将PHAsset对象转为UIImage对象

 // MARK: - 将PHAsset对象转为UIImage对象
    func PHAssetToUIImage(asset: PHAsset) -> UIImage {
        var image = UIImage()
        
        // 新建一个默认类型的图像管理器imageManager
        let imageManager = PHImageManager.default()
        
        // 新建一个PHImageRequestOptions对象
        let imageRequestOption = PHImageRequestOptions()
        
        // PHImageRequestOptions是否有效
        imageRequestOption.isSynchronous = true
        
        // 缩略图的压缩模式设置为无
        imageRequestOption.resizeMode = .none
        
        // 缩略图的质量为高质量，不管加载时间花多少
        imageRequestOption.deliveryMode = .highQualityFormat
        
        // 按照PHImageRequestOptions指定的规则取出图片
        imageManager.requestImage(for: asset, targetSize: PHImageManagerMaximumSize, contentMode: .aspectFill, options: imageRequestOption, resultHandler: {
            (result, _) -> Void in
            image = result!
        })
        return image
    }