event: Add Modifier

Modifier keys are selected of a common set of GLFW keys [1] and Web
API keys [2].

This change also renames the member Modifiers to Modifer to
follow the convension (e.g., [3]).

Updates #926

[1] https://www.glfw.org/docs/latest/group__mods.html
[2] https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/getModifierState
[3] https://godoc.org/golang.org/x/tools/go/packages#Config

event/event.go

@@ -30,9 +30,8 @@ type Event interface {
 type KeyboardKeyCharacter struct {
 	// Key is the key code of the key typed.
 	Key Key
-	// Modifiers are the modifiers pressed together with the key.
-	// TODO: this should change later from an int to an enumeration type.
-	Modifiers int
+	// Modifier is the logical-or value of the modifiers pressed together with the key.
+	Modifier Modifier
 	// Character is the character that was typed.
 	Character rune
 }
@@ -41,9 +40,8 @@ type KeyboardKeyCharacter struct {
 type KeyboardKeyDown struct {
 	// Key is the key code of the key pressed or released.
 	Key Key
-	// Modifiers are the modifiers pressed together with the key.
-	// TODO: this should change later from an int to an enumeration type.
-	Modifiers int
+	// Modifier is the logical-or value of the modifiers pressed together with the key.
+	Modifier Modifier
 }
 
 // KeyboardKeyUp is an event that occurs when a key is released on the keyboard.

event/modifier.go

@@ -0,0 +1,29 @@
+// Copyright 2019 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 event
+
+import (
+	"github.com/hajimehoshi/ebiten/internal/driver"
+)
+
+type Modifier int
+
+const (
+	ModifierShift    Modifier = Modifier(driver.ModifierShift)
+	ModifierControl  Modifier = Modifier(driver.ModifierControl)
+	ModifierAlt      Modifier = Modifier(driver.ModifierAlt)
+	ModifierCapsLock Modifier = Modifier(driver.ModifierCapsLock)
+	ModifierNumLock  Modifier = Modifier(driver.ModifierNumLock)
+)

internal/driver/modifier.go

@@ -0,0 +1,25 @@
+// Copyright 2019 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 driver
+
+type Modifier int
+
+const (
+	ModifierShift Modifier = 1 << iota
+	ModifierControl
+	ModifierAlt
+	ModifierCapsLock
+	ModifierNumLock
+)