Fix pixel alignment math (#312)

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 - Fixed an issue that could cause accessibility focus to shift unexpectedly
+- Fixed a screen-pixel alignment issue
 
 ### Changed
 - Rewrote accessibility code to avoid posting notifications, which causes poor Voice Over performance and odd focus bugs

Sources/Internal/FrameProvider.swift

@@ -230,7 +230,7 @@ final class FrameProvider {
     let origin: CGPoint
     if distanceFromAdjacentDay < 0 {
       let proposedX = adjacentDayFrame.minX - content.horizontalDayMargin - daySize.width
-      if proposedX > minX || proposedX.isEqual(to: minX, threshold: 1 / scale) {
+      if proposedX > minX || proposedX.isEqual(to: minX, screenScale: scale) {
         origin = CGPoint(x: proposedX, y: adjacentDayFrame.minY)
       } else {
         origin = CGPoint(
@@ -239,7 +239,7 @@ final class FrameProvider {
       }
     } else {
       let proposedX = adjacentDayFrame.maxX + content.horizontalDayMargin
-      if proposedX < maxX || proposedX.isEqual(to: maxX, threshold: 1 / scale) {
+      if proposedX < maxX || proposedX.isEqual(to: maxX, screenScale: scale) {
         origin = CGPoint(x: proposedX, y: adjacentDayFrame.minY)
       } else {
         origin = CGPoint(x: minX, y: adjacentDayFrame.maxY + content.verticalDayMargin)

Sources/Internal/ScreenPixelAlignment.swift

@@ -22,10 +22,12 @@ extension CGFloat {
     (self * scale).rounded() / scale
   }
 
-  /// Tests `self` for approximate equality using the threshold value. For example, 1.48 equals 1.52 if the threshold is 0.05.
-  /// `threshold` will be treated as a positive value by taking its absolute value.
-  func isEqual(to rhs: CGFloat, threshold: CGFloat) -> Bool {
-    abs(self - rhs) <= abs(threshold)
+  /// Tests `self` for approximate equality, first rounding the operands to be pixel-aligned for a screen with the given
+  /// `screenScale`. For example, 1.48 equals 1.52 if the `screenScale` is `2`.
+  func isEqual(to rhs: CGFloat, screenScale: CGFloat) -> Bool {
+    let lhs = alignedToPixel(forScreenWithScale: screenScale)
+    let rhs = rhs.alignedToPixel(forScreenWithScale: screenScale)
+    return lhs == rhs
   }
 
 }
@@ -35,13 +37,11 @@ extension CGRect {
   /// Rounds a `CGRect`'s `origin` and `size` values so that they're aligned on pixel boundaries for a screen with the provided
   /// scale.
   func alignedToPixels(forScreenWithScale scale: CGFloat) -> CGRect {
-    let alignedX = minX.alignedToPixel(forScreenWithScale: scale)
-    let alignedY = minY.alignedToPixel(forScreenWithScale: scale)
-    return CGRect(
-      x: alignedX,
-      y: alignedY,
-      width: maxX.alignedToPixel(forScreenWithScale: scale) - alignedX,
-      height: maxY.alignedToPixel(forScreenWithScale: scale) - alignedY)
+    CGRect(
+      x: minX.alignedToPixel(forScreenWithScale: scale),
+      y: minY.alignedToPixel(forScreenWithScale: scale),
+      width: width.alignedToPixel(forScreenWithScale: scale),
+      height: height.alignedToPixel(forScreenWithScale: scale))
   }
 
 }

Tests/FrameProviderTests.swift

@@ -528,10 +528,10 @@ final class FrameProviderTests: XCTestCase {
       height: 34.857142857142854)
       .alignedToPixels(forScreenWithScale: 3)
     let expectedFrameAfterMiddleDay = CGRect(
-      x: 187.33333333333334,
+      x: 187.66666666666666,
       y: 374.3333333333333,
-      width: 34.857142857142854,
-      height: 34.857142857142854)
+      width: 35,
+      height: 35)
       .alignedToPixels(forScreenWithScale: 3)
     XCTAssert(frameBeforeMiddleDay == expectedFrameBeforeMiddleDay, "Incorrect frame for day.")
     XCTAssert(frameAfterMiddleDay == expectedFrameAfterMiddleDay, "Incorrect frame for day.")

Tests/ScreenPixelAlignmentTests.swift

@@ -108,15 +108,15 @@ final class ScreenPixelAlignmentTests: XCTestCase {
 
   func test2xScaleRectAlignment() {
     let rect = CGRect(x: 5.299999, y: -19.1994, width: 20.25, height: 0.76)
-    let expectedRect = CGRect(x: 5.5, y: -19, width: 20, height: 0.5)
+    let expectedRect = CGRect(x: 5.5, y: -19, width: 20.5, height: 1)
     XCTAssert(
       rect.alignedToPixels(forScreenWithScale: 2) == expectedRect,
       "Incorrect screen pixel alignment")
   }
 
   func test3xScaleRectAlignment() {
     let rect = CGRect(x: 71.13, y: 71.19, width: 20.25, height: 2)
-    let expectedRect = CGRect(x: 71, y: 71.33333333333333, width: 20.33333333333333, height: 2)
+    let expectedRect = CGRect(x: 71, y: 71.33333333333333, width: 20.333333333333332, height: 2)
     XCTAssert(
       rect.alignedToPixels(forScreenWithScale: 3) == expectedRect,
       "Incorrect screen pixel alignment")
@@ -125,17 +125,11 @@ final class ScreenPixelAlignmentTests: XCTestCase {
   // MARK: CGFloat Approximate Comparison Tests
 
   func testApproximateEquality() {
-    XCTAssert(CGFloat(1.48).isEqual(to: 1.52, threshold: 0.05))
-    XCTAssert(!CGFloat(1.48).isEqual(to: 1.53, threshold: 0.05))
-
-    XCTAssert(CGFloat(1).isEqual(to: 10, threshold: 9))
-    XCTAssert(!CGFloat(1).isEqual(to: 11, threshold: 9))
-
-    XCTAssert(CGFloat(1).isEqual(to: 10, threshold: 9))
-    XCTAssert(!CGFloat(1).isEqual(to: 11, threshold: 9))
-
-    XCTAssert(CGFloat(1.333).isEqual(to: 1.666, threshold: 1 / 3))
-    XCTAssert(!CGFloat(1.332).isEqual(to: 1.666, threshold: 1 / 3))
+    XCTAssert(CGFloat(1.48).isEqual(to: 1.52, screenScale: 2))
+    XCTAssert(!CGFloat(1).isEqual(to: 10, screenScale: 9))
+    XCTAssert(!CGFloat(1).isEqual(to: 10, screenScale: 9))
+    XCTAssert(!CGFloat(1).isEqual(to: 9, screenScale: 9))
+    XCTAssert(!CGFloat(1.333).isEqual(to: 1.666, screenScale: 3))
   }
 
 }