如何使用html5中的键盘箭头键在画布上移动图像

Question

I m Drawing the image on canvas with this code and it successfully draw the image on canvas now i want to move the image on canvas for that i write the code i check that if the right key of my keyboard is pressed i will increment the x coordinate of an image if left key is pressed i will decrement the x coordinate but image is not moving on the canvas

player = new Image();
player.src = "game_character.png";
context.drawImage(player,player.x * wallDim + wallDim ,player.y * wallDim + wallDim ,50,50);

how to move an image on canvas

 var handleInput = function(event, keyState) {
        switch(event.which) {
              case 37: { // Left Arrow
                    keyDown.arrowLeft = keyState;
                    break;
              }
              case 38: { // Up Arrow
                    keyDown.arrowUp = keyState;
                    break;
              }
              case 39: { // Right Arrow
                    keyDown.arrowRight = keyState;
                    break;
              }
              case 40: { // Down Arrow
                    keyDown.arrowDown = keyState;
                    break;
              }
        }
  }

  /**
  * physics
  *
  * This function contains the basic logic for the maze.
  */
  var physics = function() {
       console.log("physics ");
       console.log("first condition "+keyDown.arrowRight +player.x+1);
        if(keyDown.arrowLeft && player.x-1 >= 0 && map[player.y][player.x-1] != 1) {
              player.x--;
              redraw = true;
        }

        if(keyDown.arrowUp && player.y-1 >= 0 && map[player.y-1][player.x] != 1) {
              player.y--;
              redraw = true;
        }

        if(keyDown.arrowRight && player.x+1 < map[0].length && map[player.y][player.x+1] != 1) {
              console.log("arrow right");
              player.x++;
              redraw = true;
        }

        if(keyDown.arrowDown && player.y+1 < map.length && map[player.y+1][player.x] != 1) {
              player.y++;
              redraw = true;
        }
        if(keyDown.arrowRight && player.x+1 >= map[0].length)
        {
            player.x++;
            document.getElementById("canvas_div").style.display="none";
            document.getElementById("end_screen_div").style.display="block";
            //alert("completed");
        }
  }

  /**
  * draw
  *
  * This function simply draws the current state of the game.
  */
  var draw = function() {

        // Don't redraw if nothing has changed
        if(!redraw)
              return;

        context.clearRect(0, 0, cols, rows);
        context.beginPath();

        // Draw the maze
        for(var a = 0; a < rows; a++) {
              for(var b = 0; b < cols; b++) {
                    switch(map[a][b]) {
                          case C.EMPTY: context.fillStyle = colors.empty; break;
                          case C.WALL: context.fillStyle = colors.wall; break;
                    }

                        context.fillRect(b * wallDim, a * wallDim, wallDim, wallDim); // x, y, width, height
              }
        }

        // Draw the player
     /* context.fillStyle = colors.player;
        context.arc(
              player.x * wallDim + wallDim / 2, // x position
              player.y * wallDim + wallDim / 2, // y position
              wallDim / 2, // Radius
              0, // Starting angle
              Math.PI * 2, // Ending angle
              true // antiClockwise
        );*/


    player = new Image();
    player.src = "game_character.png";

    context.drawImage(player,player.x * wallDim + wallDim ,player.y * wallDim + wallDim ,50,50);

    var firstplayer=new Image();
    firstplayer.src="top_character01.png";

    context.drawImage(firstplayer,680,0,60,60);

    var secondplayer= new Image();
    secondplayer.src="top_character02.png";

    context.drawImage(secondplayer,750,0,60,60);

    context.fill();
    context.closePath();

        redraw = false;
  }

Answer 1

In your draw method, you reinitialize the player each time :

player = new Image();
player.src = "game_character.png";

So you erase the player.x modified by your event handler.

You should initialize the player only once, outside the draw function. You can move the initialization like this :

var player = new Image();
player.src = "game_character.png";
var draw = function() {

There is absolutely no need to call player.src = "game_character.png"; inside the draw function.

As a general rule, when dealing with animation, try to remove all what you can from the draw function, which should be as fast as possible.

Answer 2

You will need to redraw the canvas each time. Something like this:

function init()
{
    canvas = document.getElementById("canvas");
    context = canvas.getContext("2d");

    x = canvas.width / 2; //align to centre of the screen
    y = canvas.height / 2; //same as above

    speed = 5; //speed for the player to move at

    width = 50; //width of the player
    height = 50; //height of the player

    playerimage = new Image();
    playerimage.src = "path/to/image/for/player"; //path to the image to use for the player

    canvas.addEventListener("keypress", update);
}

function update(event)
{
    if (event.keyCode == 38)
    {
        y -= speed; //going up
    }
    if (event.keyCode == 40)
    {
        y += speed; //going down
    }
    if (event.keyCode == 37)
    {
        x -= speed; //going left
    }
    if (event.keyCode == 39)
    {
        x += speed; //going right
    }
    render();
}

function render()
{
    context.clearRect(0, 0, canvas.width, canvas.height);
    context.drawImage(playerimage, x, y, width, height);
}

I haven't tested it, so I don't know whether it works and there may be some mistakes here and there. It should work though! If nothing else, it will (hopefully) give you an idea of one way in which you can go about doing it...