Add Screens.Empty() for checking if a Screens has no content (#1092)

encode/encode.go

@@ -54,6 +54,11 @@ func ScreensFromImages(images ...image.Image) *Screens {
 	return &screens
 }
 
+// Empty returns true if there are no render roots or images in this screen.
+func (s *Screens) Empty() bool {
+	return len(s.roots) == 0 && len(s.images) == 0
+}
+
 // Hash returns a hash of the render roots for this screen. This can be used for
 // testing whether two render trees are exactly equivalent, without having to
 // do the actual rendering.

encode/encode_test.go

@@ -162,6 +162,7 @@ func TestHash(t *testing.T) {
 
 	roots, err := app.Run(context.Background())
 	require.NoError(t, err)
+	assert.False(t, ScreensFromRoots(roots).Empty())
 
 	// ensure we can calculate a hash
 	hash, err := ScreensFromRoots(roots).Hash()
@@ -185,11 +186,33 @@ func TestHash(t *testing.T) {
 
 	// ensure we can calculate a hash on the new app
 	hash2, err := ScreensFromRoots(roots2).Hash()
+	require.NoError(t, err)
 
 	// ensure hashes are different
 	require.NotEqual(t, hash, hash2)
 }
 
+func TestHashEmptyApp(t *testing.T) {
+	app, err := runtime.NewApplet("test.star", []byte(`def main(): return []`))
+	require.NoError(t, err)
+
+	roots, err := app.Run(context.Background())
+	require.NoError(t, err)
+	assert.True(t, ScreensFromRoots(roots).Empty())
+
+	// ensure we can calculate a hash
+	hash, err := ScreensFromRoots(roots).Hash()
+	require.NoError(t, err)
+	require.True(t, len(hash) > 0)
+
+	// ensure the hash doesn't change
+	for i := 0; i < 20; i++ {
+		h, err := ScreensFromRoots(roots).Hash()
+		assert.NoError(t, err)
+		assert.Equal(t, hash, h)
+	}
+}
+
 func TestHashDelayAndMaxAge(t *testing.T) {
 	r := []render.Root{{Child: &render.Text{Content: "derp"}}}