ebiten/examples/blocks/blocks/scenemanager.go

// Copyright 2014 Hajime Hoshi
//
// 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 blocks

import (
	"github.com/hajimehoshi/ebiten/v2"
)

var (
	transitionFrom = ebiten.NewImage(ScreenWidth, ScreenHeight)
	transitionTo   = ebiten.NewImage(ScreenWidth, ScreenHeight)
)

type Scene interface {
	Update(state *GameState) error
	Draw(screen *ebiten.Image)
}

const transitionMaxCount = 20

type SceneManager struct {
	current         Scene
	next            Scene
	transitionCount int
}

type GameState struct {
	SceneManager *SceneManager
	Input        *Input
}

func (s *SceneManager) Update(input *Input) error {
	if s.transitionCount == 0 {
		return s.current.Update(&GameState{
			SceneManager: s,
			Input:        input,
		})
	}

	s.transitionCount--
	if s.transitionCount > 0 {
		return nil
	}

	s.current = s.next
	s.next = nil
	return nil
}

func (s *SceneManager) Draw(r *ebiten.Image) {
	if s.transitionCount == 0 {
		s.current.Draw(r)
		return
	}

	transitionFrom.Clear()
	s.current.Draw(transitionFrom)

	transitionTo.Clear()
	s.next.Draw(transitionTo)

	r.DrawImage(transitionFrom, nil)

	alpha := 1 - float32(s.transitionCount)/float32(transitionMaxCount)
	op := &ebiten.DrawImageOptions{}
	op.ColorScale.ScaleAlpha(alpha)
	r.DrawImage(transitionTo, op)
}

func (s *SceneManager) GoTo(scene Scene) {
	if s.current == nil {
		s.current = scene
	} else {
		s.next = scene
		s.transitionCount = transitionMaxCount
	}
}