cmd/gogio: extract endtoend driver base into a type

This type contains all the common bits, such as *testing.T, as well as
the channel and method used to wait for blocking until a frame is ready.

It also allows us to initialise this base separately from Start, which
keeps the exported method simpler to understand.

The base type is embedded into the specific driver types, so that the
code remains simple. While at it, start embedding *testing.T too, so
that we can write d.Fatalf instead of d.t.Fatalf. The drivers will only
have a small number of exported methods as per the interface, so it's
easy to keep those from colliding with the method set on T.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>

cmd/gogio/e2e_test.go

@@ -20,14 +20,15 @@ const appid = "localhost.gogio.endtoend"
 // tests on. None of its methods return any errors, as the errors are directly
 // reported to testing.T via methods like Fatal.
 type TestDriver interface {
-	// Start provides the test driver with a testing.T, as well as the path
-	// to the Gio app to use for the test. The driver should attempt to run
-	// the app with the given width and height, and the platform's
+	initBase(*testing.T)
+
+	// Start opens the Gio app found at path. The driver should attempt to
+	// run the app with the given width and height, and the platform's
 	// background should be white.
 	//
 	// When the function returns, the gio app must be ready to use on the
 	// platform, with its initial frame fully drawn.
-	Start(t *testing.T, path string, width, height int)
+	Start(path string, width, height int)
 
 	// Screenshot takes a screenshot of the Gio app on the platform.
 	Screenshot() image.Image
@@ -38,6 +39,17 @@ type TestDriver interface {
 	Click(x, y int)
 }
 
+type driverBase struct {
+	*testing.T
+
+	frameNotifs chan bool
+}
+
+func (d *driverBase) initBase(t *testing.T) {
+	d.T = t
+	d.frameNotifs = make(chan bool, 1)
+}
+
 func TestEndToEnd(t *testing.T) {
 	if testing.Short() {
 		t.Skipf("end-to-end tests tend to be slow")
@@ -66,9 +78,11 @@ func TestEndToEnd(t *testing.T) {
 }
 
 func runEndToEndTest(t *testing.T, driver TestDriver) {
+	driver.initBase(t)
+
 	size := image.Point{X: 800, Y: 600}
 	t.Log("starting driver and gio app")
-	driver.Start(t, "testdata/red.go", size.X, size.Y)
+	driver.Start("testdata/red.go", size.X, size.Y)
 
 	// The colors are split in four rectangular sections. Check the corners
 	// of each of the sections. We check the corners left to right, top to
@@ -146,8 +160,8 @@ func wantColor(t *testing.T, img image.Image, x, y int, want color.Color) {
 	}
 }
 
-func waitForFrame(t *testing.T, frameNotifs <-chan bool) {
-	t.Helper()
+func (d *driverBase) waitForFrame() {
+	d.Helper()
 
 	// Unfortunately, there isn't a way to select on a test failing, since
 	// testing.T doesn't have anything like a context or a "done" channel.
@@ -158,8 +172,8 @@ func waitForFrame(t *testing.T, frameNotifs <-chan bool) {
 	// For now, a static short timeout is better than nothing. 2s is plenty
 	// for our simple test app to render on any device.
 	select {
-	case <-frameNotifs:
+	case <-d.frameNotifs:
 	case <-time.After(5 * time.Second):
-		t.Fatalf("timed out waiting for a frame to be ready")
+		d.Fatalf("timed out waiting for a frame to be ready")
 	}
 }