internal/ui: implement (*Monitor).Size for mobiles

Closes #2935

internal/ui/ui_android.go

@@ -18,15 +18,19 @@ package ui
 #include <jni.h>
 #include <stdlib.h>
 
-// Basically same as:
+// The following JNI code works as this pseudo Java code:
 //
 //     WindowService windowService = context.getSystemService(Context.WINDOW_SERVICE);
 //     Display display = windowManager.getDefaultDisplay();
 //     DisplayMetrics displayMetrics = new DisplayMetrics();
 //     display.getRealMetrics(displayMetrics);
-//     this.deviceScale = displayMetrics.density;
+//     return displayMetrics.widthPixels, displayMetrics.heightPixels, displayMetrics.density;
 //
-static float deviceScale(uintptr_t java_vm, uintptr_t jni_env, uintptr_t ctx) {
+static void displayInfo(int* width, int* height, float* scale, uintptr_t java_vm, uintptr_t jni_env, uintptr_t ctx) {
+  *width = 0;
+  *height = 0;
+  *scale = 1;
+
   JavaVM* vm = (JavaVM*)java_vm;
   JNIEnv* env = (JNIEnv*)jni_env;
   jobject context = (jobject)ctx;
@@ -64,7 +68,15 @@ static float deviceScale(uintptr_t java_vm, uintptr_t jni_env, uintptr_t ctx) {
       env, display,
       (*env)->GetMethodID(env, android_view_Display, "getRealMetrics", "(Landroid/util/DisplayMetrics;)V"),
       displayMetrics);
-  const float density =
+  *width =
+      (*env)->GetIntField(
+          env, displayMetrics,
+          (*env)->GetFieldID(env, android_util_DisplayMetrics, "widthPixels", "I"));
+  *height =
+      (*env)->GetIntField(
+          env, displayMetrics,
+          (*env)->GetFieldID(env, android_util_DisplayMetrics, "heightPixels", "I"));
+  *scale =
       (*env)->GetFloatField(
           env, displayMetrics,
           (*env)->GetFieldID(env, android_util_DisplayMetrics, "density", "F"));
@@ -78,15 +90,12 @@ static float deviceScale(uintptr_t java_vm, uintptr_t jni_env, uintptr_t ctx) {
   (*env)->DeleteLocalRef(env, windowManager);
   (*env)->DeleteLocalRef(env, display);
   (*env)->DeleteLocalRef(env, displayMetrics);
-
-  return density;
 }
 */
 import "C"
 
 import (
 	"errors"
-	"fmt"
 
 	"github.com/ebitengine/gomobile/app"
 
@@ -119,18 +128,27 @@ func (*graphicsDriverCreatorImpl) newPlayStation5() (graphicsdriver.Graphics, er
 	return nil, errors.New("ui: PlayStation 5 is not supported in this environment")
 }
 
-func (u *UserInterface) deviceScaleFactor() float64 {
-	var s float64
-	if err := app.RunOnJVM(func(vm, env, ctx uintptr) error {
-		// TODO: This might be crash when this is called from init(). How can we detect this?
-		s = float64(C.deviceScale(C.uintptr_t(vm), C.uintptr_t(env), C.uintptr_t(ctx)))
-		return nil
-	}); err != nil {
-		panic(fmt.Sprintf("devicescale: error %v", err))
-	}
-	return s
-}
-
 func dipToNativePixels(x float64, scale float64) float64 {
 	return x * scale
 }
+
+func dipFromNativePixels(x float64, scale float64) float64 {
+	return x / scale
+}
+
+func (u *UserInterface) displayInfo() (int, int, float64, bool) {
+	var cWidth, cHeight C.int
+	var cScale C.float
+	if err := app.RunOnJVM(func(vm, env, ctx uintptr) error {
+		C.displayInfo(&cWidth, &cHeight, &cScale, C.uintptr_t(vm), C.uintptr_t(env), C.uintptr_t(ctx))
+		return nil
+	}); err != nil {
+		// JVM is not ready yet.
+		// TODO: Fix gomobile to detect the error type for this case.
+		return 0, 0, 1, false
+	}
+	scale := float64(cScale)
+	width := int(dipFromNativePixels(float64(cWidth), scale))
+	height := int(dipFromNativePixels(float64(cHeight), scale))
+	return width, height, scale, true
+}

internal/ui/ui_ios.go

@@ -19,31 +19,43 @@ package ui
 //
 // #import <UIKit/UIKit.h>
 //
-// static double devicePixelRatioOnMainThread(UIView* view) {
+// static void displayInfoOnMainThread(float* width, float* height, float* scale, UIView* view) {
+//   *width = 0;
+//   *height = 0;
+//   *scale = 1;
 //   UIWindow* window = view.window;
 //   if (!window) {
-//     return 1;
+//     return;
 //   }
 //   UIWindowScene* scene = window.windowScene;
 //   if (!scene) {
-//     return 1;
+//     return;
 //   }
-//   return scene.screen.nativeScale;
+//   CGRect bounds = scene.screen.bounds;
+//   *width = bounds.size.width;
+//   *height = bounds.size.height;
+//   *scale = scene.screen.nativeScale;
 // }
 //
-// static double devicePixelRatio(uintptr_t viewPtr) {
+// static void displayInfo(float* width, float* height, float* scale, uintptr_t viewPtr) {
+//   *width = 0;
+//   *height = 0;
+//   *scale = 1;
 //   if (!viewPtr) {
-//     return 1;
+//     return;
 //   }
 //   UIView* view = (__bridge UIView*)(void*)viewPtr;
 //   if ([NSThread isMainThread]) {
-//     return devicePixelRatioOnMainThread(view);
+//     displayInfoOnMainThread(width, height, scale, view);
+//     return;
 //   }
-//   __block double scale;
+//   __block float w, h, s;
 //   dispatch_sync(dispatch_get_main_queue(), ^{
-//     scale = devicePixelRatioOnMainThread(view);
+//     displayInfoOnMainThread(&w, &h, &s, view);
 //   });
-//   return scale;
+//   *width = w;
+//   *height = h;
+//   *scale = s;
 // }
 import "C"
 
@@ -113,10 +125,24 @@ func (u *UserInterface) IsGL() (bool, error) {
 	return u.GraphicsLibrary() == GraphicsLibraryOpenGL, nil
 }
 
-func (u *UserInterface) deviceScaleFactor() float64 {
-	return float64(C.devicePixelRatio(C.uintptr_t(u.uiView.Load())))
-}
-
 func dipToNativePixels(x float64, scale float64) float64 {
 	return x
 }
+
+func dipFromNativePixels(x float64, scale float64) float64 {
+	return x
+}
+
+func (u *UserInterface) displayInfo() (int, int, float64, bool) {
+	view := u.uiView.Load()
+	if view == 0 {
+		return 0, 0, 1, false
+	}
+
+	var cWidth, cHeight, cScale C.float
+	C.displayInfo(&cWidth, &cHeight, &cScale, C.uintptr_t(view))
+	scale := float64(cScale)
+	width := int(dipFromNativePixels(float64(cWidth), scale))
+	height := int(dipFromNativePixels(float64(cHeight), scale))
+	return width, height, scale, true
+}

internal/ui/ui_mobile.go

@@ -268,8 +268,10 @@ func (u *UserInterface) Window() Window {
 }
 
 type Monitor struct {
-	deviceScaleFactor     float64
-	deviceScaleFactorOnce sync.Once
+	width             int
+	height            int
+	deviceScaleFactor float64
+	inited            atomic.Bool
 
 	m sync.Mutex
 }
@@ -280,22 +282,35 @@ func (m *Monitor) Name() string {
 	return ""
 }
 
-func (m *Monitor) DeviceScaleFactor() float64 {
+func (m *Monitor) ensureInit() {
+	if m.inited.Load() {
+		return
+	}
+
 	m.m.Lock()
 	defer m.m.Unlock()
+	// Re-check the state since the state might be changed while locking.
+	if m.inited.Load() {
+		return
+	}
+	width, height, scale, ok := theUI.displayInfo()
+	if !ok {
+		return
+	}
+	m.width = width
+	m.height = height
+	m.deviceScaleFactor = scale
+	m.inited.Store(true)
+}
 
-	// The device scale factor can be obtained after the main function starts, especially on Android.
-	// Initialize this lazily.
-	m.deviceScaleFactorOnce.Do(func() {
-		// Assume that the device scale factor never changes on mobiles.
-		m.deviceScaleFactor = theUI.deviceScaleFactor()
-	})
+func (m *Monitor) DeviceScaleFactor() float64 {
+	m.ensureInit()
 	return m.deviceScaleFactor
 }
 
 func (m *Monitor) Size() (int, int) {
-	// TODO: Return a valid value.
-	return 0, 0
+	m.ensureInit()
+	return m.width, m.height
 }
 
 func (u *UserInterface) AppendMonitors(mons []*Monitor) []*Monitor {

monitor.go

@@ -32,8 +32,7 @@ func (m *MonitorType) Name() string {
 // DeviceScaleFactor returns a meaningful value on high-DPI display environment,
 // otherwise DeviceScaleFactor returns 1.
 //
-// DeviceScaleFactor might panic on init function on some devices like Android.
-// Then, it is not recommended to call DeviceScaleFactor from init functions.
+// On mobiles, DeviceScaleFactor returns 1 before the game starts e.g. in init functions.
 func (m *MonitorType) DeviceScaleFactor() float64 {
 	return (*ui.Monitor)(m).DeviceScaleFactor()
 }
@@ -42,7 +41,7 @@ func (m *MonitorType) DeviceScaleFactor() float64 {
 // This is the same as the screen size in fullscreen mode.
 // The returned value can be given to SetSize function if the perfectly fit fullscreen is needed.
 //
-// On mobiles, Size returns (0, 0) so far.
+// On mobiles, Size returns (0, 0) before the game starts e.g. in init functions.
 //
 // Size's use cases are limited. If you are making a fullscreen application, you can use RunGame and
 // the Game interface's Layout function instead. If you are making a not-fullscreen application but the application's