Fix NPE in handleTouchMove when activePath is null

scratchoff/src/main/java/com/jackpocket/scratchoff/ScratchableLayoutDrawer.java

@@ -60,6 +60,8 @@ public interface Delegate {
 
     private Long activeClearTag = 0L;
 
+    private boolean activePathRecoveryEnabled = false;
+
     private WeakReference<View> initializeLayoutTarget = new WeakReference<>(null);
     private ViewTreeObserver.OnGlobalLayoutListener initializationGlobalLayoutListener;
 
@@ -443,4 +445,13 @@ public ScratchableLayoutDrawer setClearAnimationInterpolator(Interpolator clearA
 
         return this;
     }
+
+    @SuppressWarnings("WeakerAccess")
+    public ScratchableLayoutDrawer setActivePathRecoveryEnabled(boolean activePathRecoveryEnabled) {
+        this.activePathRecoveryEnabled = activePathRecoveryEnabled;
+
+        pathManager.setActivePathRecoveryEnabled(activePathRecoveryEnabled);
+
+        return this;
+    }
 }

scratchoff/src/main/java/com/jackpocket/scratchoff/ScratchoffController.java

@@ -79,6 +79,8 @@ public interface ThresholdChangedListener {
     private final LinkedBlockingQueue<ScratchPathPoint> history = new LinkedBlockingQueue<ScratchPathPoint>();
     private boolean stateRestorationEnabled;
 
+    private boolean activePathRecoveryEnabled = false;
+
     /**
      * Create a new {@link ScratchoffController} instance targeting a scratchable layout.
      */
@@ -152,7 +154,8 @@ public ScratchoffController attach() {
     protected ScratchableLayoutDrawer createLayoutDrawer() {
         return new ScratchableLayoutDrawer(this)
             .setClearAnimationDurationMs(clearAnimationDurationMs)
-            .setClearAnimationInterpolator(clearAnimationInterpolator);
+            .setClearAnimationInterpolator(clearAnimationInterpolator)
+            .setActivePathRecoveryEnabled(activePathRecoveryEnabled);
     }
 
     protected ScratchoffThresholdProcessor createThresholdProcessor() {
@@ -480,6 +483,22 @@ public ScratchoffController setStateRestorationEnabled(boolean stateRestorationE
         return this;
     }
 
+    /**
+     * Set whether or not to recover from a null active {@link android.graphics.Path} during
+     * a touch-move event, instead of allowing a {@link NullPointerException} to be thrown.
+     * This can occur when a move event arrives without a preceding down event,
+     * e.g. due to race conditions during layout callbacks.
+     * The default for this value is false for the original (crashing) behavior.
+     */
+    public ScratchoffController setActivePathRecoveryEnabled(
+        boolean activePathRecoveryEnabled
+    ) {
+
+        this.activePathRecoveryEnabled = activePathRecoveryEnabled;
+
+        return this;
+    }
+
     public View getViewBehind() {
         return behindView.get();
     }

scratchoff/src/main/java/com/jackpocket/scratchoff/paths/ScratchPathManager.java

@@ -18,6 +18,8 @@ public class ScratchPathManager implements ScratchPathPointsAggregator {
     private final ArrayList<Path> paths = new ArrayList<>();
     private float scale = 1f;
 
+    private boolean activePathRecoveryEnabled = false;
+
     public ScratchPathManager() {
     }
 
@@ -27,6 +29,12 @@ public ScratchPathManager setScale(float scale) {
         return this;
     }
 
+    public ScratchPathManager setActivePathRecoveryEnabled(boolean activePathRecoveryEnabled) {
+        this.activePathRecoveryEnabled = activePathRecoveryEnabled;
+
+        return this;
+    }
+
     @Override
     public void addScratchPathPoints(Collection<ScratchPathPoint> events) {
         for (ScratchPathPoint event : events) {
@@ -82,6 +90,17 @@ protected void handleTouchMove(int pointerIndex, float x, float y) {
 
         Path activePath = this.activePaths[pointerIndex];
 
+        // Recover from a null activePath caused by a move event arriving without
+        // a preceding down event (e.g. due to race conditions during layout callbacks)
+        if (activePath == null) {
+            if (!activePathRecoveryEnabled) {
+                return;
+            }
+
+            createPath(pointerIndex, x, y);
+            activePath = this.activePaths[pointerIndex];
+        }
+
         // If the active Path has been drawn, it would have been reset to an empty state
         if (activePath.isEmpty()) {
             activePath.moveTo(x, y);

scratchoff/src/test/java/com/jackpocket/scratchoff/paths/ScratchPathManagerTests.kt

@@ -61,6 +61,29 @@ class ScratchPathManagerTests {
         assertEquals(value, pathBounds.isEmpty)
     }
 
+    @Test
+    fun testMoveWithNullActivePathIsIgnoredWhenRecoveryDisabled() {
+        val manager = ScratchPathManager()
+
+        assertEquals(0, manager.paths.size)
+
+        manager.handleTouchMove(0, 1f, 1f)
+
+        assertEquals(0, manager.paths.size)
+    }
+
+    @Test
+    fun testMoveWithNullActivePathCreatesPathWhenRecoveryEnabled() {
+        val manager = ScratchPathManager()
+        manager.setActivePathRecoveryEnabled(true)
+
+        assertEquals(0, manager.paths.size)
+
+        manager.handleTouchMove(0, 1f, 1f)
+
+        assertEquals(1, manager.paths.size)
+    }
+
     @Test
     fun testTouchDownAndMovePassthroughIncludesScalar() {
         val manager = object: ScratchPathManager() {