Improve hit test performance by optimizing X axis

Example/AccessibilitySnapshot.xcodeproj/xcshareddata/xcbaselines/607FACE41AFB9204008FA782.xcbaseline/EDA548A6-E453-4CBA-9366-15D413356833.plist

@@ -11,7 +11,7 @@
 				<key>com.apple.XCTPerformanceMetric_WallClockTime</key>
 				<dict>
 					<key>baselineAverage</key>
-					<real>17.200000</real>
+					<real>5.930000</real>
 					<key>baselineIntegrationDisplayName</key>
 					<string>Local Baseline</string>
 				</dict>

Example/SnapshotTests/HitTargetTests.swift

@@ -50,7 +50,8 @@ final class HitTargetTests: SnapshotTestCase {
                     for: buttonTraitsViewController.view,
                     useMonochromeSnapshot: true,
                     viewRenderingMode: .drawHierarchyInRect,
-                    maxPermissibleMissedRegionHeight: 4
+                    maxPermissibleMissedRegionWidth: 1,
+                    maxPermissibleMissedRegionHeight: 1
                 )
             } catch {
                 XCTFail("Utility should not fail to generate snapshot image")

Sources/AccessibilitySnapshot/Core/Swift/Classes/AccessibilitySnapshotView.swift

@@ -59,6 +59,8 @@ public final class AccessibilitySnapshotView: UIView {
         /// `.renderLayerInContext` view rendering mode
         case containedViewHasUnsupportedTransform(transform: CATransform3D)
 
+        case containedViewHasZeroSize(viewSize: CGSize)
+
     }
 
     // MARK: - Life Cycle

Sources/AccessibilitySnapshot/Core/Swift/Classes/HitTargetSnapshotView.swift

@@ -50,6 +50,7 @@ public enum HitTargetSnapshotUtility {
         useMonochromeSnapshot: Bool,
         viewRenderingMode: AccessibilitySnapshotView.ViewRenderingMode,
         colors: [UIColor] = AccessibilitySnapshotView.defaultMarkerColors,
+        maxPermissibleMissedRegionWidth: CGFloat = 0,
         maxPermissibleMissedRegionHeight: CGFloat = 0
     ) throws -> UIImage {
         let colors = colors.map { $0.withAlphaComponent(0.2) }
@@ -62,12 +63,17 @@ public enum HitTargetSnapshotUtility {
             viewRenderingMode: viewRenderingMode
         )
 
+        guard view.bounds.width > 0 && view.bounds.height > 0 else {
+            throw AccessibilitySnapshotView.Error.containedViewHasZeroSize(viewSize: view.bounds.size)
+        }
+
         return renderer.image { context in
             viewImage.draw(in: bounds)
 
             var viewToColorMap: [UIView: UIColor] = [:]
             let pixelWidth: CGFloat = 1 / UIScreen.main.scale
 
+            let maxPermissibleMissedRegionWidth = max(pixelWidth, floor(maxPermissibleMissedRegionWidth))
             let maxPermissibleMissedRegionHeight = max(pixelWidth, floor(maxPermissibleMissedRegionHeight))
 
             func drawScanLineSegment(
@@ -111,32 +117,55 @@ public enum HitTargetSnapshotUtility {
 
             typealias ScanLine = [(xRange: ClosedRange<CGFloat>, view: UIView?)]
 
+            // In some cases striding by 1/3 can result in the `to` value being included due to a floating point rouding
+            // error, in particular when dealing with bounds with a negative y origin. By striding to a value slightly
+            // less than the desired stop (small enough to be less than the density of any screen in the foreseeable
+            // future), we can avoid this rounding problem.
+            let stopEpsilon: CGFloat = 0.0001
+
             func scanLine(y: CGFloat) -> ScanLine {
                 var scanLine: ScanLine = []
-                var lastHit: (CGFloat, UIView?)? = nil
+                var lastHit: (CGFloat, UIView?) = (
+                    bounds.minX,
+                    view.hitTest(CGPoint(x: bounds.minX + touchOffset, y: y), with: nil)
+                )
+
+                func updateForHit(_ hitView: UIView?, at x: CGFloat) {
+                    if hitView == lastHit.1 {
+                        // We're still hitting the same view. Nothing to update.
+                        return
+
+                    } else {
+                        // We've moved on to a new view, so draw the scan line for the previous view.
+                        scanLine.append(((lastHit.0...x), lastHit.1))
+                        lastHit = (x, hitView)
+
+                    }
+                }
 
                 // Step through every pixel along the X axis.
-                for x in stride(from: bounds.minX, to: bounds.maxX, by: pixelWidth) {
+                for x in stride(from: bounds.minX, to: bounds.maxX, by: maxPermissibleMissedRegionWidth) {
                     let hitView = view.hitTest(CGPoint(x: x + touchOffset, y: y), with: nil)
 
-                    if let lastHit = lastHit, hitView == lastHit.1 {
+                    if hitView == lastHit.1 {
                         // We're still hitting the same view. Keep scanning.
                         continue
 
-                    } else if let previousHit = lastHit {
-                        // We've moved on to a new view, so draw the scan line for the previous view.
-                        scanLine.append(((previousHit.0...x), previousHit.1))
-                        lastHit = (x, hitView)
-
                     } else {
-                        // We've started a new view's region.
-                        lastHit = (x, hitView)
+                        // The last iteration of the loop hit test at (x - maxPermissibleMissedRegionWidth), so we want
+                        // to start one pixel in front of that.
+                        let startX = x - maxPermissibleMissedRegionWidth + pixelWidth
+
+                        for stepX in stride(from: startX, through: x, by: pixelWidth) {
+                            let stepHitView = view.hitTest(CGPoint(x: stepX + touchOffset, y: y), with: nil)
+                            updateForHit(stepHitView, at: stepX)
+                        }
                     }
                 }
 
                 // Finish the scan line if necessary.
-                if let lastHit = lastHit, let lastHitView = lastHit.1 {
-                    scanLine.append(((lastHit.0...bounds.maxX), lastHitView))
+                if lastHit.0 != bounds.maxX {
+                    scanLine.append(((lastHit.0...bounds.maxX), lastHit.1))
                 }
 
                 return scanLine
@@ -161,12 +190,6 @@ public enum HitTargetSnapshotUtility {
                     }
             }
 
-            // In some cases striding by 1/3 can result in the `to` value being included due to a floating point rouding
-            // error, in particular when dealing with bounds with a negative y origin. By striding to a value slightly
-            // less than the desired stop (small enough to be less than the density of any screen in the foreseeable
-            // future), we can avoid this rounding problem.
-            let stopEpsilon: CGFloat = 0.0001
-
             // Step through every full point along the Y axis and check if it's equal to the above line. If so, draw the
             // line at a full point width. If not, step through the pixel lines and draw each individually.
             var previousScanLine: (y: CGFloat, scanLine: ScanLine)? = nil