后调用

- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary<NSString *,id> *)info {
    //获取选择的媒体类型
    NSString *mediaType=[info objectForKey:UIImagePickerControllerMediaType];
    if ([mediaType isEqualToString:(NSString *)kUTTypeMovie]) {
        NSURL *videoURL = [info objectForKey:UIImagePickerControllerMediaURL];
        NSString *path = [[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, true) lastObject] stringByAppendingPathComponent:[NSString stringWithFormat:@"%ld_compressedVideo.mp4",time(NULL)]];
        NSLog(@"compressedVideoSavePath : %@",path);
        //压缩
        AVURLAsset *avAsset = [[AVURLAsset alloc] initWithURL:videoURL options:nil];
        NSArray *compatiblePresets = [AVAssetExportSession exportPresetsCompatibleWithAsset:avAsset];
        if ([compatiblePresets containsObject:AVAssetExportPresetHighestQuality]) {
            AVAssetExportSession *exportSession = [[AVAssetExportSession alloc] initWithAsset:avAsset presetName:AVAssetExportPreset640x480];
            //输出URL
            exportSession.outputURL = [NSURL fileURLWithPath:path];
            //优化网络
            exportSession.shouldOptimizeForNetworkUse = true;
            //转换后的格式
            exportSession.outputFileType = AVFileTypeMPEG4;
            //异步导出
            [exportSession exportAsynchronouslyWithCompletionHandler:^{
                // 如果导出的状态为完成
                if ([exportSession status] == AVAssetExportSessionStatusCompleted) {
                    [self saveVideo:[NSURL fileURLWithPath:path]];
                    NSLog(@"压缩完毕,压缩后大小 %f MB",[self fileSize:[NSURL fileURLWithPath:path]]);
                }else{
                    NSLog(@"当前压缩进度:%f",exportSession.progress);
                }
                NSLog(@"%@",exportSession.error);
                
            }];
        }
    }
    
    [self dismissViewControllerAnimated:YES completion:nil];
}

存入相册

- (void)saveVideo:(NSURL *)outputFileURL
{
    //ALAssetsLibrary提供了我们对iOS设备中的相片、视频的访问。
    ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
    [library writeVideoAtPathToSavedPhotosAlbum:outputFileURL completionBlock:^(NSURL *assetURL, NSError *error) {
        if (error) {
            NSLog(@"保存视频失败:%@",error);
        } else {
            NSLog(@"保存视频到相册成功");
        }
    }];
}

获取压缩后的大小

- (CGFloat)fileSize:(NSURL *)path
{
    return [[NSData dataWithContentsOfURL:path] length]/1024.00 /1024.00;
}