示例代码:

#import "ViewController.h"
#import <AVFoundation/AVFoundation.h>

@interface ViewController ()<UINavigationControllerDelegate, UIImagePickerControllerDelegate>

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    // 获取视频
    
    // 对该视频进行压缩
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    
    // 创建图片控制器
    UIImagePickerController *pickerController = [[UIImagePickerController alloc]init];

    /** 设置控制器类型
     *       UIImagePickerControllerSourceTypePhotoLibrary,    相簿
             UIImagePickerControllerSourceTypeCamera,          相机
             UIImagePickerControllerSourceTypeSavedPhotosAlbum 照片
     */
    pickerController.sourceType = UIImagePickerControllerSourceTypePhotoLibrary;
    
    /* 设置媒体类型  : 如果想要获取视频文件,必须要设置媒体类型,否则弹出的控制器下默认不显示视频文件
     
         UIImagePickerControllerSourceTypePhotoLibrary,
         UIImagePickerControllerSourceTypeCamera,
         UIImagePickerControllerSourceTypeSavedPhotosAlbum
     */
    NSArray *mediaTypeArr = [UIImagePickerController availableMediaTypesForSourceType:UIImagePickerControllerSourceTypePhotoLibrary];
    pickerController.mediaTypes = mediaTypeArr;
    


    
    // modal展示
    [self presentViewController:pickerController animated:YES completion:nil];
    
    // 设置代理
    pickerController.delegate = self;
}

- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary<NSString *,id> *)info{
    
    
    /*  NSLog(@"%@",info);  Log信息:
     2016-07-27 20:19:40.828 视频压缩[3828:3690420] {
     UIImagePickerControllerMediaType = "public.movie";
     UIImagePickerControllerMediaURL = "file:///private/var/mobile/Containers/Data/Application/6D369842-32F4-455C-9F40-8B55D44263C8/tmp/trim.C17F3DC5-CF34-4F72-B319-CD28B957431B.MOV";
     UIImagePickerControllerReferenceURL = "assets-library://asset/asset.MOV?id=9603CD2B-5692-417F-BC8F-6CA0C780113C&ext=MOV";
     }
     */
    
    // 进行资源转出
    
    // 获取资源对象
    AVAsset *asset = [AVAsset assetWithURL:info[UIImagePickerControllerMediaURL]];
    
    // 创建资源转出会话  参数1: 转出的资源文件  参数2: 压缩质量
    /*
         AVAssetExportPresetLowQuality        低质量
         AVAssetExportPresetMediumQuality     中质量
         AVAssetExportPresetHighestQuality    高质量
     */
    AVAssetExportSession *session = [AVAssetExportSession exportSessionWithAsset:asset presetName:AVAssetExportPresetLowQuality];
    
    // 设置转出文件名
    /*  不设置转出文件名,会崩溃报错:
     ***  Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'outputURL cannot be NULL'
     同样,不是设置转储文件类型,也会报错:
     ***  Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'outputFileType cannot be NULL'
     */
    NSURL *outPutURL = [[NSURL fileURLWithPath:NSSearchPathForDirectoriesInDomains(NSLibraryDirectory, NSUserDomainMask, YES).lastObject] URLByAppendingPathComponent:@"转出视频.mp4"];
    session.outputURL = outPutURL;
    // 设置转出文件类型
    
    // 通过该方法可以获取会话转出的可支持类型
    [session determineCompatibleFileTypesWithCompletionHandler:^(NSArray<NSString *> * _Nonnull compatibleFileTypes) {
        NSLog(@"%@",compatibleFileTypes);
        
        /**
         *  Log信息:
         2016-07-27 20:45:15.584 视频压缩[3873:3699090] (
                "com.apple.quicktime-movie",
                "public.mpeg-4"
         )
         */
    }];

    session.outputFileType = @"public.mpeg-4";
    
    
    // 进行转出操作
    [session exportAsynchronouslyWithCompletionHandler:^{
        
        NSLog(@"完成视频压缩");
    }];
    
    [self dismissViewControllerAnimated:YES completion:nil];
    
    
    
}

@end

通过Xcode可以获取到真机的应用沙盒,就可以找到压缩后的视频