[Android] Initialize the Handler fields eagerly (#4461)

## Description

`Tap`, `LongPress`, `Fling`, `Pan` and `Hover` gesture handlers kept their `Handler` as a nullable field created on first use, guarded by null checks and `!!` at every call site (with a `TODO: lazy init` left in `Tap` since the Kotlin conversion).

`Handler` object was never cleared anyway (except `LongPress`, where it used to serve as a flag, and `Hover`).

Also adds a comment explaining the 4 ms delay in `HoverGestureHandler`.

## Test plan

- `:react-native-gesture-handler:compileDebugKotlin` builds clean in basic-example
- `yarn format:android` passes

<details>
<summary>Tested on the following code:</summary>

```tsx
import React, { useCallback, useState } from 'react';
import { StyleSheet, Text, View } from 'react-native';
import {
  Directions,
  GestureDetector,
  useFlingGesture,
  useHoverGesture,
  useLongPressGesture,
  usePanGesture,
  useTapGesture,
} from 'react-native-gesture-handler';
import { scheduleOnRN } from 'react-native-worklets';

type Counters = { begin: number; act: number; end: number; fail: number };
type Results = Record<string, Counters>;

const EMPTY: Counters = { begin: 0, act: 0, end: 0, fail: 0 };

export default function EmptyExample() {
  const [results, setResults] = useState<Results>({});

  const report = useCallback((key: string, kind: keyof Counters) => {
    setResults((prev) => {
      const current = prev[key] ?? EMPTY;
      return { ...prev, [key]: { ...current, [kind]: current[kind] + 1 } };
    });
  }, []);

  const tap = useTapGesture({
    onBegin: () => scheduleOnRN(report, 'tap', 'begin'),
    onActivate: () => scheduleOnRN(report, 'tap', 'act'),
    onFinalize: (e) =>
      scheduleOnRN(report, 'tap', e.canceled ? 'fail' : 'end'),
  });

  const doubleTap = useTapGesture({
    numberOfTaps: 2,
    onBegin: () => scheduleOnRN(report, 'doubleTap', 'begin'),
    onActivate: () => scheduleOnRN(report, 'doubleTap', 'act'),
    onFinalize: (e) =>
      scheduleOnRN(report, 'doubleTap', e.canceled ? 'fail' : 'end'),
  });

  const longPress = useLongPressGesture({
    minDurationMs: 400,
    onBegin: () => scheduleOnRN(report, 'longPress', 'begin'),
    onActivate: () => scheduleOnRN(report, 'longPress', 'act'),
    onFinalize: (e) =>
      scheduleOnRN(report, 'longPress', e.canceled ? 'fail' : 'end'),
  });

  const fling = useFlingGesture({
    direction: Directions.RIGHT,
    onBegin: () => scheduleOnRN(report, 'fling', 'begin'),
    onActivate: () => scheduleOnRN(report, 'fling', 'act'),
    onFinalize: (e) =>
      scheduleOnRN(report, 'fling', e.canceled ? 'fail' : 'end'),
  });

  const pan = usePanGesture({
    activateAfterLongPress: 400,
    onBegin: () => scheduleOnRN(report, 'pan', 'begin'),
    onActivate: () => scheduleOnRN(report, 'pan', 'act'),
    onFinalize: (e) =>
      scheduleOnRN(report, 'pan', e.canceled ? 'fail' : 'end'),
  });

  const hover = useHoverGesture({
    onBegin: () => scheduleOnRN(report, 'hover', 'begin'),
    onActivate: () => scheduleOnRN(report, 'hover', 'act'),
    onFinalize: (e) =>
      scheduleOnRN(report, 'hover', e.canceled ? 'fail' : 'end'),
  });

  const rows: { key: string; label: string; gesture: any }[] = [
    { key: 'tap', label: 'Tap', gesture: tap },
    { key: 'doubleTap', label: 'DoubleTap', gesture: doubleTap },
    { key: 'longPress', label: 'LongPress 400ms', gesture: longPress },
    { key: 'fling', label: 'Fling right', gesture: fling },
    { key: 'pan', label: 'Pan holdActivate 400ms', gesture: pan },
    { key: 'hover', label: 'Hover', gesture: hover },
  ];

  return (
    <View style={styles.container}>
      {rows.map(({ key, label, gesture }) => {
        const c = results[key] ?? EMPTY;
        return (
          <GestureDetector key={key} gesture={gesture}>
            <View style={styles.box}>
              <Text style={styles.label}>{label}</Text>
              <Text style={styles.status}>
                {`${key} b:${c.begin} a:${c.act} e:${c.end} f:${c.fail}`}
              </Text>
            </View>
          </GestureDetector>
        );
      })}
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    padding: 12,
    gap: 10,
  },
  box: {
    height: 88,
    borderRadius: 12,
    backgroundColor: '#dbe4ff',
    justifyContent: 'center',
    alignItems: 'center',
  },
  label: {
    fontSize: 18,
    fontWeight: '600',
  },
  status: {
    fontSize: 15,
    fontVariant: ['tabular-nums'],
  },
});
```


</details>

Author: m-bert

packages/react-native-gesture-handler/android/src/main/java/com/swmansion/gesturehandler/core/FlingGestureHandler.kt

@@ -14,7 +14,7 @@ class FlingGestureHandler : GestureHandler() {
 
   private val maxDurationMs = DEFAULT_MAX_DURATION_MS
   private val minVelocity = DEFAULT_MIN_VELOCITY
-  private var handler: Handler? = null
+  private val handler = Handler(Looper.getMainLooper())
   private var maxNumberOfPointersSimultaneously = 0
   private val failDelayed = Runnable { fail() }
   private var velocityTracker: VelocityTracker? = null
@@ -29,12 +29,8 @@ class FlingGestureHandler : GestureHandler() {
     velocityTracker = VelocityTracker.obtain()
     begin()
     maxNumberOfPointersSimultaneously = 1
-    if (handler == null) {
-      handler = Handler(Looper.getMainLooper()) // lazy delegate?
-    } else {
-      handler!!.removeCallbacksAndMessages(null)
-    }
-    handler!!.postDelayed(failDelayed, maxDurationMs)
+    handler.removeCallbacksAndMessages(null)
+    handler.postDelayed(failDelayed, maxDurationMs)
   }
 
   private fun tryEndFling(event: MotionEvent): Boolean {
@@ -69,7 +65,7 @@ class FlingGestureHandler : GestureHandler() {
       isAligned &&
       isFast
     ) {
-      handler!!.removeCallbacksAndMessages(null)
+      handler.removeCallbacksAndMessages(null)
       activate()
       true
     } else {
@@ -109,13 +105,13 @@ class FlingGestureHandler : GestureHandler() {
   }
 
   override fun onCancel() {
-    handler?.removeCallbacksAndMessages(null)
+    handler.removeCallbacksAndMessages(null)
   }
 
   override fun onReset() {
     velocityTracker?.recycle()
     velocityTracker = null
-    handler?.removeCallbacksAndMessages(null)
+    handler.removeCallbacksAndMessages(null)
   }
 
   private fun addVelocityMovement(tracker: VelocityTracker?, event: MotionEvent) {

packages/react-native-gesture-handler/android/src/main/java/com/swmansion/gesturehandler/core/HoverGestureHandler.kt

@@ -12,7 +12,7 @@ import com.swmansion.gesturehandler.react.events.eventbuilders.HoverGestureHandl
 class HoverGestureHandler : GestureHandler() {
   override val isContinuous = true
 
-  private var handler: Handler? = null
+  private val handler = Handler(Looper.getMainLooper())
   private var finishRunnable = Runnable { finish() }
   var stylusData: StylusData = StylusData()
     private set
@@ -85,8 +85,7 @@ class HoverGestureHandler : GestureHandler() {
 
   override fun onHandle(event: MotionEvent, sourceEvent: MotionEvent) {
     if (event.action == MotionEvent.ACTION_DOWN) {
-      handler?.removeCallbacksAndMessages(null)
-      handler = null
+      handler.removeCallbacksAndMessages(null)
     } else if (event.action == MotionEvent.ACTION_UP) {
       if (!isWithinBounds) {
         finish()
@@ -97,11 +96,9 @@ class HoverGestureHandler : GestureHandler() {
   override fun onHandleHover(event: MotionEvent, sourceEvent: MotionEvent) {
     when {
       event.action == MotionEvent.ACTION_HOVER_EXIT -> {
-        if (handler == null) {
-          handler = Handler(Looper.getMainLooper())
-        }
-
-        handler!!.postDelayed(finishRunnable, 4)
+        // Touching down synthesizes HOVER_EXIT right before ACTION_DOWN, so finish
+        // with a slight delay - the DOWN cancels it and hover survives the press.
+        handler.postDelayed(finishRunnable, 4)
       }
 
       !isWithinBounds -> {

packages/react-native-gesture-handler/android/src/main/java/com/swmansion/gesturehandler/core/LongPressGestureHandler.kt

@@ -20,7 +20,7 @@ class LongPressGestureHandler(context: Context) : GestureHandler() {
   private var startY = 0f
   private var startTime: Long = 0
   private var previousTime: Long = 0
-  private var handler: Handler? = null
+  private val handler = Handler(Looper.getMainLooper())
   private var currentPointers = 0
 
   init {
@@ -112,9 +112,9 @@ class LongPressGestureHandler(context: Context) : GestureHandler() {
           sourceEvent.actionMasked == MotionEvent.ACTION_BUTTON_PRESS
         )
     ) {
-      handler = Handler(Looper.getMainLooper())
+      handler.removeCallbacksAndMessages(null)
       if (minDurationMs > 0) {
-        handler!!.postDelayed({ activate() }, minDurationMs)
+        handler.postDelayed({ activate() }, minDurationMs)
       } else if (minDurationMs == 0L) {
         activate()
       }
@@ -124,10 +124,7 @@ class LongPressGestureHandler(context: Context) : GestureHandler() {
     ) {
       currentPointers--
 
-      handler?.let {
-        it.removeCallbacksAndMessages(null)
-        handler = null
-      }
+      handler.removeCallbacksAndMessages(null)
 
       if (state == STATE_ACTIVE) {
         end()
@@ -164,10 +161,7 @@ class LongPressGestureHandler(context: Context) : GestureHandler() {
   }
 
   override fun onStateChange(newState: Int, previousState: Int) {
-    handler?.let {
-      it.removeCallbacksAndMessages(null)
-      handler = null
-    }
+    handler.removeCallbacksAndMessages(null)
   }
 
   override fun dispatchStateChange(newState: Int, prevState: Int) {

packages/react-native-gesture-handler/android/src/main/java/com/swmansion/gesturehandler/core/PanGestureHandler.kt

@@ -60,7 +60,7 @@ class PanGestureHandler(context: Context?) : GestureHandler() {
   private var averageTouches = false
   private var activateAfterLongPress = DEFAULT_ACTIVATE_AFTER_LONG_PRESS
   private val activateDelayed = Runnable { activate() }
-  private var handler: Handler? = null
+  private val handler = Handler(Looper.getMainLooper())
   var stylusData: StylusData = StylusData()
     private set
 
@@ -140,7 +140,7 @@ class PanGestureHandler(context: Context?) : GestureHandler() {
     val dy = lastY - startY + offsetY
 
     if (activateAfterLongPress > 0 && dx * dx + dy * dy > defaultMinDist * defaultMinDist) {
-      handler?.removeCallbacksAndMessages(null)
+      handler.removeCallbacksAndMessages(null)
       return true
     }
     if (failOffsetXStart != MAX_VALUE_IGNORE && dx < failOffsetXStart) {
@@ -200,10 +200,7 @@ class PanGestureHandler(context: Context?) : GestureHandler() {
       begin()
 
       if (activateAfterLongPress > 0) {
-        if (handler == null) {
-          handler = Handler(Looper.getMainLooper())
-        }
-        handler!!.postDelayed(activateDelayed, activateAfterLongPress)
+        handler.postDelayed(activateDelayed, activateAfterLongPress)
       }
     } else if (velocityTracker != null) {
       addVelocityMovement(velocityTracker, sourceEvent)
@@ -252,11 +249,11 @@ class PanGestureHandler(context: Context?) : GestureHandler() {
   }
 
   override fun onCancel() {
-    handler?.removeCallbacksAndMessages(null)
+    handler.removeCallbacksAndMessages(null)
   }
 
   override fun onReset() {
-    handler?.removeCallbacksAndMessages(null)
+    handler.removeCallbacksAndMessages(null)
     velocityTracker?.let {
       it.recycle()
       velocityTracker = null

packages/react-native-gesture-handler/android/src/main/java/com/swmansion/gesturehandler/core/TapGestureHandler.kt

@@ -26,7 +26,7 @@ class TapGestureHandler : GestureHandler() {
   private var offsetY = 0f
   private var lastX = 0f
   private var lastY = 0f
-  private var handler: Handler? = null
+  private val handler = Handler(Looper.getMainLooper())
   private var tapsSoFar = 0
   private val failDelayed = Runnable { fail() }
 
@@ -47,24 +47,16 @@ class TapGestureHandler : GestureHandler() {
   }
 
   private fun startTap() {
-    if (handler == null) {
-      handler = Handler(Looper.getMainLooper()) // TODO: lazy init (handle else branch correctly)
-    } else {
-      handler!!.removeCallbacksAndMessages(null)
-    }
-    handler!!.postDelayed(failDelayed, maxDurationMs)
+    handler.removeCallbacksAndMessages(null)
+    handler.postDelayed(failDelayed, maxDurationMs)
   }
 
   private fun endTap() {
-    if (handler == null) {
-      handler = Handler(Looper.getMainLooper())
-    } else {
-      handler!!.removeCallbacksAndMessages(null)
-    }
+    handler.removeCallbacksAndMessages(null)
     if (++tapsSoFar == numberOfTaps && currentMaxNumberOfPointers >= minNumberOfPointers) {
       activate()
     } else {
-      handler!!.postDelayed(failDelayed, maxDelayMs)
+      handler.postDelayed(failDelayed, maxDelayMs)
     }
   }
 
@@ -139,13 +131,13 @@ class TapGestureHandler : GestureHandler() {
   }
 
   override fun onCancel() {
-    handler?.removeCallbacksAndMessages(null)
+    handler.removeCallbacksAndMessages(null)
   }
 
   override fun onReset() {
     tapsSoFar = 0
     currentMaxNumberOfPointers = 0
-    handler?.removeCallbacksAndMessages(null)
+    handler.removeCallbacksAndMessages(null)
   }
 
   class Factory : GestureHandler.Factory<TapGestureHandler>() {