Add joystick plugin (#39)

https://github.com/sony/flutter-elinux-plugins/issues/35

packages/joystick/lib/joystick.dart

@@ -0,0 +1,73 @@
+// Copyright 2021 Sony Group Corporation. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// ignore_for_file: public_member_api_docs
+
+import 'dart:ffi';
+import 'package:ffi/ffi.dart';
+
+DynamicLibrary _dylib = DynamicLibrary.open('libjoystick_plugin.so');
+
+/// See: <linux/joystick.h>
+/// struct js_event {
+/// 	__u32 time;	/* event timestamp in milliseconds */
+/// 	__s16 value;	/* value */
+/// 	__u8 type;	/* event type */
+/// 	__u8 number;	/* axis/button number */
+/// };
+class JSEvent extends Struct {
+  @Uint32()
+  external int time;
+  @Int16()
+  external int value;
+  @Uint8()
+  external int type;
+  @Uint8()
+  external int number;
+}
+
+/// button pressed/released. See [JS_EVENT_BUTTON] in <linux/joystick.h>
+const int JS_EVENT_BUTTON = 0x01;
+
+/// joystick moved. See [JS_EVENT_AXIS] in <linux/joystick.h>
+const int JS_EVENT_AXIS = 0x02;
+
+/// initial state of device. See [JS_EVENT_INIT] in <linux/joystick.h>
+const int JS_EVENT_INIT = 0x80;
+
+typedef JoystickOpenNative = Int32 Function(Pointer<Utf8>);
+typedef JoystickOpen = int Function(Pointer<Utf8>);
+
+/// Opens joystick device.
+final JoystickOpen joystickOpen = _dylib
+    .lookup<NativeFunction<JoystickOpenNative>>('joystick_open')
+    .asFunction();
+
+typedef JoystickReadNative = Int32 Function(Int32 fd, Pointer<JSEvent>);
+typedef JoystickRead = int Function(int fd, Pointer<JSEvent>);
+
+/// Reads joystick input data.
+final JoystickRead joystickRead = _dylib
+    .lookup<NativeFunction<JoystickReadNative>>('joystick_read')
+    .asFunction();
+
+/// Returns true if no events.
+bool joystickInputIsInactive(JSEvent ev) {
+  return (ev.type & JS_EVENT_INIT) != 0;
+}
+
+/// Returns true if the event was caused by a button press.
+bool joystickInputIsButton(JSEvent ev) {
+  return (ev.type & JS_EVENT_BUTTON) != 0;
+}
+
+/// Returns true if the event was caused by an axis movement.
+bool joystickInputIsAxis(JSEvent ev) {
+  return (ev.type & JS_EVENT_AXIS) != 0;
+}
+
+/// Returns true if the button is pressed.
+bool joystickButtonIsPressed(JSEvent ev) {
+  return (ev.value & 1) != 0;
+}