HTML5 Canvas鼠标与键盘事件

演示HTML5 Canvas鼠标事件，获取Canvas对象上的鼠标坐标，演示键盘事件
通过键盘控制Canvas上对象移动。

Canvas对象支持所有的JavaScript的鼠标事件，包括鼠标点击(MouseClick), 鼠标按下

(Mouse Down), 鼠标抬起(Mouse Up),鼠标移动( Mouse Move)

对Canvas添加鼠标事件方式有两种，一种方式是通过API来完成：

// mouse event

canvas.addEventListener("mousedown",doMouseDown,false);

canvas.addEventListener('mousemove', doMouseMove,false);

canvas.addEventListener('mouseup',  doMouseUp, false);

另外一种方式在JavaScript中称为反模式：

canvas. = function(e){

}

canvas. = function(e){

}

canvas. = function(e){

}

获取鼠标在Canvas对象上坐标：

由于Canvas上鼠标事件中不能直接获取鼠标在Canvas的坐标，所获取的都是基于整个

屏幕的坐标。所以通过鼠标事件e.pageX与e.pageY来获取鼠标位置，然后通过

Canvas. getBoundingClientRect()来获取Canvas对象相对屏幕的相对位置，通过计算

得到鼠标在Canvas的坐标，代码如下：

function getPointOnCanvas(canvas, x, y) {

    var bbox =canvas.getBoundingClientRect();

    return { x: x- bbox.left *(canvas.width / bbox.width),

            y:y - bbox.top  * (canvas.height / bbox.height)

            };

}

键盘事件：

HTML5 Canvas本身不支持键盘事件监听与获取，常用的有两种方法来解决这个问题：

一：通过windows对象来实现Canvas键盘事件监听与处理
// key event - use window as object

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

二：通过在Canvas对象上添加其它支持键盘事件的DOM元素实现键盘事件支持

<canvas id="event_canvas"tabindex="0"></canvas>

// key event - use DOM element asobject

canvas.addEventListener('keydown', doKeyDown,true);

canvas.focus(); 

其中tabindex为HTML5 DOM元素，支持键盘事件。

演示，一个可以根据键盘上下左右移动的矩形块:

一个完整的鼠标与键盘事件演示代码如下：

		var tempContext = null; // global variable 2d context 		var started = false; 		var mText_canvas = null; 		var x = 0, y =0; 		window.add 		window.onload = function() { 			var canvas = document.getElementById("event_canvas"); 			console.log(canvas.parentNode.clientWidth); 			canvas.width = canvas.parentNode.clientWidth; 			canvas.height = canvas.parentNode.clientHeight; 			 			if (!canvas.getContext) { 			    console.log("Canvas not supported. Please install a HTML5 compatible browser."); 			    return; 			} 			 			// get 2D context of canvas and draw rectangel 			tempContext = canvas.getContext("2d"); 			tempContext.fillStyle="blue"; 			x = canvas.width/2; 			y = canvas.height/2; 			tempContext.fillRect(x, y, 80, 40); 	 	        // key event - use DOM element as object 	        canvas.addEventListener('keydown', doKeyDown, true); 	        canvas.focus();   	        // key event - use window as object 	        window.addEventListener('keydown', doKeyDown, true); 	         	        // mouse event 	        canvas.addEventListener("mousedown", doMouseDown, false); 	        canvas.addEventListener('mousemove', doMouseMove, false); 	        canvas.addEventListener('mouseup',   doMouseUp, false); 		} 		 		function getPointOnCanvas(canvas, x, y) { 			var bbox = canvas.getBoundingClientRect(); 			return { x: x - bbox.left * (canvas.width  / bbox.width), 					y: y - bbox.top  * (canvas.height / bbox.height) 					}; 		} 		 		function doKeyDown(e) { 			var keyID = e.keyCode ? e.keyCode :e.which; 			if(keyID === 38 || keyID === 87)  { // up arrow and W 				clearCanvas(); 				y = y - 10; 				tempContext.fillRect(x, y, 80, 40); 				e.preventDefault(); 			} 			if(keyID === 39 || keyID === 68)  { // right arrow and D 				clearCanvas(); 				x = x + 10; 				tempContext.fillRect(x, y, 80, 40); 				e.preventDefault(); 			} 			if(keyID === 40 || keyID === 83)  { // down arrow and S 				clearCanvas(); 				y = y + 10; 				tempContext.fillRect(x, y, 80, 40); 				e.preventDefault(); 			} 			if(keyID === 37 || keyID === 65)  { // left arrow and A 				clearCanvas(); 				x = x - 10; 				tempContext.fillRect(x, y, 80, 40); 				e.preventDefault(); 			} 		} 		 		function clearCanvas() { 			tempContext.clearRect(0, 0, 500, 500) 		} 		 		function doMouseDown(event) { 			var x = event.pageX; 			var y = event.pageY; 			var canvas = event.target; 			var loc = getPointOnCanvas(canvas, x, y); 			console.log("mouse down at point( x:" + loc.x + ", y:" + loc.y + ")"); 			tempContext.beginPath(); 			tempContext.moveTo(loc.x, loc.y); 			started = true; 		} 		 		function doMouseMove(event) { 			var x = event.pageX; 			var y = event.pageY; 			var canvas = event.target; 			var loc = getPointOnCanvas(canvas, x, y); 			if (started) { 				tempContext.lineTo(loc.x, loc.y); 				tempContext.stroke(); 			} 		} 		 		function doMouseUp(event) { 			console.log("mouse up now"); 		    if (started) { 		    	doMouseMove(event); 		        started = false; 			} 		}

<body> 	<h2>HTML Canvas Event Demo - By Gloomy Fish</h2> 	<pre>Press W, A, S, D keys to move</pre> 	<div id="my_painter"> 		<canvas id="event_canvas" tabindex="0"></canvas> 	</div> </body>