Allow WebGL tile layers to be constructed without a source

src/ol/layer/WebGLTile.js

@@ -2,6 +2,7 @@
  * @module ol/layer/WebGLTile
  */
 import BaseTileLayer from './BaseTile.js';
+import LayerProperty from '../layer/Property.js';
 import WebGLTileLayerRenderer, {
   Attributes,
   Uniforms,
@@ -298,14 +299,28 @@ class WebGLTileLayer extends BaseTileLayer {
      * @private
      */
     this.styleVariables_ = this.style_.variables || {};
+
+    this.addChangeListener(LayerProperty.SOURCE, this.handleSourceUpdate_);
+  }
+
+  /**
+   * @private
+   */
+  handleSourceUpdate_() {
+    this.setStyle(this.style_);
+  }
+
+  /**
+   * @private
+   * @return {number} The number of source bands.
+   */
+  getSourceBandCount_() {
+    const source = this.getSource();
+    return source && 'bandCount' in source ? source.bandCount : 4;
   }
 
   createRenderer() {
-    const source = this.getSource();
-    const parsedStyle = parseStyle(
-      this.style_,
-      'bandCount' in source ? source.bandCount : 4
-    );
+    const parsedStyle = parseStyle(this.style_, this.getSourceBandCount_());
 
     return new WebGLTileLayerRenderer(this, {
       vertexShader: parsedStyle.vertexShader,
@@ -323,11 +338,7 @@ class WebGLTileLayer extends BaseTileLayer {
    */
   setStyle(style) {
     this.style_ = style;
-    const source = this.getSource();
-    const parsedStyle = parseStyle(
-      this.style_,
-      'bandCount' in source ? source.bandCount : 4
-    );
+    const parsedStyle = parseStyle(this.style_, this.getSourceBandCount_());
     const renderer = this.getRenderer();
     renderer.reset({
       vertexShader: parsedStyle.vertexShader,

test/browser/spec/ol/layer/WebGLTile.test.js

@@ -339,4 +339,38 @@ describe('ol/layer/WebGLTile', function () {
     }
     expect(incorrectStyle).to.throwException(); // missing 'blue' in styleVariables
   });
+
+  it('works if the layer is constructed without a source', (done) => {
+    const sourceless = new WebGLTileLayer({
+      className: 'testlayer',
+      style: {
+        variables: {
+          r: 0,
+          g: 255,
+          b: 0,
+        },
+        color: ['color', ['var', 'r'], ['var', 'g'], ['var', 'b']],
+      },
+    });
+
+    map.addLayer(sourceless);
+
+    sourceless.setSource(
+      new DataTileSource({
+        loader(z, x, y) {
+          return new ImageData(256, 256);
+        },
+      })
+    );
+
+    let called = false;
+    layer.on('postrender', (event) => {
+      called = true;
+    });
+
+    map.once('rendercomplete', () => {
+      expect(called).to.be(true);
+      done();
+    });
+  });
 });