internal/gamepad: ignore the very first MotionEvent with 0 value for Android

On Android, MotionEvent with 0 values might come for axes when connecting
a gamepad, even though a user didn't touch any axes. This is problematic
especially for tirgger axes, where the default value should be -1.

This change fixes the issue by adding a new state `axesReady` to check
if an axis is really touched or not. If an axis is not touched yet,
a button value for a standard (trigger) button always returns 0.

This change also removes an old hack to initialize axis values for
triggers.

Closes #2598

cmd/ebitenmobile/_files/EbitenView.java

@@ -244,12 +244,6 @@ public class EbitenView extends ViewGroup implements InputManager.InputDeviceLis
         int axisMask = getAxisMask(inputDevice);
 
         Ebitenmobileview.onGamepadAdded(deviceId, inputDevice.getName(), gamepad.axes.size(), gamepad.hats.size()/2, descriptor, vendorId, productId, buttonMask, axisMask);
-
-        // Initialize the trigger axes values explicitly, or the initial button values would be 0.5 instead of 0.
-        if (gamepad.axes.size() >= 6) {
-            Ebitenmobileview.onGamepadAxisChanged(deviceId, 4, -1);
-            Ebitenmobileview.onGamepadAxisChanged(deviceId, 5, -1);
-        }
     }
 
     // The implementation is copied from SDL:

internal/gamepad/extern_android.go

@@ -45,6 +45,7 @@ func (g *gamepads) addAndroidGamepad(androidDeviceID int, name, sdlID string, ax
 	gp := g.add(name, sdlID)
 	gp.native = &nativeGamepadImpl{
 		androidDeviceID: androidDeviceID,
+		axesReady:       make([]bool, axisCount),
 		axes:            make([]float64, axisCount),
 		buttons:         make([]bool, gamepaddb.SDLControllerButtonMax+1),
 		hats:            make([]int, hatCount),
@@ -108,6 +109,13 @@ func (g *Gamepad) updateAndroidGamepadAxis(axis int, value float64) {
 		return
 	}
 	n.axes[axis] = value
+
+	// MotionEvent with 0 value can be sent when a gamepad is connected even though an axis is not touched (#2598).
+	// This is problematic when an axis is a trigger button where -1 should be the default value.
+	// When MotionEvent with non-0 value is sent, it seems fine to assume that the axis is actually touched and ready.
+	if value != 0 {
+		n.axesReady[axis] = true
+	}
 }
 
 func (g *Gamepad) updateAndroidGamepadButton(button Button, pressed bool) {

internal/gamepad/gamepad.go

@@ -243,6 +243,7 @@ type nativeGamepad interface {
 	axisCount() int
 	buttonCount() int
 	hatCount() int
+	isAxisReady(axis int) bool
 	axisValue(axis int) float64
 	buttonValue(button int) float64
 	isButtonPressed(button int) bool
@@ -296,6 +297,14 @@ func (g *Gamepad) HatCount() int {
 	return g.native.hatCount()
 }
 
+// IsAxisReady is concurrent-safe.
+func (g *Gamepad) IsAxisReady(axis int) bool {
+	g.m.Lock()
+	defer g.m.Unlock()
+
+	return g.native.isAxisReady(axis)
+}
+
 // Axis is concurrent-safe.
 func (g *Gamepad) Axis(axis int) float64 {
 	g.m.Lock()

internal/gamepad/gamepad_android.go

@@ -37,6 +37,7 @@ func (*nativeGamepadsImpl) update(gamepads *gamepads) error {
 type nativeGamepadImpl struct {
 	androidDeviceID int
 
+	axesReady []bool
 	axes      []float64
 	buttons   []bool
 	hats      []int
@@ -71,6 +72,13 @@ func (g *nativeGamepadImpl) hatCount() int {
 	return len(g.hats)
 }
 
+func (g *nativeGamepadImpl) isAxisReady(axis int) bool {
+	if axis < 0 || axis >= len(g.axesReady) {
+		return false
+	}
+	return g.axesReady[axis]
+}
+
 func (g *nativeGamepadImpl) axisValue(axis int) float64 {
 	if axis < 0 || axis >= len(g.axes) {
 		return 0

internal/gamepad/gamepad_darwin.go

@@ -399,6 +399,10 @@ func (g *nativeGamepadImpl) hatCount() int {
 	return len(g.hatValues)
 }
 
+func (g *nativeGamepadImpl) isAxisReady(axis int) bool {
+	return axis >= 0 && axis < g.axisCount()
+}
+
 func (g *nativeGamepadImpl) axisValue(axis int) float64 {
 	if axis < 0 || axis >= len(g.axisValues) {
 		return 0

internal/gamepad/gamepad_desktop_windows.go

@@ -719,6 +719,10 @@ func (g *nativeGamepadDesktop) hatCount() int {
 	return 1
 }
 
+func (g *nativeGamepadDesktop) isAxisReady(axis int) bool {
+	return axis >= 0 && axis < g.axisCount()
+}
+
 func (g *nativeGamepadDesktop) axisValue(axis int) float64 {
 	if g.usesDInput() {
 		if axis < 0 || axis >= len(g.dinputAxes) {

internal/gamepad/gamepad_ios.go

@@ -76,6 +76,10 @@ func (g *nativeGamepadImpl) hatCount() int {
 	return len(g.hats)
 }
 
+func (g *nativeGamepadImpl) isAxisReady(axis int) bool {
+	return axis >= 0 && axis < g.axisCount()
+}
+
 func (g *nativeGamepadImpl) axisValue(axis int) float64 {
 	if axis < 0 || axis >= len(g.axes) {
 		return 0

internal/gamepad/gamepad_js.go

@@ -152,6 +152,10 @@ func (g *nativeGamepadImpl) hatCount() int {
 	return 0
 }
 
+func (g *nativeGamepadImpl) isAxisReady(axis int) bool {
+	return axis >= 0 && axis < g.axisCount()
+}
+
 func (g *nativeGamepadImpl) axisValue(axis int) float64 {
 	axes := g.value.Get("axes")
 	if axis < 0 || axis >= axes.Length() {

internal/gamepad/gamepad_linux.go

@@ -592,6 +592,10 @@ func (g *nativeGamepadImpl) hatCount() int {
 	return g.hatCount_
 }
 
+func (g *nativeGamepadImpl) isAxisReady(axis int) bool {
+	return axis >= 0 && axis < g.axisCount()
+}
+
 func (g *nativeGamepadImpl) axisValue(axis int) float64 {
 	if axis < 0 || axis >= g.axisCount_ {
 		return 0

internal/gamepad/gamepad_nintendosdk.go

@@ -146,6 +146,10 @@ func (g *nativeGamepadImpl) hatCount() int {
 	return 0
 }
 
+func (g *nativeGamepadImpl) isAxisReady(axis int) bool {
+	return axis >= 0 && axis < g.axisCount()
+}
+
 func (g *nativeGamepadImpl) axisValue(axis int) float64 {
 	if axis < 0 || axis >= len(g.axisValues) {
 		return 0

internal/gamepad/gamepad_null.go

@@ -66,6 +66,10 @@ func (*nativeGamepadImpl) hatCount() int {
 	return 0
 }
 
+func (g *nativeGamepadImpl) isAxisReady(axis int) bool {
+	return false
+}
+
 func (*nativeGamepadImpl) axisValue(axis int) float64 {
 	return 0
 }

internal/gamepad/gamepad_xbox_windows.go

@@ -190,6 +190,10 @@ func (n *nativeGamepadXbox) hatCount() int {
 	return 0
 }
 
+func (g *nativeGamepadXbox) isAxisReady(axis int) bool {
+	return axis >= 0 && axis < g.axisCount()
+}
+
 func (n *nativeGamepadXbox) axisValue(axis int) float64 {
 	switch gamepaddb.StandardAxis(axis) {
 	case gamepaddb.StandardAxisLeftStickHorizontal:

internal/gamepaddb/gamepaddb.go

@@ -390,6 +390,7 @@ func HasStandardLayoutMapping(id string) bool {
 }
 
 type GamepadState interface {
+	IsAxisReady(index int) bool
 	Axis(index int) float64
 	Button(index int) bool
 	Hat(index int) int
@@ -430,6 +431,9 @@ func StandardAxisValue(id string, axis StandardAxis, state GamepadState) float64
 
 	switch mapping.Type {
 	case mappingTypeAxis:
+		if !state.IsAxisReady(mapping.Index) {
+			return 0
+		}
 		v := state.Axis(mapping.Index)*mapping.AxisScale + mapping.AxisOffset
 		if v > 1 {
 			return 1
@@ -486,6 +490,9 @@ func standardButtonValue(id string, button StandardButton, state GamepadState) f
 
 	switch mapping.Type {
 	case mappingTypeAxis:
+		if !state.IsAxisReady(mapping.Index) {
+			return 0
+		}
 		v := state.Axis(mapping.Index)*mapping.AxisScale + mapping.AxisOffset
 		if v > 1 {
 			v = 1