Fix onBegin and onTouchesDown events ordering on Android

android/lib/src/main/java/com/swmansion/gesturehandler/GestureHandler.kt

@@ -46,6 +46,7 @@ open class GestureHandler<ConcreteGestureHandlerT : GestureHandler<ConcreteGestu
   var trackedPointersCount = 0
     private set
   private val trackedPointers: Array<PointerData?> = Array(MAX_POINTERS_COUNT) { null }
+  private var deferredPointerEventSender: (() -> Unit)? = null
   var needsPointerData = false
 
 
@@ -447,14 +448,36 @@ open class GestureHandler<ConcreteGestureHandlerT : GestureHandler<ConcreteGestu
   }
 
   fun updatePointerData(event: MotionEvent) {
-    if (event.actionMasked == MotionEvent.ACTION_DOWN || event.actionMasked == MotionEvent.ACTION_POINTER_DOWN) {
-      dispatchTouchDownEvent(event)
-      dispatchTouchMoveEvent(event)
-    } else if (event.actionMasked == MotionEvent.ACTION_UP || event.actionMasked == MotionEvent.ACTION_POINTER_UP) {
-      dispatchTouchMoveEvent(event)
-      dispatchTouchUpEvent(event)
-    } else if (event.actionMasked == MotionEvent.ACTION_MOVE) {
-      dispatchTouchMoveEvent(event)
+    // Touch events are sent before the handler itself has a chance to process them. This means that
+    // the first `onTouchesDown` event is sent before a gesture begins, activation in callback for
+    // this event causes problems because the handler doesn't have a chance to initialize itself
+    // with starting values of pointer, resulting in wrong positioning or trying to send NaN values.
+    // The simplest (I think) solution to that, while preserving the correct ordering of events on
+    // the JS side, is to defer sending the first `onTouchesDown` event until the gesture is
+    // initialized and sends `BEGIN` event. When that happens we can also send deferred touch event
+    // preserving the correct flow.
+
+    val eventSender = {
+      when (event.actionMasked) {
+          MotionEvent.ACTION_DOWN, MotionEvent.ACTION_POINTER_DOWN -> {
+            dispatchTouchDownEvent(event)
+            dispatchTouchMoveEvent(event)
+          }
+          MotionEvent.ACTION_UP, MotionEvent.ACTION_POINTER_UP -> {
+            dispatchTouchMoveEvent(event)
+            dispatchTouchUpEvent(event)
+          }
+          MotionEvent.ACTION_MOVE -> {
+            dispatchTouchMoveEvent(event)
+          }
+      }
+    }
+
+    if (this.state != STATE_UNDETERMINED) {
+      eventSender()
+    } else {
+      // this is the first event, we want to defer it
+      this.deferredPointerEventSender = eventSender
     }
   }
 
@@ -648,6 +671,19 @@ open class GestureHandler<ConcreteGestureHandlerT : GestureHandler<ConcreteGestu
   }
 
   fun begin() {
+    // double state check as it might change because of the touch event
+    if (state == STATE_UNDETERMINED && this.deferredPointerEventSender != null) {
+      // if there was a deferred touch event, we want to send it before sending `BEGAN` event, to
+      // preserve the correct event flow.
+
+      // set `deferredPointerEventSender` to null before invoking the sender, as the current state
+      // is still UNDETERMINED, and there is a possibility that `begin` will be called on the JS
+      // which would result in an infinite loop
+      val sender = this.deferredPointerEventSender
+      this.deferredPointerEventSender = null
+      sender!!()
+    }
+
     if (state == STATE_UNDETERMINED) {
       moveToState(STATE_BEGAN)
     }

android/lib/src/main/java/com/swmansion/gesturehandler/GestureHandlerOrchestrator.kt

@@ -253,21 +253,10 @@ class GestureHandlerOrchestrator(
 
     val action = sourceEvent.actionMasked
     val event = transformEventToViewCoords(handler.view, MotionEvent.obtain(sourceEvent))
-
-    // Touch events are sent before the handler itself has a chance to process them,
-    // mainly because `onTouchesUp` shoul be send befor gesture finishes. This means that
-    // the first `onTouchesDown` event is sent before a gesture begins, activation in 
-    // callback for this event causes problems because the handler doesn't have a chance
-    // to initialize itself with starting values of pointer (in pan this causes translation
-    // to be equal to the coordinates of the pointer). The simplest solution is to send
-    // the first `onTouchesDown` event after the handler processes it and changes state
-    // to `BEGAN`.
-    if (handler.needsPointerData && handler.state != 0) {
-      handler.updatePointerData(event)
-    }
+
+    handler.updatePointerData(event)
 
     if (!handler.isAwaiting || action != MotionEvent.ACTION_MOVE) {
-      val isFirstEvent = handler.state == 0
       handler.handle(event, sourceEvent)
       if (handler.isActive) {
         // After handler is done waiting for other one to fail its progress should be
@@ -284,10 +273,6 @@ class GestureHandlerOrchestrator(
         handler.dispatchHandlerUpdate(event)
       }
 
-      if (handler.needsPointerData && isFirstEvent) {
-        handler.updatePointerData(event)
-      }
-
       // if event was of type UP or POINTER_UP we request handler to stop tracking now that
       // the event has been dispatched
       if (action == MotionEvent.ACTION_UP || action == MotionEvent.ACTION_POINTER_UP) {

android/src/main/java/com/swmansion/gesturehandler/react/RNGestureHandlerModule.kt

@@ -434,7 +434,12 @@ class RNGestureHandlerModule(reactContext: ReactApplicationContext?) :
   override fun setGestureHandlerState(handlerTag: Int, newState: Int) {
     registry.getHandler(handlerTag)?.let { handler ->
       when (newState) {
-        GestureHandler.STATE_ACTIVE -> handler.activate(force = true)
+        GestureHandler.STATE_ACTIVE -> {
+          // also call begin to force correct state flow in case the `activate` is called in the
+          // UNDETERMINED state
+          handler.begin()
+          handler.activate(force = true)
+        }
         GestureHandler.STATE_BEGAN -> handler.begin()
         GestureHandler.STATE_END -> handler.end()
         GestureHandler.STATE_FAILED -> handler.fail()

ios/Handlers/RNFlingHandler.m

@@ -2,12 +2,12 @@
 
 @interface RNBetterSwipeGestureRecognizer : UISwipeGestureRecognizer
 
-- (id)initWithGestureHandler:(RNGestureHandler*)gestureHandler;
+- (id)initWithGestureHandler:(RNGestureHandler *)gestureHandler;
 
 @end
 
 @implementation RNBetterSwipeGestureRecognizer {
-  __weak RNGestureHandler* _gestureHandler;
+  __weak RNGestureHandler *_gestureHandler;
   CGPoint _lastPoint; // location of the most recently updated touch, relative to the view
   bool _hasBegan; // whether the `BEGAN` event has been sent
 }
@@ -27,6 +27,7 @@ - (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
   _lastPoint = [[[touches allObjects] objectAtIndex:0] locationInView:_gestureHandler.recognizer.view];
   [_gestureHandler reset];
   [super touchesBegan:touches withEvent:event];
+  [_gestureHandler.pointerTracker touchesBegan:touches withEvent:event];
 
   // self.numberOfTouches doesn't work for this because in case than one finger is required,
   // when holding one finger on the screen and tapping with the second one, numberOfTouches is equal
@@ -35,8 +36,6 @@ - (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
     [self triggerAction];
     _hasBegan = YES;
   }
-
-  [_gestureHandler.pointerTracker touchesBegan:touches withEvent:event];
 }
 
 - (void)touchesMoved:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
@@ -73,7 +72,8 @@ - (void)reset
   [super reset];
 }
 
-- (CGPoint)getLastLocation {
+- (CGPoint)getLastLocation
+{
   // I think keeping the location of only one touch is enough since it would be used to determine the direction
   // of the movement, and if it's wrong the recognizer fails anyway.
   // In case the location of all touches is required, touch events are the way to go
@@ -103,48 +103,50 @@ - (void)resetConfig
 
 - (void)configure:(NSDictionary *)config
 {
-    [super configure:config];
-    UISwipeGestureRecognizer *recognizer = (UISwipeGestureRecognizer *)_recognizer;
-
-    id prop = config[@"direction"];
-    if (prop != nil) {
-        recognizer.direction = [RCTConvert NSInteger:prop];
-    }
-    
+  [super configure:config];
+  UISwipeGestureRecognizer *recognizer = (UISwipeGestureRecognizer *)_recognizer;
+
+  id prop = config[@"direction"];
+  if (prop != nil) {
+    recognizer.direction = [RCTConvert NSInteger:prop];
+  }
+
 #if !TARGET_OS_TV
-    prop = config[@"numberOfPointers"];
-    if (prop != nil) {
-        recognizer.numberOfTouchesRequired = [RCTConvert NSInteger:prop];
-    }
+  prop = config[@"numberOfPointers"];
+  if (prop != nil) {
+    recognizer.numberOfTouchesRequired = [RCTConvert NSInteger:prop];
+  }
 #endif
 }
 
 - (BOOL)gestureRecognizerShouldBegin:(UIGestureRecognizer *)gestureRecognizer
 {
-    RNGestureHandlerState savedState = _lastState;
-    BOOL shouldBegin = [super gestureRecognizerShouldBegin:gestureRecognizer];
-    _lastState = savedState;
-    
-    return shouldBegin;
+  RNGestureHandlerState savedState = _lastState;
+  BOOL shouldBegin = [super gestureRecognizerShouldBegin:gestureRecognizer];
+  _lastState = savedState;
+
+  return shouldBegin;
 }
 
 - (RNGestureHandlerEventExtraData *)eventExtraData:(id)_recognizer
 {
-    // For some weird reason [recognizer locationInView:recognizer.view.window] returns (0, 0).
-    // To calculate the correct absolute position, first calculate the absolute position of the
-    // view inside the root view controller (https://stackoverflow.com/a/7448573) and then
-    // add the relative touch position to it.
-
-    RNBetterSwipeGestureRecognizer *recognizer = (RNBetterSwipeGestureRecognizer *)_recognizer;
-
-    CGPoint viewAbsolutePosition = [recognizer.view convertPoint:recognizer.view.bounds.origin toView:[UIApplication sharedApplication].keyWindow.rootViewController.view];
-    CGPoint locationInView = [recognizer getLastLocation];
-
-    return [RNGestureHandlerEventExtraData
-            forPosition:locationInView
-            withAbsolutePosition:CGPointMake(viewAbsolutePosition.x + locationInView.x, viewAbsolutePosition.y + locationInView.y)
-            withNumberOfTouches:recognizer.numberOfTouches];
+  // For some weird reason [recognizer locationInView:recognizer.view.window] returns (0, 0).
+  // To calculate the correct absolute position, first calculate the absolute position of the
+  // view inside the root view controller (https://stackoverflow.com/a/7448573) and then
+  // add the relative touch position to it.
+
+  RNBetterSwipeGestureRecognizer *recognizer = (RNBetterSwipeGestureRecognizer *)_recognizer;
+
+  CGPoint viewAbsolutePosition =
+      [recognizer.view convertPoint:recognizer.view.bounds.origin
+                             toView:[UIApplication sharedApplication].keyWindow.rootViewController.view];
+  CGPoint locationInView = [recognizer getLastLocation];
+
+  return [RNGestureHandlerEventExtraData
+               forPosition:locationInView
+      withAbsolutePosition:CGPointMake(
+                               viewAbsolutePosition.x + locationInView.x, viewAbsolutePosition.y + locationInView.y)
+       withNumberOfTouches:recognizer.numberOfTouches];
 }
 
 @end
-