flutter自定义地图Marker完美展示图片

世人都说雪景美

寒风冻脚无人疼

只道是一身正气

结论

参考Flutter集成高德地图并添加自定义Maker先实现自定义Marker。如果自定义Marker中用到了图片，那么会碰到图片没有被绘制到Marker的问题，此时需要通过precacheImage来预加载图片，从而解决此问题。

一、背景

在高德地图上需要展示每一辆单车的电量。而amap_flutter_map确没有提供自定义Marker的方法，只有一个static BitmapDescriptor *fromBytes*(Uint8List byteData). 所以需要将定制的Widget转为图片，然后图片转为字节流，来实现自定义Marker。

二、自定义Marker

1. 定义widget

创建业务需求的widget，如下：

static Future<Widget> _createMarkerView(BuildContext context, String name,String imageName,
      {bool selected = false, MarkerTitleType markerTitleType = MarkerTitleType.small}) async {
    var height = markerTitleType.height;
    if (selected) {
      height *= 2;
    }
    var width = markerTitleType.width;

    return Container(
      height: height,
      constraints: BoxConstraints(minWidth: width),
      alignment: Alignment.center,
      decoration: BoxDecoration(
        image: DecorationImage(image: AssetImage(imageName)),
      ),
      child: Directionality(
        textDirection: TextDirection.ltr,
        child: Text(
          name,
          style: TextStyle(fontSize: selected ? 22 : 14, color: Colors.white),
          maxLines: 1,
          overflow: TextOverflow.ellipsis,
          textAlign: TextAlign.center,
        ),
      ),
    );
  }

很简单的一个背景图片和一个标题。其中这个图片，因为是从Images资源中加载，会偶现加载失败的问题。

展示失败

2. `widget`转`ByteData`

不展示widget到窗口，直接将widget保存为图片。代码如下：

  static Future<ByteData?> widgetToByteData(Widget widget, String imageName,
      {Alignment alignment = Alignment.center,
      Size size = const Size(double.maxFinite, double.maxFinite),
      double devicePixelRatio = 1.0,
      double pixelRatio = 1.0,
      required BuildContext context}) async {
    RenderRepaintBoundary repaintBoundary = RenderRepaintBoundary();
    RenderView renderView = RenderView(
      child: RenderPositionedBox(alignment: alignment, child: repaintBoundary),
      configuration: ViewConfiguration(
        size: size,
        devicePixelRatio: devicePixelRatio,
      ),
      view: View.of(context),
    );

    PipelineOwner pipelineOwner = PipelineOwner();
    pipelineOwner.rootNode = renderView;
    renderView.prepareInitialFrame();

    BuildOwner buildOwner = BuildOwner(focusManager: FocusManager());
    RenderObjectToWidgetElement rootElement = RenderObjectToWidgetAdapter(
      container: repaintBoundary,
      child: widget,
    ).attachToRenderTree(buildOwner);
    await precacheImage(AssetImage(imageName), rootElement);
    buildOwner.buildScope(rootElement);
    buildOwner.finalizeTree();

    pipelineOwner.flushLayout();
    pipelineOwner.flushCompositingBits();
    pipelineOwner.flushPaint();

    ui.Image image = await repaintBoundary.toImage(pixelRatio: pixelRatio);
    ByteData? byteData = await image.toByteData(format: ui.ImageByteFormat.png);

    return byteData;
  }

其中的 await precacheImage(AssetImage(imageName), rootElement); 是解决图片偶现加载失败的关键。

3. 返回`BitmapDescriptor`

创建Marker的时候需要一个BitmapDescriptor,代码如下：

var data = await MapConverting.widgetToByteData(view, imageName,
        context: context,
        devicePixelRatio: AMapUtil.devicePixelRatio,
        pixelRatio: AMapUtil.devicePixelRatio,
        size: Size(selected ? width * 1.5 : width, selected ? height * 1.2 : height));
    var uInt8List = data?.buffer.asUint8List();
    if (null != uInt8List) {
      bitmapDescriptor = BitmapDescriptor.fromBytes(uInt8List);

      _createdBitmapDescriptor[key] = bitmapDescriptor;
    } else {
      bitmapDescriptor = await BitmapDescriptor.fromAssetImage(imageConfiguration, MyImages.imagesIcParkingNormal);
    }

    return bitmapDescriptor;

为了方便BitmapDescriptor的管理，可以创建一个BitmapDescriptorFactory类，添加一个static final Map<String, BitmapDescriptor> *_createdBitmapDescriptor* = {};将创建过的自定义BitmapDescriptorFactory做一个全局缓存，来解决重复创建的问题。