i82973 scroll mouse wheel support (flutter#44724)

Fixes flutter/flutter#82973

Mouse scroll wheel support for android. 

I chose to not cache the vertical and horizontal scale factors that come from view configuration. The primary reason is that in the current code path context is only used when the user scrolls which was the unimplemented feature. This smaller blast radius I decided was worth the additional calls. A secondary reason is that when the scale factors are changed is not a well documented path nor is there a lifecycle callback to listen to. Scroll factor is cached on api 25 and below because that more closely mirrors the behavior I see in pre 25 versions of android scroll view. 

Note flutter takes longer to "see" a mouse that then scrolls than android

Fixes #flutter/flutter/82973

Todo list prior to merge

## Links 
* Some pre api 26 scroll factor code 
    - https://cs.android.com/android/platform/superproject/main/+/main:frameworks/base/core/java/android/view/View.java?q=function:getVerticalScrollFactor%20filepath:android%2Fview%2FView.java&ss=android%2Fplatform%2Fsuperproject%2Fmain 
    - https://cs.android.com/android/_/android/platform/frameworks/base/+/main:core/java/android/widget/ScrollView.java;l=930;drc=2fe301db7555bccf53e465436d4cb7442c803df3;bpv=0;bpt=0
* Post api 26 scroll factor code 
    - https://cs.android.com/android/_/android/platform/frameworks/base/+/main:core/java/android/widget/ScrollView.java;l=361;drc=406e0f655387f27dda874c2b975fb0ddbd61aa13;bpv=0;bpt=0 
    - https://developer.android.com/reference/android/view/ViewConfiguration#getScaledVerticalScrollFactor()

shell/platform/android/io/flutter/embedding/android/AndroidTouchProcessor.java

@@ -1,9 +1,13 @@
 package io.flutter.embedding.android;
 
+import android.annotation.TargetApi;
+import android.content.Context;
 import android.graphics.Matrix;
 import android.os.Build;
+import android.util.TypedValue;
 import android.view.InputDevice;
 import android.view.MotionEvent;
+import android.view.ViewConfiguration;
 import androidx.annotation.IntDef;
 import androidx.annotation.NonNull;
 import androidx.annotation.VisibleForTesting;
@@ -80,6 +84,10 @@ public class AndroidTouchProcessor {
   private static final int POINTER_DATA_FIELD_COUNT = 35;
   @VisibleForTesting static final int BYTES_PER_FIELD = 8;
 
+  // Default if context is null, chosen to ensure reasonable speed scrolling.
+  @VisibleForTesting static final int DEFAULT_VERTICAL_SCROLL_FACTOR = 48;
+  @VisibleForTesting static final int DEFAULT_HORIZONTAL_SCROLL_FACTOR = 48;
+
   // This value must match the value in framework's platform_view.dart.
   // This flag indicates whether the original Android pointer events were batched together.
   private static final int POINTER_DATA_FLAG_BATCHED = 1;
@@ -93,6 +101,9 @@ public class AndroidTouchProcessor {
 
   private final Map<Integer, float[]> ongoingPans = new HashMap<>();
 
+  // Only used on api 25 and below to avoid requerying display metrics.
+  private int cachedVerticalScrollFactor;
+
   /**
    * Constructs an {@code AndroidTouchProcessor} that will send touch event data to the Flutter
    * execution context represented by the given {@link FlutterRenderer}.
@@ -181,9 +192,10 @@ public boolean onTouchEvent(@NonNull MotionEvent event, @NonNull Matrix transfor
    * wheel movements, etc.
    *
    * @param event The generic motion event being processed.
+   * @param context For use by ViewConfiguration.get(context) to scale input.
    * @return True if the event was handled.
    */
-  public boolean onGenericMotionEvent(@NonNull MotionEvent event) {
+  public boolean onGenericMotionEvent(@NonNull MotionEvent event, @NonNull Context context) {
     // Method isFromSource is only available in API 18+ (Jelly Bean MR2)
     // Mouse hover support is not implemented for API < 18.
     boolean isPointerEvent =
@@ -192,7 +204,9 @@ public boolean onGenericMotionEvent(@NonNull MotionEvent event) {
     boolean isMovementEvent =
         (event.getActionMasked() == MotionEvent.ACTION_HOVER_MOVE
             || event.getActionMasked() == MotionEvent.ACTION_SCROLL);
-    if (!isPointerEvent || !isMovementEvent) {
+    if (isPointerEvent && isMovementEvent) {
+      // Continue.
+    } else {
       return false;
     }
 
@@ -203,26 +217,43 @@ public boolean onGenericMotionEvent(@NonNull MotionEvent event) {
     packet.order(ByteOrder.LITTLE_ENDIAN);
 
     // ACTION_HOVER_MOVE always applies to a single pointer only.
-    addPointerForIndex(event, event.getActionIndex(), pointerChange, 0, IDENTITY_TRANSFORM, packet);
+    addPointerForIndex(
+        event, event.getActionIndex(), pointerChange, 0, IDENTITY_TRANSFORM, packet, context);
     if (packet.position() % (POINTER_DATA_FIELD_COUNT * BYTES_PER_FIELD) != 0) {
       throw new AssertionError("Packet position is not on field boundary.");
     }
     renderer.dispatchPointerDataPacket(packet, packet.position());
     return true;
   }
 
-  // TODO(mattcarroll): consider creating a PointerPacket class instead of using a procedure that
-  // mutates inputs.
+  /// Calls addPointerForIndex with null for context.
+  ///
+  /// Without context the scroll wheel will not mimick android's scroll speed.
   private void addPointerForIndex(
       MotionEvent event,
       int pointerIndex,
       int pointerChange,
       int pointerData,
       Matrix transformMatrix,
       ByteBuffer packet) {
+    addPointerForIndex(
+        event, pointerIndex, pointerChange, pointerData, transformMatrix, packet, null);
+  }
+
+  // TODO: consider creating a PointerPacket class instead of using a procedure that
+  // mutates inputs. https://github.com/flutter/flutter/issues/132853
+  private void addPointerForIndex(
+      MotionEvent event,
+      int pointerIndex,
+      int pointerChange,
+      int pointerData,
+      Matrix transformMatrix,
+      ByteBuffer packet,
+      Context context) {
     if (pointerChange == -1) {
       return;
     }
+    final int pointerId = event.getPointerId(pointerIndex);
 
     int pointerKind = getPointerDeviceTypeForToolType(event.getToolType(pointerIndex));
     // We use this in lieu of using event.getRawX and event.getRawY as we wish to support
@@ -238,16 +269,21 @@ private void addPointerForIndex(
         // Some implementations translate trackpad scrolling into a mouse down-move-up event
         // sequence with buttons: 0, such as ARC on a Chromebook. See #11420, a legacy
         // implementation that uses the same condition but converts differently.
-        ongoingPans.put(event.getPointerId(pointerIndex), viewToScreenCoords);
+        ongoingPans.put(pointerId, viewToScreenCoords);
       }
     } else if (pointerKind == PointerDeviceKind.STYLUS) {
+      // Returns converted android button state into flutter framework normalized state
+      // and updates ongoingPans for chromebook trackpad scrolling.
+      // See
+      // https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/gestures/events.dart
+      // for target button constants.
       buttons = (event.getButtonState() >> 4) & 0xF;
     } else {
       buttons = 0;
     }
 
     int panZoomType = -1;
-    boolean isTrackpadPan = ongoingPans.containsKey(event.getPointerId(pointerIndex));
+    boolean isTrackpadPan = ongoingPans.containsKey(pointerId);
     if (isTrackpadPan) {
       panZoomType = getPointerChangeForPanZoom(pointerChange);
       if (panZoomType == -1) {
@@ -278,13 +314,13 @@ private void addPointerForIndex(
       packet.putLong(pointerKind); // kind
     }
     packet.putLong(signalKind); // signal_kind
-    packet.putLong(event.getPointerId(pointerIndex)); // device
+    packet.putLong(pointerId); // device
     packet.putLong(0); // pointer_identifier, will be generated in pointer_data_packet_converter.cc.
 
     if (isTrackpadPan) {
-      float[] panStart = ongoingPans.get(event.getPointerId(pointerIndex));
-      packet.putDouble(panStart[0]);
-      packet.putDouble(panStart[1]);
+      float[] panStart = ongoingPans.get(pointerId);
+      packet.putDouble(panStart[0]); // physical_x
+      packet.putDouble(panStart[1]); // physical_y
     } else {
       packet.putDouble(viewToScreenCoords[0]); // physical_x
       packet.putDouble(viewToScreenCoords[1]); // physical_y
@@ -341,16 +377,30 @@ private void addPointerForIndex(
 
     packet.putLong(pointerData); // platformData
 
+    // See android scrollview for inspiration.
+    // https://cs.android.com/android/platform/superproject/main/+/main:frameworks/base/core/java/android/widget/ScrollView.java?q=function:onGenericMotionEvent%20filepath:widget%2FScrollView.java&ss=android%2Fplatform%2Fsuperproject%2Fmain
     if (signalKind == PointerSignalKind.SCROLL) {
-      packet.putDouble(-event.getAxisValue(MotionEvent.AXIS_HSCROLL)); // scroll_delta_x
-      packet.putDouble(-event.getAxisValue(MotionEvent.AXIS_VSCROLL)); // scroll_delta_y
+      double horizontalScaleFactor = DEFAULT_HORIZONTAL_SCROLL_FACTOR;
+      double verticalScaleFactor = DEFAULT_VERTICAL_SCROLL_FACTOR;
+      if (context != null) {
+        horizontalScaleFactor = getHorizontalScrollFactor(context);
+        verticalScaleFactor = getVerticalScrollFactor(context);
+      }
+      // We flip the sign of the scroll value below because it aligns the pixel value with the
+      // scroll direction in native android.
+      final double horizontalScrollPixels =
+          horizontalScaleFactor * -event.getAxisValue(MotionEvent.AXIS_HSCROLL, pointerIndex);
+      final double verticalScrollPixels =
+          verticalScaleFactor * -event.getAxisValue(MotionEvent.AXIS_VSCROLL, pointerIndex);
+      packet.putDouble(horizontalScrollPixels); // scroll_delta_x
+      packet.putDouble(verticalScrollPixels); // scroll_delta_y
     } else {
       packet.putDouble(0.0); // scroll_delta_x
-      packet.putDouble(0.0); // scroll_delta_x
+      packet.putDouble(0.0); // scroll_delta_y
     }
 
     if (isTrackpadPan) {
-      float[] panStart = ongoingPans.get(event.getPointerId(pointerIndex));
+      float[] panStart = ongoingPans.get(pointerId);
       packet.putDouble(viewToScreenCoords[0] - panStart[0]);
       packet.putDouble(viewToScreenCoords[1] - panStart[1]);
     } else {
@@ -363,8 +413,46 @@ private void addPointerForIndex(
     packet.putDouble(0.0); // rotation
 
     if (isTrackpadPan && (panZoomType == PointerChange.PAN_ZOOM_END)) {
-      ongoingPans.remove(event.getPointerId(pointerIndex));
+      ongoingPans.remove(pointerId);
+    }
+  }
+
+  private float getHorizontalScrollFactor(@NonNull Context context) {
+    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
+      return ViewConfiguration.get(context).getScaledHorizontalScrollFactor();
+    } else {
+      // Vertical scroll factor is not a typo. This is what View.java does in android.
+      return getVerticalScrollFactorPre26(context);
+    }
+  }
+
+  private float getVerticalScrollFactor(@NonNull Context context) {
+    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
+      return getVerticalScrollFactorAbove26(context);
+    } else {
+      return getVerticalScrollFactorPre26(context);
+    }
+  }
+
+  @TargetApi(26)
+  private float getVerticalScrollFactorAbove26(@NonNull Context context) {
+    return ViewConfiguration.get(context).getScaledVerticalScrollFactor();
+  }
+
+  // See
+  // https://cs.android.com/android/platform/superproject/main/+/main:frameworks/base/core/java/android/view/View.java?q=function:getVerticalScrollFactor%20filepath:android%2Fview%2FView.java&ss=android%2Fplatform%2Fsuperproject%2Fmain
+  private int getVerticalScrollFactorPre26(@NonNull Context context) {
+    if (cachedVerticalScrollFactor == 0) {
+      TypedValue outValue = new TypedValue();
+      if (!context
+          .getTheme()
+          .resolveAttribute(android.R.attr.listPreferredItemHeight, outValue, true)) {
+        return DEFAULT_VERTICAL_SCROLL_FACTOR;
+      }
+      cachedVerticalScrollFactor =
+          (int) outValue.getDimension(context.getResources().getDisplayMetrics());
     }
+    return cachedVerticalScrollFactor;
   }
 
   @PointerChange

shell/platform/android/io/flutter/embedding/android/FlutterView.java

@@ -952,7 +952,8 @@ public boolean onTouchEvent(@NonNull MotionEvent event) {
   @Override
   public boolean onGenericMotionEvent(@NonNull MotionEvent event) {
     boolean handled =
-        isAttachedToFlutterEngine() && androidTouchProcessor.onGenericMotionEvent(event);
+        isAttachedToFlutterEngine()
+            && androidTouchProcessor.onGenericMotionEvent(event, getContext());
     return handled ? true : super.onGenericMotionEvent(event);
   }
 

shell/platform/android/io/flutter/view/FlutterView.java

@@ -482,7 +482,8 @@ public boolean onHoverEvent(MotionEvent event) {
    */
   @Override
   public boolean onGenericMotionEvent(MotionEvent event) {
-    boolean handled = isAttached() && androidTouchProcessor.onGenericMotionEvent(event);
+    boolean handled =
+        isAttached() && androidTouchProcessor.onGenericMotionEvent(event, getContext());
     return handled ? true : super.onGenericMotionEvent(event);
   }