PointerDownEvent and PointerMoveEvent default `buttons` to 1 (#30579)

* Down and Move have default buttons = kPrimary.

packages/flutter/lib/src/gestures/events.dart

@@ -8,20 +8,30 @@ import 'package:flutter/foundation.dart';
 
 export 'dart:ui' show Offset, PointerDeviceKind;
 
-/// The bit of [PointerEvent.buttons] that corresponds to a unified behavior of
-/// "basic operation".
+/// The bit of [PointerEvent.buttons] that corresponds to the "primary
+/// action" on any device.
 ///
-/// It is equivalent to:
+/// More specifially,
 ///
-///  * [kTouchContact]: The pointer contacts the touch screen.
-///  * [kStylusContact]: The stylus contacts the screen.
-///  * [kPrimaryMouseButton]: The primary mouse button.
+///  * For touch screen, it's when the pointer contacts the screen.
+///  * For stylus and inverted stylus, it's when the pen contacts the screen.
+///  * For mouse, it's when the primary button is pressed.
+///
+/// See also:
+///
+///  * [kTouchContact]: an alias of this constant when used by touch screen.
+///  * [kStylusContact]: an alias of this constant when used by stylus.
+///  * [kPrimaryMouseButton]: an alias of this constant when used by mouse.
 const int kPrimaryButton = 0x01;
 
 /// The bit of [PointerEvent.buttons] that corresponds to the primary mouse button.
 ///
 /// The primary mouse button is typically the left button on the top of the
 /// mouse but can be reconfigured to be a different physical button.
+///
+/// See also:
+///
+///  * [kTouchContact]: an alias of this constant when used by touch screen.
 const int kPrimaryMouseButton = kPrimaryButton;
 
 /// The bit of [PointerEvent.buttons] that corresponds to the secondary mouse button.
@@ -32,6 +42,10 @@ const int kSecondaryMouseButton = 0x02;
 
 /// The bit of [PointerEvent.buttons] that corresponds to when a stylus
 /// contacting the screen.
+///
+/// See also:
+///
+///  * [kPrimaryButton]: an alias of this constant for any device.
 const int kStylusContact = kPrimaryButton;
 
 /// The bit of [PointerEvent.buttons] that corresponds to the primary stylus button.
@@ -67,6 +81,10 @@ const int kForwardMouseButton = 0x10;
 
 /// The bit of [PointerEvent.buttons] that corresponds to the pointer contacting
 /// a touch screen.
+///
+/// See also:
+///
+///  * [kPrimaryButton]: an alias of this constant for any device.
 const int kTouchContact = kPrimaryButton;
 
 /// The bit of [PointerEvent.buttons] that corresponds to the nth mouse button.
@@ -672,7 +690,7 @@ class PointerDownEvent extends PointerEvent {
     PointerDeviceKind kind = PointerDeviceKind.touch,
     int device = 0,
     Offset position = Offset.zero,
-    int buttons = 0,
+    int buttons = kPrimaryButton,
     bool obscured = false,
     double pressure = 1.0,
     double pressureMin = 1.0,
@@ -727,7 +745,7 @@ class PointerMoveEvent extends PointerEvent {
     int device = 0,
     Offset position = Offset.zero,
     Offset delta = Offset.zero,
-    int buttons = 0,
+    int buttons = kPrimaryButton,
     bool obscured = false,
     double pressure = 1.0,
     double pressureMin = 1.0,

packages/flutter/test/gestures/events_test.dart

@@ -19,4 +19,18 @@ void main() {
     expect(nthMouseButton(2), kSecondaryMouseButton);
     expect(nthStylusButton(2), kSecondaryStylusButton);
   });
+
+  group('Default values of PointerEvents:', () {
+    // Some parameters are intentionally set to a non-trivial value.
+
+    test('PointerDownEvent', () {
+      const PointerDownEvent event = PointerDownEvent();
+      expect(event.buttons, kPrimaryButton);
+    });
+
+    test('PointerMoveEvent', () {
+      const PointerMoveEvent event = PointerMoveEvent();
+      expect(event.buttons, kPrimaryButton);
+    });
+  });
 }