在 canvas 中移动导入的图像

Question

I have a problem with moving image when importing it into a canvas.将动态图像导入 canvas 时遇到问题。 I tried this code but it doesn't work.我试过这段代码，但它不起作用。 The code simply drag an image into the canvas and it worked but when I tried to move it it doesn't move.该代码只是将图像拖入 canvas 并且它可以工作，但是当我尝试移动它时它不会移动。 Can anyone tell me where is the error.谁能告诉我错误在哪里。 Here is the code:这是代码：

<body>
<section>

<div>
<canvas id="canvas" width="300" height="200" style="border:1px solid" >
This text is displayed if your browser does not support HTML5 Canvas.
</canvas>
<input  type="button" name="save" value="Drag Image" onClick="drag()"/>

</div>

<script type="text/javascript">

function drag(){
context.fillText("Drop an image onto the canvas", 50, 30);

var dx = 5;
var dy = 5;
var x = 170
var y = 100;
var img = new Image();
        hasText = true,
    clearCanvas = function () {
    if (hasText) {context.clearRect(0, 0, canvas.width, canvas.height);
    hasText = false;
                                }
                            };
        img.addEventListener("load", function() {
        clearCanvas();
    context.drawImage(img, x, y);
    }, false);


        // To enable drag and drop
    canvas.addEventListener("dragover", function (evt) {
       evt.preventDefault();}, false);

      // Handle dropped image file - only Firefox and Google Chrome
      canvas.addEventListener("drop", function (evt) {
     var files = evt.dataTransfer.files;
     if (files.length > 0) {
     var file = files[0];
     if (typeof FileReader !== "undefined" && file.type.indexOf("image") != -1) {
    var reader = new FileReader();

    reader.onload = function (evt) {
    img.src = evt.target.result;
   };
   reader.readAsDataURL(file);}
    }
   evt.preventDefault();}, false);



function doKeyDown(evt){
  switch (evt.keyCode) {
    case 38:  /* Up arrow was pressed */
        if (y - dy > 0){ 
        y -= dy;
      }
      break;
    case 40:  /* Down arrow was pressed */
        if (y + dy < c){ 
        y += dy;
      }
      break;
    case 37:  /* Left arrow was pressed */
        if (x - dx > 0){ 
        x -= dx;
      }
      break;
    case 39:  /* Right arrow was pressed */
        if (x + dx < canvas.width){ 
        x += dx;
      }
      break;
  }
}
function draw(){
clearCanvas();
context.drawImage(img, x, y);}

img.addEventListener('keydown',doKeyDown,true);
}
</script>
<script type="text/javascript">

canvas = document.getElementById("canvas");
context = canvas.getContext("2d");
setInterval(draw,5);

</script>

</section>
</body>
</html>

Answer 1

There are some small syntax errors that caused some errors, if you are using firefox you should try taking a look into using either the built in javascript error console for firefox ( Tools -> Error Console ) or firebug or if you use chrome the built in console ( Tools -> Javascript console ) and that should help you debug you're script.有一些小的语法错误会导致一些错误，如果您使用的是 firefox，您应该尝试使用 firefox 的内置javascript错误控制台或如果您使用内置的错误控制台（ Tools -> Error Console ）控制台（ Tools -> Javascript console ），这应该可以帮助您调试脚本。 It seems that keydown events attached to the canvas or canvas elements don't trigger, settings the keydown event listener to attach to the window instead seems to make it function properly. It seems that keydown events attached to the canvas or canvas elements don't trigger, settings the keydown event listener to attach to the window instead seems to make it function properly. Also the loading code doesn't seem to work in chrome, but it would be something to look into as a separate issue.此外，加载代码似乎不适用于 chrome，但这将是一个单独的问题。

<html>
<head>
<title>Test Page</title>
</head>
<body>

<canvas id="canvas" width="300" height="200" style="border:1px solid" >
    This text is displayed if your browser does not support HTML5 Canvas.
</canvas>
<input  type="button" name="save" value="Drag Image" onClick="drag()"/>

<script type="text/javascript">
var canvas = document.getElementById("canvas");
var context = canvas.getContext("2d");

function drag(){
    context.fillText("Drop an image onto the canvas", 50, 30);
}

var dx = 5;
var dy = 5;
var x = 170
var y = 100;
var img = new Image();
hasText = true;

clearCanvas = function () {
    if (hasText) {
        context.clearRect(0, 0, canvas.width, canvas.height);
        hasText = false;
    } else {
        context.clearRect(0, 0, canvas.width, canvas.height);
    }
};

img.addEventListener("load", function() {
    clearCanvas();
    context.drawImage(img, x, y);
}, false);

// To enable drag and drop
canvas.addEventListener("dragover", function (evt) {
    evt.preventDefault();
}, false);

// Handle dropped image file - only Firefox and Google Chrome
canvas.addEventListener("drop", function (evt) {
var files = evt.dataTransfer.files;
if (files.length > 0) {
    var file = files[0];
    if (typeof FileReader !== "undefined" && file.type.indexOf("image") != -1) {
        var reader = new FileReader();
        reader.onload = function (evt) {
            img.src = evt.target.result;
        };
        reader.readAsDataURL(file);
    }
}
evt.preventDefault();
}, false);

function doKeyDown(evt){
    console.log(evt.keyCode);
    switch (evt.keyCode) {
    case 38:  /* Up arrow was pressed */
        if (y - dy > 0){ 
            y -= dy;
        }
        break;
    case 40:  /* Down arrow was pressed */
        if (y + dy < canvas.height){ 
            y += dy;
        }
        break;
    case 37:  /* Left arrow was pressed */
        if (x - dx > 0){ 
            x -= dx;
        }
        break;
    case 39:  /* Right arrow was pressed */
        if (x + dx < canvas.width){ 
            x += dx;
        }
        break;
    }
    draw();
}

function draw(){
    clearCanvas();
    context.drawImage(img, x, y);
}

window.addEventListener('keydown', doKeyDown, false);

//setInterval(draw,5);
</script>
</body>
</html>

在 canvas 中移动导入的图像

问题描述

1 个解决方案

解决方案1
1 2011-06-28 21:53:39

在 canvas 中移动导入的图像

问题描述

1 个解决方案

解决方案1 1 2011-06-28 21:53:39

解决方案1
1 2011-06-28 21:53:39