JavaScript 游戏视口

Question

你好，我是一个新手，想通过跟随w3schools游戏教程来学习更多关于JavaScript的知识。

我决定做一个平台游戏，但是我一直没有弄清楚如何添加一个视口来跟随玩家在画布外面移动。

有人能帮帮我吗？提前感谢。

Fiddle

//Background
var Background;

//Objects
var Player;
var Obstacle;

//Mobile buttons
var UpBtn;
var DownBtn;
var LeftBtn;
var RightBtn;
//End

function startGame() {
    Background = new component(656, 270, "gray", 0, 0);

    Player = new component(30, 30, "blue", 200, 75);

    Obstacle = new component(10,100, "red", 300, 170);

  //Mobile buttons
    UpBtn = new component(0, 0, "black", 50, 160);    
    DownBtn = new component(0, 0, "black", 50, 220);    
    LeftBtn = new component(30, 30, "black", 20, 200);    
    RightBtn = new component(30, 30, "black", 90, 200); 
    //End
    myGameArea.start();
}

var myGameArea = {
    canvas : document.createElement("canvas"),
    start : function() {
        this.canvas.width = 480;
        this.canvas.height = 270;
        this.context = this.canvas.getContext("2d");
        document.body.insertBefore(this.canvas, document.body.childNodes[0]);
        this.interval = setInterval(updateGameArea, 20);
        //Mobile buttons
        window.addEventListener('mousedown', function (e) {
            myGameArea.x = e.pageX;
            myGameArea.y = e.pageY;
        })
        window.addEventListener('mouseup', function (e) {
            myGameArea.x = false;
            myGameArea.y = false;
        })
        window.addEventListener('touchstart', function (e) {
            myGameArea.x = e.pageX;
            myGameArea.y = e.pageY;
        })
        window.addEventListener('touchend', function (e) {
            myGameArea.x = false;
            myGameArea.y = false;
        })
        //End


        //Keyboard
        window.addEventListener('keydown', function (e) {
          myGameArea.keys = (myGameArea.keys || []);
          myGameArea.keys[e.keyCode] = true;
        })
        window.addEventListener('keyup', function (e) {
          myGameArea.keys[e.keyCode] = false; 
        })
        //End

    },
    stop : function() {
        clearInterval(this.interval);
    },    
    clear : function() {
        this.context.clearRect(0, 0, this.canvas.width, this.canvas.height);
    },
    dead : function() {
      myGameArea.stop();
      startGame();
    }
}

function component(width, height, color, x, y, type) {
    this.type = type;
    if (type == "image") {
    this.image = new Image();
    this.image.src = color;
    }
    this.width = width;
    this.height = height;
    this.x = x;
    this.y = y;
    speed = 4;
    this.speedX = 0;
    this.speedY = 0;    
    this.gravity = 0.175;
    this.gravitySpeed = 0;
    this.bounce = 1;
    this.update = function() {

        ctx = myGameArea.context;
        if (type == "image") {
            ctx.drawImage(this.image, 
                this.x, 
                this.y,
                this.width, this.height);
    } else {
        ctx.fillStyle = color;
        ctx.fillRect(this.x, this.y, this.width, this.height);
    }
}
    this.newPos = function() {
        this.gravitySpeed += this.gravity;
        this.x += this.speedX;
        this.y += this.speedY + this.gravitySpeed;
        this.hitBottom();
    }
    this.hitBottom = function() {
        var rockbottom = myGameArea.canvas.height - this.height;
        if (this.y > rockbottom) {
            this.y = rockbottom;
            this.gravitySpeed = -(this.gravitySpeed * this.bounce);
        }
    }
//Mobile buttons
    this.clicked = function() {
        var myleft = this.x;
        var myright = this.x + (this.width);
        var mytop = this.y;
        var mybottom = this.y + (this.height);
        var clicked = true;
        if ((mybottom < myGameArea.y) || (mytop > myGameArea.y) || (myright < myGameArea.x) || (myleft > myGameArea.x)) {
            clicked = false;
        }
        return clicked;
    }
    //End

      this.crashWith = function(otherobj) {
    var myleft = this.x;
    var myright = this.x + (this.width);
    var mytop = this.y;
    var mybottom = this.y + (this.height);
    var otherleft = otherobj.x;
    var otherright = otherobj.x + (otherobj.width);
    var othertop = otherobj.y;
    var otherbottom = otherobj.y + (otherobj.height);
    var crash = true;
    if ((mybottom < othertop) ||
    (mytop > otherbottom) ||
    (myright < otherleft) ||
    (myleft > otherright)) {
      crash = false;
    }
    return crash;
  }
}

function updateGameArea() {

    if (Player.crashWith(Obstacle)) {
    myGameArea.dead();
  } else {
    myGameArea.clear();
    Obstacle.update();
  }
    //Mobile buttons
        if (myGameArea.x && myGameArea.y) {
        /*if (myUpBtn.clicked()) {
            Player.y -= this.speed;
        }
        if (myDownBtn.clicked()) {
            Player.y += this.speed;
        }*/
        if (LeftBtn.clicked()) {
            Player.x += -this.speed;
        }
        if (RightBtn.clicked()) {
            Player.x += this.speed;
        }
    }
    UpBtn.update();        
    DownBtn.update();        
    LeftBtn.update();     
    RightBtn.update();                                
    Player.update();

    //End

    //Keyboard
  Player.speedX = 0;
  Player.speedY = 0; 

  if (myGameArea.keys && myGameArea.keys[37]) {Player.speedX = -this.speed; }
  if (myGameArea.keys && myGameArea.keys[39]) {Player.speedX = this.speed; }
  //if (myGameArea.keys && myGameArea.keys[38]) {Player.speedY = -this.speed; }
  //if (myGameArea.keys && myGameArea.keys[40]) {Player.speedY = this.speed; }
  //End
    Background.newPos(); 
    Background.update();
    Player.newPos();
    Player.update();

    Obstacle.update();

    //UpBtn.update();        
    //DownBtn.update();        
    LeftBtn.update();     
    RightBtn.update();
}

- Zaedian

---

你想让蓝色玩家不超出边界吗？ - Omer

是的，还有下面的答案，我的手机控件（按钮）停止工作了。 :/ 尝试修复，但无法弄清楚。 - Zaedian

抱歉，我没有注意到黑色方块是按钮。我已经更新了答案来修复这个问题。我创建了一个属性（isFixed）来处理这个问题。 - Daniel Barral

Answer 1

你可以创建一个变量来存储canvas中的offsetX：

var offsetX = 0;

然后，在updateGameArea()方法中，将此偏移量更新为以玩家x坐标为中心：

offsetX = Player.x - myGameArea.canvas.width/2;

此外，在绘制组件时应减去 offsetX 值：

ctx.fillRect(
    this.x - (this.isFixed ? 0 : offsetX), // <--- here
    this.y, 
    this.width, 
    this.height);

我还创建了一个新属性来告诉组件是否固定在屏幕上：

this.isFixed = false;

将“isFixed = true”设置为控制按钮和背景：

Background.isFixed=true;
LeftBtn.isFixed=true;
RightBtn.isFixed=true;

这里是更新后的代码（Fiddle）： https://jsfiddle.net/xkbvphjz/13/ 完整代码：

//Background
var Background;

//Objects
var Player;
var Obstacle;

//Mobile buttons
var UpBtn;
var DownBtn;
var LeftBtn;
var RightBtn;
//End

var offsetX = 0;

function startGame() {
    Background = new component(656, 270, "gray", 0, 0);
        Background.isFixed=true;

    Player = new component(30, 30, "blue", 200, 75);

    Obstacle = new component(10,100, "red", 300, 170);

  //Mobile buttons
    UpBtn = new component(0, 0, "black", 50, 160);    
    DownBtn = new component(0, 0, "black", 50, 220);    
    LeftBtn = new component(30, 30, "black", 20, 200);
    LeftBtn.isFixed=true;
    RightBtn = new component(30, 30, "black", 90, 200); 
    RightBtn.isFixed=true;
    //End
    myGameArea.start();
}

var myGameArea = {
    canvas : document.createElement("canvas"),
    start : function() {
        this.canvas.width = 480;
        this.canvas.height = 270;
        this.context = this.canvas.getContext("2d");
        document.body.insertBefore(this.canvas, document.body.childNodes[0]);
        this.interval = setInterval(updateGameArea, 20);
        //Mobile buttons
        window.addEventListener('mousedown', function (e) {
            myGameArea.x = e.pageX;
            myGameArea.y = e.pageY;
        })
        window.addEventListener('mouseup', function (e) {
            myGameArea.x = false;
            myGameArea.y = false;
        })
        window.addEventListener('touchstart', function (e) {
            myGameArea.x = e.pageX;
            myGameArea.y = e.pageY;
        })
        window.addEventListener('touchend', function (e) {
            myGameArea.x = false;
            myGameArea.y = false;
        })
        //End


        //Keyboard
        window.addEventListener('keydown', function (e) {
          myGameArea.keys = (myGameArea.keys || []);
          myGameArea.keys[e.keyCode] = true;
        })
        window.addEventListener('keyup', function (e) {
          myGameArea.keys[e.keyCode] = false; 
        })
        //End

    },
    stop : function() {
        clearInterval(this.interval);
    },    
    clear : function() {
        this.context.clearRect(0, 0, this.canvas.width, this.canvas.height);
    },
    dead : function() {
      myGameArea.stop();
      startGame();
    }
}

function component(width, height, color, x, y, type, isFixed) {
    this.type = type;
    if (type == "image") {
    this.image = new Image();
    this.image.src = color;
    }
    this.width = width;
    this.height = height;
    this.x = x;
    this.y = y;
    speed = 4;
    this.speedX = 0;
    this.speedY = 0;    
    this.gravity = 0.175;
    this.gravitySpeed = 0;
    this.bounce = 1;
    this.isFixed = false;
    this.update = function() {

        ctx = myGameArea.context;
        if (type == "image") {
            ctx.drawImage(this.image, 
                this.x - (this.isFixed ? 0 : offsetX), 
                this.y,
                this.width, this.height);
    } else {
        ctx.fillStyle = color;
        ctx.fillRect(
            this.x - (this.isFixed ? 0 : offsetX),
            this.y, 
          this.width, 
          this.height);
    }
}
    this.newPos = function() {
            if (!this.isFixed) {
          this.gravitySpeed += this.gravity;
          this.x += this.speedX;
          this.y += this.speedY + this.gravitySpeed;
          this.hitBottom();
          //offsetX += this.speedX;
        }
    }
    this.hitBottom = function() {
        var rockbottom = myGameArea.canvas.height - this.height;
        if (this.y > rockbottom) {
            this.y = rockbottom;
            this.gravitySpeed = -(this.gravitySpeed * this.bounce);
        }
    }
//Mobile buttons
    this.clicked = function() {
        var myleft = this.x;
        var myright = this.x + (this.width);
        var mytop = this.y;
        var mybottom = this.y + (this.height);
        var clicked = true;
        if ((mybottom < myGameArea.y) || (mytop > myGameArea.y) || (myright < myGameArea.x) || (myleft > myGameArea.x)) {
            clicked = false;
        }
        return clicked;
    }
    //End

      this.crashWith = function(otherobj) {
    var myleft = this.x;
    var myright = this.x + (this.width);
    var mytop = this.y;
    var mybottom = this.y + (this.height);
    var otherleft = otherobj.x;
    var otherright = otherobj.x + (otherobj.width);
    var othertop = otherobj.y;
    var otherbottom = otherobj.y + (otherobj.height);
    var crash = true;
    if ((mybottom < othertop) ||
    (mytop > otherbottom) ||
    (myright < otherleft) ||
    (myleft > otherright)) {
      crash = false;
    }
    return crash;
  }
}

function updateGameArea() {

    if (Player.crashWith(Obstacle)) {
    myGameArea.dead();
  } else {
    myGameArea.clear();
    Obstacle.update();
  }
    //Mobile buttons
        if (myGameArea.x && myGameArea.y) {
        /*if (myUpBtn.clicked()) {
            Player.y -= this.speed;
        }
        if (myDownBtn.clicked()) {
            Player.y += this.speed;
        }*/
        if (LeftBtn.clicked()) {
            Player.x += -this.speed;
        }
        if (RightBtn.clicked()) {
            Player.x += this.speed;
        }
    }
    UpBtn.update();        
    DownBtn.update();        
    LeftBtn.update();     
    RightBtn.update();                                
    Player.update();

    offsetX = Player.x - myGameArea.canvas.width/2;

    //End

    //Keyboard
  Player.speedX = 0;
  Player.speedY = 0; 

  if (myGameArea.keys && myGameArea.keys[37]) {Player.speedX = -this.speed; }
  if (myGameArea.keys && myGameArea.keys[39]) {Player.speedX = this.speed; }
  //if (myGameArea.keys && myGameArea.keys[38]) {Player.speedY = -this.speed; }
  //if (myGameArea.keys && myGameArea.keys[40]) {Player.speedY = this.speed; }
  //End
    Background.newPos(); 
    Background.update();
    Player.newPos();
    Player.update();

    Obstacle.update();

    //UpBtn.update();        
    //DownBtn.update();        
    LeftBtn.update();     
    RightBtn.update();
}

- Daniel Barral