mobile: Change the unit from px to dp on Android (#241)

internal/ui/ui_android.go

@@ -14,6 +14,61 @@
 
 package ui
 
-func deviceScale() float64 {
+/*
-	return 1
+
+#include <jni.h>
+#include <stdlib.h>
+
+// Basically same as `getResources().getDisplayMetrics().density`;
+static float deviceScale(uintptr_t java_vm, uintptr_t jni_env, uintptr_t ctx) {
+  JavaVM* vm = (JavaVM*)java_vm;
+  JNIEnv* env = (JNIEnv*)jni_env;
+  jobject context = (jobject)ctx;
+
+  const jclass android_content_ContextWrapper =
+      (*env)->FindClass(env, "android/content/ContextWrapper");
+  const jclass android_content_res_Resources =
+      (*env)->FindClass(env, "android/content/res/Resources");
+  const jclass android_util_DisplayMetrics =
+      (*env)->FindClass(env, "android/util/DisplayMetrics");
+
+  const jobject resources =
+      (*env)->CallObjectMethod(
+          env, context,
+          (*env)->GetMethodID(env, android_content_ContextWrapper, "getResources", "()Landroid/content/res/Resources;"));
+  const jobject displayMetrics =
+      (*env)->CallObjectMethod(
+          env, resources,
+          (*env)->GetMethodID(env, android_content_res_Resources, "getDisplayMetrics", "()Landroid/util/DisplayMetrics;"));
+  const float density =
+      (*env)->GetFloatField(
+          env, displayMetrics,
+          (*env)->GetFieldID(env, android_util_DisplayMetrics, "density", "F"));
+  return density;
+}
+
+*/
+import "C"
+
+import (
+	"fmt"
+
+	"github.com/hajimehoshi/ebiten/internal/jni"
+)
+
+var (
+	androidDeviceScale = 0.0
+)
+
+func deviceScale() float64 {
+	if 0 < androidDeviceScale {
+		return androidDeviceScale
+	}
+	if err := jni.RunOnJVM(func(vm, env, ctx uintptr) error {
+		androidDeviceScale = float64(C.deviceScale(C.uintptr_t(vm), C.uintptr_t(env), C.uintptr_t(ctx)))
+		return nil
+	}); err != nil {
+		panic(fmt.Sprintf("ui: error %v", err))
+	}
+	return androidDeviceScale
 }

mobile/mobile.go

@@ -22,9 +22,7 @@ import (
 //
 // Different from ebiten.Run, this invokes only the game loop and not the main (UI) loop.
 //
-// On Android, width/height is in pixels (px).
+// The unit of width/height is device-independent pixel (dp on Android and pointer on iOS).
-//
-// On iOS, width/height is in points.
 func Start(f func(*ebiten.Image) error, width, height int, scale float64, title string) error {
 	return start(f, width, height, scale, title)
 }
@@ -54,6 +52,8 @@ func Update() error {
 //         }
 //         return true;
 //     }
+//
+// The coodinate x/y is in dp.
 func UpdateTouchesOnAndroid(action int, id int, x, y int) {
 	updateTouchesOnAndroid(action, id, x, y)
 }