使用触摸屏设备在画布上绘图答案

【问题标题】：Drawing on canvas using touchscreen devices使用触摸屏设备在画布上绘图
【发布时间】：2016-08-20 04:24:52
【问题描述】：

我一直在做一些搜索，看看是否有可能允许触摸屏用户通过原始 JavaScript 在画布标签上绘图。到目前为止，我运气不佳，最终又回到了一个无法交互的画布上。我有一个 div 里面的画布。关于我如何做到这一点的任何建议？我尝试了一些教程，它们似乎只适用于桌面设备和点击事件，而不是当用户在画布上移动手指时。

【问题讨论】：

标签： javascript html html5-canvas touchscreen

【解决方案1】：

<html>
<head>
<title>Sketchpad</title>

<script type="text/javascript">
    // Variables for referencing the canvas and 2dcanvas context
    var canvas,ctx;

    // Variables to keep track of the mouse position and left-button status 
    var mouseX,mouseY,mouseDown=0;

    // Draws a dot at a specific position on the supplied canvas name
    // Parameters are: A canvas context, the x position, the y position, the size of the dot
    function drawDot(ctx,x,y,size) {
        // Let's use black by setting RGB values to 0, and 255 alpha (completely opaque)
        r=0; g=0; b=0; a=255;

        // Select a fill style
        ctx.fillStyle = "rgba("+r+","+g+","+b+","+(a/255)+")";

        // Draw a filled circle
        ctx.beginPath();
        ctx.arc(x, y, size, 0, Math.PI*2, true); 
        ctx.closePath();
        ctx.fill();
    } 

    // Clear the canvas context using the canvas width and height
    function clearCanvas(canvas,ctx) {
        ctx.clearRect(0, 0, canvas.width, canvas.height);
    }

    // Keep track of the mouse button being pressed and draw a dot at current location
    function sketchpad_mouseDown() {
        mouseDown=1;
        drawDot(ctx,mouseX,mouseY,12);
    }

    // Keep track of the mouse button being released
    function sketchpad_mouseUp() {
        mouseDown=0;
    }

    // Keep track of the mouse position and draw a dot if mouse button is currently pressed
    function sketchpad_mouseMove(e) { 
        // Update the mouse co-ordinates when moved
        getMousePos(e);

        // Draw a dot if the mouse button is currently being pressed
        if (mouseDown==1) {
            drawDot(ctx,mouseX,mouseY,12);
        }
    }

    // Get the current mouse position relative to the top-left of the canvas
    function getMousePos(e) {
        if (!e)
            var e = event;

        if (e.offsetX) {
            mouseX = e.offsetX;
            mouseY = e.offsetY;
        }
        else if (e.layerX) {
            mouseX = e.layerX;
            mouseY = e.layerY;
        }
     }


    // Set-up the canvas and add our event handlers after the page has loaded
    function init() {
        // Get the specific canvas element from the HTML document
        canvas = document.getElementById('sketchpad');

        // If the browser supports the canvas tag, get the 2d drawing context for this canvas
        if (canvas.getContext)
            ctx = canvas.getContext('2d');

        // Check that we have a valid context to draw on/with before adding event handlers
        if (ctx) {
            canvas.addEventListener('mousedown', sketchpad_mouseDown, false);
            canvas.addEventListener('mousemove', sketchpad_mouseMove, false);
            window.addEventListener('mouseup', sketchpad_mouseUp, false);
        }
    }
</script>

<style>
/* Some CSS styling */
#sketchpadapp {
    /* Prevent nearby text being highlighted when accidentally dragging mouse outside confines of the canvas */
    -webkit-touch-callout: none;
    -webkit-user-select: none;
    -khtml-user-select: none;
    -moz-user-select: none;
    -ms-user-select: none;
    user-select: none;
}


#sketchpad {
    float:left;
    border:2px solid #888;
    border-radius:4px;
    position:relative; /* Necessary for correct mouse co-ords in Firefox */
}
</style>
</head>

<body onload="init()">
    <div id="sketchpadapp">
        <div class="leftside1">
             

             <input type="submit" value="Clear Sketchpad" onclick="clearCanvas(canvas,ctx);">
        </div>
        <div class="rightside">
            <canvas id="sketchpad" height="300" width="400">
            </canvas>
        </div>
    </div>
</body>
</html>

【讨论】：

太好了！起初它在触摸屏上效果不佳，但我用 touchmove 替换了 EventListeners，这似乎成功了。无论如何，我会继续玩弄它以掌握它。谢谢。
@userrandomuser，您究竟是如何“用 touchmove 替换事件监听器”的？我需要做同样的改变，但我不确定如何:)
@JoseManuelAbarcaRodríguez 在 ctx 的 if 语句中，我用 touchstart 替换了 mousedown，用 touchmove 替换了 mousemove，用 touchend 替换了 mouseup。由于已经很久了，不得不挖掘代码。
@userrandomuser，我按照你说的做了，效果很好，非常感谢 ^_º
@JoseManuelAbarcaRodríguez 没问题：D