diff --git a/lib/matplotlib/backends/backend_qt5.py b/lib/matplotlib/backends/backend_qt5.py
index c27c876373a0..3abe934a79f9 100644
--- a/lib/matplotlib/backends/backend_qt5.py
+++ b/lib/matplotlib/backends/backend_qt5.py
@@ -227,12 +227,8 @@ def __init__(self, figure):
self._update_figure_dpi()
# In cases with mixed resolution displays, we need to be careful if the
# dpi_ratio changes - in this case we need to resize the canvas
- # accordingly. We could watch for screenChanged events from Qt, but
- # the issue is that we can't guarantee this will be emitted *before*
- # the first paintEvent for the canvas, so instead we keep track of the
- # dpi_ratio value here and in paintEvent we resize the canvas if
- # needed.
- self._dpi_ratio_prev = None
+ # accordingly.
+ self._dpi_ratio_prev = self._dpi_ratio
self._draw_pending = False
self._is_drawing = False
@@ -253,12 +249,9 @@ def _update_figure_dpi(self):
def _dpi_ratio(self):
return _devicePixelRatioF(self)
- def _update_dpi(self):
- # As described in __init__ above, we need to be careful in cases with
- # mixed resolution displays if dpi_ratio is changing between painting
- # events.
- # Return whether we triggered a resizeEvent (and thus a paintEvent)
- # from within this function.
+ def _update_pixel_ratio(self):
+ # We need to be careful in cases with mixed resolution displays if
+ # dpi_ratio changes.
if self._dpi_ratio != self._dpi_ratio_prev:
# We need to update the figure DPI.
self._update_figure_dpi()
@@ -270,8 +263,20 @@ def _update_dpi(self):
self.resizeEvent(event)
# resizeEvent triggers a paintEvent itself, so we exit this one
# (after making sure that the event is immediately handled).
- return True
- return False
+
+ def _update_screen(self, screen):
+ # Handler for changes to a window's attached screen.
+ self._update_pixel_ratio()
+ if screen is not None:
+ screen.physicalDotsPerInchChanged.connect(self._update_pixel_ratio)
+ screen.logicalDotsPerInchChanged.connect(self._update_pixel_ratio)
+
+ def showEvent(self, event):
+ # Set up correct pixel ratio, and connect to any signal changes for it,
+ # once the window is shown (and thus has these attributes).
+ window = self.window().windowHandle()
+ window.screenChanged.connect(self._update_screen)
+ self._update_screen(window.screen())
def get_width_height(self):
w, h = FigureCanvasBase.get_width_height(self)
@@ -364,10 +369,6 @@ def keyReleaseEvent(self, event):
FigureCanvasBase.key_release_event(self, key, guiEvent=event)
def resizeEvent(self, event):
- # _dpi_ratio_prev will be set the first time the canvas is painted, and
- # the rendered buffer is useless before anyways.
- if self._dpi_ratio_prev is None:
- return
w = event.size().width() * self._dpi_ratio
h = event.size().height() * self._dpi_ratio
dpival = self.figure.dpi
diff --git a/lib/matplotlib/backends/backend_qt5agg.py b/lib/matplotlib/backends/backend_qt5agg.py
index 80e670b3f3e4..90eafca5334d 100644
--- a/lib/matplotlib/backends/backend_qt5agg.py
+++ b/lib/matplotlib/backends/backend_qt5agg.py
@@ -27,9 +27,6 @@ def paintEvent(self, event):
In Qt, all drawing should be done inside of here when a widget is
shown onscreen.
"""
- if self._update_dpi():
- # The dpi update triggered its own paintEvent.
- return
self._draw_idle() # Only does something if a draw is pending.
# If the canvas does not have a renderer, then give up and wait for
diff --git a/lib/matplotlib/backends/backend_qt5cairo.py b/lib/matplotlib/backends/backend_qt5cairo.py
index c79b0bc22260..a375d1f4d1e2 100644
--- a/lib/matplotlib/backends/backend_qt5cairo.py
+++ b/lib/matplotlib/backends/backend_qt5cairo.py
@@ -17,7 +17,6 @@ def draw(self):
super().draw()
def paintEvent(self, event):
- self._update_dpi()
dpi_ratio = self._dpi_ratio
width = int(dpi_ratio * self.width())
height = int(dpi_ratio * self.height())
diff --git a/lib/matplotlib/tests/test_backend_qt.py b/lib/matplotlib/tests/test_backend_qt.py
index ab90da74d51e..afc6711158a5 100644
--- a/lib/matplotlib/tests/test_backend_qt.py
+++ b/lib/matplotlib/tests/test_backend_qt.py
@@ -158,32 +158,38 @@ def on_key_press(event):
@pytest.mark.backend('Qt5Agg')
-def test_dpi_ratio_change():
+def test_pixel_ratio_change():
"""
- Make sure that if _dpi_ratio changes, the figure dpi changes but the
+ Make sure that if the pixel ratio changes, the figure dpi changes but the
widget remains the same physical size.
"""
- prop = 'matplotlib.backends.backend_qt5.FigureCanvasQT._dpi_ratio'
-
- with mock.patch(prop, new_callable=mock.PropertyMock) as p:
-
+ prop = 'matplotlib.backends.backend_qt5.FigureCanvasQT.devicePixelRatioF'
+ with mock.patch(prop) as p:
p.return_value = 3
fig = plt.figure(figsize=(5, 2), dpi=120)
qt_canvas = fig.canvas
qt_canvas.show()
- from matplotlib.backends.backend_qt5 import qApp
+ def set_pixel_ratio(ratio):
+ p.return_value = ratio
+ # Make sure the mocking worked
+ assert qt_canvas._dpi_ratio == ratio
- # Make sure the mocking worked
- assert qt_canvas._dpi_ratio == 3
+ # The value here doesn't matter, as we can't mock the C++ QScreen
+ # object, but can override the functional wrapper around it.
+ # Emitting this event is simply to trigger the DPI change handler
+ # in Matplotlib in the same manner that it would occur normally.
+ screen.logicalDotsPerInchChanged.emit(96)
- size = qt_canvas.size()
+ qt_canvas.draw()
+ qt_canvas.flush_events()
qt_canvas.manager.show()
- qt_canvas.draw()
- qApp.processEvents()
+ size = qt_canvas.size()
+ screen = qt_canvas.window().windowHandle().screen()
+ set_pixel_ratio(3)
# The DPI and the renderer width/height change
assert fig.dpi == 360
@@ -196,17 +202,7 @@ def test_dpi_ratio_change():
assert qt_canvas.get_width_height() == (600, 240)
assert (fig.get_size_inches() == (5, 2)).all()
- p.return_value = 2
-
- assert qt_canvas._dpi_ratio == 2
-
- qt_canvas.draw()
- qApp.processEvents()
- # this second processEvents is required to fully run the draw.
- # On `update` we notice the DPI has changed and trigger a
- # resize event to refresh, the second processEvents is
- # required to process that and fully update the window sizes.
- qApp.processEvents()
+ set_pixel_ratio(2)
# The DPI and the renderer width/height change
assert fig.dpi == 240
@@ -219,17 +215,7 @@ def test_dpi_ratio_change():
assert qt_canvas.get_width_height() == (600, 240)
assert (fig.get_size_inches() == (5, 2)).all()
- p.return_value = 1.5
-
- assert qt_canvas._dpi_ratio == 1.5
-
- qt_canvas.draw()
- qApp.processEvents()
- # this second processEvents is required to fully run the draw.
- # On `update` we notice the DPI has changed and trigger a
- # resize event to refresh, the second processEvents is
- # required to process that and fully update the window sizes.
- qApp.processEvents()
+ set_pixel_ratio(1.5)
# The DPI and the renderer width/height change
assert fig.dpi == 180
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