ui: Don't panic at DeviceScaleFactor in init

Fixes #597

internal/devicescale/devicescale.go

@@ -24,12 +24,10 @@ var (
 )
 
 func DeviceScale() float64 {
-	s := 0.0
 	m.Lock()
+	defer m.Unlock()
 	if scale == 0.0 {
 		scale = impl()
 	}
-	s = scale
-	m.Unlock()
-	return s
+	return scale
 }

internal/devicescale/impl_android.go

@@ -64,6 +64,9 @@ import (
 )
 
 func impl() float64 {
+	if !jni.IsJVMAvailable() {
+		return 0
+	}
 	s := 0.0
 	if err := jni.RunOnJVM(func(vm, env, ctx uintptr) error {
 		s = float64(C.deviceScale(C.uintptr_t(vm), C.uintptr_t(env), C.uintptr_t(ctx)))

internal/jni/available_android.go

@@ -0,0 +1,34 @@
+// Copyright 2018 The Ebiten Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package jni
+
+/*
+#include <jni.h>
+#include <stdbool.h>
+
+JavaVM* current_vm;
+
+static bool isJVMAvailable() {
+	return current_vm != NULL;
+}
+*/
+import "C"
+
+// IsJVMAvailable returns a boolean value indicating whether JVM is available or not.
+//
+// In init functions, JVM is not available.
+func IsJVMAvailable() bool {
+	return bool(C.isJVMAvailable())
+}

run.go

@@ -466,7 +466,11 @@ func SetWindowIcon(iconImages []image.Image) {
 // DeviceScaleFactor returns a meaningful value on high-DPI display environment,
 // otherwise DeviceScaleFactor returns 1.
 //
-// This function is concurrent-safe.
+// DeviceScaleFactor might return 0 on init function on some devices like Android.
+// Don't expect DeviceScaleFactor returns a valid value until main starts.
+// Then, it is not recommended to call DeviceScaleFactor from init functions.
+//
+// DeviceScaleFactor is concurrent-safe.
 func DeviceScaleFactor() float64 {
 	return devicescale.DeviceScale()
 }