TST: Test number of draw events from a blitted animation

This adds a test that only one draw_event is called when using
blitting with animations. There will be more than one draw_event
if blitting is not implemented properly and instead calls a full
redraw of the canvas each time.

Author: greglucas

lib/matplotlib/tests/test_backends_interactive.py

@@ -413,3 +413,72 @@ def _lazy_headless():
 @pytest.mark.backend('QtAgg', skip_on_importerror=True)
 def test_lazy_linux_headless():
     proc = _run_helper(_lazy_headless, timeout=_test_timeout, MPLBACKEND="")
+
+
+def _test_number_of_draws_script():
+    import matplotlib.pyplot as plt
+
+    fig, ax = plt.subplots()
+
+    # animated=True tells matplotlib to only draw the artist when we
+    # explicitly request it
+    ln, = ax.plot([0, 1], [1, 2], animated=True)
+
+    # make sure the window is raised, but the script keeps going
+    plt.show(block=False)
+    plt.pause(0.3)
+    # Connect to draw_event to count the occurrences
+    fig.canvas.mpl_connect('draw_event', print)
+
+    # get copy of entire figure (everything inside fig.bbox)
+    # sans animated artist
+    bg = fig.canvas.copy_from_bbox(fig.bbox)
+    # draw the animated artist, this uses a cached renderer
+    ax.draw_artist(ln)
+    # show the result to the screen
+    fig.canvas.blit(fig.bbox)
+
+    for j in range(10):
+        # reset the background back in the canvas state, screen unchanged
+        fig.canvas.restore_region(bg)
+        # Create a **new** artist here, this is poor usage of blitting
+        # but good for testing to make sure that this doesn't create
+        # excessive draws
+        ln, = ax.plot([0, 1], [1, 2])
+        # render the artist, updating the canvas state, but not the screen
+        ax.draw_artist(ln)
+        # copy the image to the GUI state, but screen might not changed yet
+        fig.canvas.blit(fig.bbox)
+        # flush any pending GUI events, re-painting the screen if needed
+        fig.canvas.flush_events()
+
+    # Let the event loop process everything before leaving
+    plt.pause(0.1)
+
+
+_blit_backends = _get_testable_interactive_backends()
+for param in _blit_backends:
+    backend = param.values[0]["MPLBACKEND"]
+    if backend == "gtk3cairo":
+        # copy_from_bbox only works when rendering to an ImageSurface
+        param.marks.append(
+            pytest.mark.skip("gtk3cairo does not support blitting"))
+    elif backend == "wx":
+        param.marks.append(
+            pytest.mark.skip("wx does not support blitting"))
+
+
+@pytest.mark.parametrize("env", _blit_backends)
+# subprocesses can struggle to get the display, so rerun a few times
+@pytest.mark.flaky(reruns=4)
+def test_blitting_events(env):
+    proc = _run_helper(_test_number_of_draws_script,
+                       timeout=_test_timeout,
+                       **env)
+
+    # Count the number of draw_events we got. We could count some initial
+    # canvas draws (which vary in number by backend), but the critical
+    # check here is that it isn't 10 draws, which would be called if
+    # blitting is not properly implemented
+    ndraws = proc.stdout.count("DrawEvent")
+    assert 0 < ndraws < 5