HarmonyOS  选择相册的图片后，返回了图片的uri，如何根据uri将图片转为base64

let packOpts: image.PackingOption = { format: "image/jpeg", quality: 98 };
let imagePackerApi = image.createImagePacker();

const context: Context = getContext(this);
const resourceMgr = context.resourceManager;
let imageBuffer = await resourceMgr.getMediaContent($r('app.media.ic_low'));
const imageSource: image.ImageSource = image.createImageSource(imageBuffer.buffer);
let decodingOptions: image.DecodingOptions = {
editable: true,
desiredPixelFormat: 3,
rotate: 90
}
// 创建pixelMap并进行简单的旋转和缩放

imageSource.createPixelMap(decodingOptions).then((pixelMap: PixelMap) => {

this.pixelMap = pixelMap
// pixelmap 转base64 先要有一个packing的过程
imagePackerApi.packing(pixelMap, packOpts).then((data: ArrayBuffer) => {
let base64Str = buffer.from(data).toString('base64')
let resultBase64Str = "data:image/png;base64," + base64Str
this.resultBase64Str = resultBase64Str
}).catch((error: BusinessError) => {
console.error('Failed to pack the image. And the error is: ' + error);
})
pixelMap.release();
})
//重点提示： pixelmap转base64 不能直接转，先要加一个packing的过程，下面的方法是一个错误的实例，转出来的base64字符串不对。

let pixelMap = await imageSource.createPixelMap(opts);
console.log("pixelMap.getPixelBytesNumber():" + pixelMap.getPixelBytesNumber())
let arrayBuffer = new ArrayBuffer(pixelMap.getPixelBytesNumber());
pixelMap.readPixelsToBuffer(arrayBuffer)
console.log("arrayBuffer" + arrayBuffer)
let base64Str = buffer.from(arrayBuffer).toString(Constants.BASE_64);
let resultBase64Str = "data:image/png;base64," + base64Str