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。