Dispose GC with Canvas

CanvasContext is wrapping a "GC" NativeObject that needs to be
disposed. While the Canvas is set as the "parent" of the GC, the
usual mechanism that automatically disposes children of a widget
is not in effect here, so the GC needs to be disposed explicitly.

Fix #2130

Change-Id: Ic71e7b99a5160d37c636cfaffc8d371691ee849f

src/tabris/CanvasContext.js

@@ -197,10 +197,12 @@ defineMethod('drawImage', 3, /** @this {CanvasContext} */ function(image, x1, y1
 
 CanvasContext.getContext = function(canvas, width, height) {
   if (!canvas._gc) {
+    const gc = new GC({parent: canvas});
+    canvas.on('dispose', () => gc.dispose());
     Object.defineProperty(canvas, '_gc', {
       enumerable: false,
       writable: false,
-      value: new GC({parent: canvas})
+      value: gc
     });
   }
   if (!canvas._ctx) {

test/tabris/CanvasContext.test.js

@@ -92,13 +92,28 @@ describe('CanvasContext', function() {
       expect(createCalls[0].properties.parent).to.equal(canvas.cid);
     });
 
+    it('disposes native GC when parent disposes', function() {
+      canvas.getContext('2d', 100, 200);
+      const id = client.calls({op: 'create', type: 'tabris.GC'})[0].id;
+      tabris.flush();
+
+      canvas.dispose();
+
+      console.log(client.calls());
+
+      const disposeCalls = client.calls({op: 'destroy'});
+      expect(disposeCalls.length).to.equal(2);
+      expect(disposeCalls[0].id).to.equal(id);
+      expect(disposeCalls[1].id).to.equal(canvas.cid);
+    });
+
     it('creates and returns graphics context', function() {
       ctx = canvas.getContext('2d', 100, 200);
 
       expect(ctx).to.be.an.instanceof(CanvasContext);
     });
 
-    it('returns same instance everytime', function() {
+    it('returns same instance every time', function() {
       const ctx1 = canvas.getContext('2d', 100, 200);
 
       const ctx2 = canvas.getContext('2d', 100, 200);