call setData when children disappear

Previously, if you had a Layer that had Features as children,
and then the props changed such that the Layer no longer had
children, <source>.setData would not be called to remove the
Features from the source.

To fix this, I've changed Layer so that it will always call
setData when rendered. This should be fine because Layer#render
will only be called if the component should update.

I've added tests as well.

src/__tests__/layer.test.tsx

@@ -7,12 +7,18 @@ describe('Layer', () => {
   let LayerWithContext: any;
   let addLayerMock = jest.fn();
   let addSourceMock = jest.fn();
+  let setDataMock = jest.fn();
   let children: any[];
+  let childrenWithOneFeature: any[];
+  let feature: Object;
 
   beforeEach(() => {
     addLayerMock = jest.fn();
     addSourceMock = jest.fn();
+    setDataMock = jest.fn();
+    feature = { coordinates: [-123, 45] };
     children = [{ props: {}}];
+    childrenWithOneFeature = [{ props: feature }];
 
     LayerWithContext = withContext({
       map: React.PropTypes.object
@@ -21,7 +27,7 @@ describe('Layer', () => {
         addSource: addSourceMock,
         addLayer: addLayerMock,
         on: jest.fn(),
-        getSource: jest.fn().mockReturnValue({ setData: jest.fn() })
+        getSource: jest.fn().mockReturnValue({ setData: setDataMock })
       }
     }))(Layer);
   });
@@ -58,4 +64,35 @@ describe('Layer', () => {
     }]);
   });
 
+  it('Should set features based on children', () => {
+    const layer = mount(
+      <LayerWithContext
+        children={childrenWithOneFeature}
+      /> as React.ReactElement<any>
+    );
+
+    expect(setDataMock.mock.calls[0]).toEqual([{
+      'type': 'FeatureCollection',
+      'features': [{
+        'geometry': {...feature, 'type': 'Point'},
+        'properties': {id: 0},
+        'type': 'Feature'
+      }]
+    }])
+  });
+
+  it('Should set features to empty array when children disappear', () => {
+    const layer = mount(
+      <LayerWithContext
+        children={childrenWithOneFeature}
+      /> as React.ReactElement<any>
+    );
+
+    layer.setProps({ children: undefined });
+
+    expect(setDataMock.mock.calls[1]).toEqual([{
+      'type': 'FeatureCollection',
+      'features': []
+    }])
+  });
 });

src/layer.ts

@@ -208,20 +208,17 @@ export default class Layer extends React.PureComponent<Props, void> {
 
   public render() {
     const { map } = this.context;
+    const children = ([] as any).concat(this.props.children || []);
 
-    if (this.props.children) {
-      const children = ([] as any).concat(this.props.children);
+    const features = children
+      .map(({ props }: any, id: string) => this.makeFeature(props, id))
+      .filter(Boolean);
 
-      const features = children
-        .map(({ props }: any, id: string) => this.makeFeature(props, id))
-        .filter(Boolean);
-
-      const source = map.getSource(this.props.sourceId || this.id);
-      (source as MapboxGL.GeoJSONSource).setData({
-        type: 'FeatureCollection',
-        features
-      });
-    }
+    const source = map.getSource(this.props.sourceId || this.id);
+    (source as MapboxGL.GeoJSONSource).setData({
+      type: 'FeatureCollection',
+      features
+    });
 
     return null;
   }