Fix reset_transform in Cocoa/iOS

android/tests_backend/widgets/canvas.py

@@ -11,6 +11,7 @@
 
 class CanvasProbe(SimpleProbe):
     native_class = DrawHandlerView
+    screenshot_reset_transform = False
 
     def reference_variant(self, reference):
         if reference in {"multiline_text", "write_text", "write_text_and_path"}:

changes/4044.bugfix.md

@@ -0,0 +1 @@
+Canvas's `reset_transform` method now correctly resets the transformation on Mac and iOS, instead of clearing the current state.

cocoa/src/toga_cocoa/libs/core_graphics.py

@@ -35,8 +35,26 @@ class CGAffineTransform(Structure):
         ("ty", CGFloat),
     ]
 
+    def __eq__(self, other):
+        try:
+            return (
+                self.a == other.a
+                and self.b == other.b
+                and self.c == other.c
+                and self.d == other.d
+                and self.tx == other.tx
+                and self.ty == other.ty
+            )
+        except AttributeError:
+            return False
+
+
+CGAffineTransformIdentity = CGAffineTransform.in_dll(
+    core_graphics, "CGAffineTransformIdentity"
+)
 
-core_graphics.CGAffineTransformIdentity = CGAffineTransform
+core_graphics.CGAffineTransformConcat.argtypes = [CGAffineTransform, CGAffineTransform]
+core_graphics.CGAffineTransformConcat.restype = CGAffineTransform
 core_graphics.CGAffineTransformInvert.restype = CGAffineTransform
 core_graphics.CGAffineTransformInvert.argtypes = [CGAffineTransform]
 core_graphics.CGAffineTransformMakeScale.restype = CGAffineTransform

cocoa/src/toga_cocoa/widgets/canvas.py

@@ -1,4 +1,5 @@
 from math import ceil
+from warnings import warn
 
 from rubicon.objc import CGSize, objc_method, objc_property
 from travertino.size import at_least
@@ -7,6 +8,7 @@
 from toga.constants import Baseline, FillRule
 from toga_cocoa.colors import native_color
 from toga_cocoa.libs import (
+    CGAffineTransformIdentity,
     CGFloat,
     CGPathDrawingMode,
     CGRectMake,
@@ -38,6 +40,17 @@ class TogaCanvas(NSView):
     @objc_method
     def drawRect_(self, rect: NSRect) -> None:
         context = NSGraphicsContext.currentContext.CGContext
+
+        # Store this so we can restore the original if needed.
+        self.original_matrix = core_graphics.CGContextGetCTM(context)
+        self.using_standard_coords = self.original_matrix == CGAffineTransformIdentity
+        # If we're not in the normal coordinate space (presumably because we're
+        # rendering to a cache), save the inverse as well.
+        if not self.using_standard_coords:
+            self.inverse_original = core_graphics.CGAffineTransformInvert(
+                core_graphics.CGContextGetCTM(context)
+            )
+
         self.interface.context._draw(self.impl, draw_context=context)
 
     @objc_method
@@ -244,12 +257,39 @@ def translate(self, tx, ty, draw_context, **kwargs):
         core_graphics.CGContextTranslateCTM(draw_context, tx, ty)
 
     def reset_transform(self, draw_context, **kwargs):
-        # Restore the "clean" state of the graphics context.
-        core_graphics.CGContextRestoreGState(draw_context)
-        # CoreGraphics has a stack-based state representation,
-        # so ensure that there is a new, clean version of the "clean"
-        # state on the stack.
-        core_graphics.CGContextSaveGState(draw_context)
+        # Core Graphics has no built-in ability to assign to or reset the transformation
+        # matrix.
+        current = core_graphics.CGContextGetCTM(draw_context)
+
+        if current == self.native.original_matrix:
+            # No resetting necessary.
+            return
+
+        if self.native.using_standard_coords:
+            transform = current
+        else:
+            # The *original* transform matrix isn't the standard identity; this is
+            # probably because we're rendering to a cache. So we need to know the
+            # transform from what we started with.
+            transform = core_graphics.CGAffineTransformConcat(
+                current,
+                self.native.inverse_original,
+            )
+
+        inverse_transform = core_graphics.CGAffineTransformInvert(transform)
+        if inverse_transform == transform:
+            # When a matrix has no inverse, TransformInvert returns it unchanged.
+            warn(
+                (
+                    "No way to reset transform on macOS if current transform has no "
+                    "inverse. Did you scale to 0, perhaps?"
+                ),
+                RuntimeWarning,
+                stacklevel=2,
+            )
+            return
+
+        core_graphics.CGContextConcatCTM(draw_context, inverse_transform)
 
     # Text
     def _render_string(self, text, font, **kwargs):

cocoa/tests_backend/widgets/canvas.py

@@ -13,6 +13,7 @@
 
 class CanvasProbe(SimpleProbe):
     native_class = NSView
+    screenshot_reset_transform = True
 
     @property
     def background_color(self):

gtk/tests_backend/widgets/canvas.py

@@ -9,6 +9,7 @@
 
 class CanvasProbe(SimpleProbe):
     native_class = Gtk.DrawingArea
+    screenshot_reset_transform = False
 
     def reference_variant(self, reference):
         if reference == "multiline_text":

iOS/tests_backend/widgets/canvas.py

@@ -22,6 +22,7 @@ def locationInView(self, view) -> NSPoint:
 
 class CanvasProbe(SimpleProbe):
     native_class = UIView
+    screenshot_reset_transform = True
 
     def reference_variant(self, reference):
         # System fonts and sizes are platform specific

qt/tests_backend/widgets/canvas.py

@@ -10,6 +10,7 @@
 
 class CanvasProbe(SimpleProbe):
     native_class = TogaCanvas
+    screenshot_reset_transform = False
 
     def reference_variant(self, reference):
         if reference in {"multiline_text", "write_text", "write_text_and_path"}: