调用系统的相册

第一步遵守协议，UIImagePickerControllerDelegate,UINavigationControllerDelegate

第二步：创建picker对象

    UIImagePickerController * picker = [[UIImagePickerController alloc]init];
第三步 ：打开的模式三种

    picker.sourceType = UIImagePickerControllerSourceTypePhotoLibrary;

    UIImagePickerControllerSourceTypePhotoLibrary,
    UIImagePickerControllerSourceTypeCamera,
    UIImagePickerControllerSourceTypeSavedPhotosAlbum

第四步：设置代理

    picker.delegate = self;
第五步：进入相册

    [self presentViewController:picker animated:YES completion:nil];
选中相册中某一种照片的协议方法为

-(void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info
{
        //获取选中的图片
    UIImage * selectImage = [info objectForKey:UIImagePickerControllerOriginalImage];
    self.imageView.image = selectImage;
    //将picker隐藏
    [picker dismissViewControllerAnimated:YES completion:nil];
}