[英]drawing a rectangle without clearing the canvas? (eventlisteners?)
我對javascript很陌生。 我試圖使用戶能夠繪制自己的對象,並且在網上找到了一個包含鉛筆和矩形工具的腳本。 每次在使用矩形工具時移動鼠標時,都會清除畫布。
我試圖通過在第一個畫布上添加第二個畫布來解決此問題,但是它不起作用,老實說,我也不知道為什么。
這是我當前的腳本版本: http : //jsfiddle.net/Scilens/mACuK/
我認為為canvas2添加事件偵聽器將有所幫助,但顯然這可以:
canvas.addEventListener('mousedown', ev_canvas, false);
canvas.addEventListener('mousemove', ev_canvas, false);
canvas.addEventListener('mouseup', ev_canvas, false);
canvas2.addEventListener('mousedown', ev_canvas, false);
canvas2.addEventListener('mousemove', ev_canvas, false);
canvas2.addEventListener('mouseup', ev_canvas, false);
這不是正確的方法嗎? 那是問題嗎? 我還要怎么辦? = /我已經為此苦苦掙扎了很長一段時間,所以如果您能幫助我,我將非常高興!
您可以使用臨時繪圖畫布覆蓋永久畫布
通用程序為:
MOUSEDOWN:
設置isDown標志以指示繪圖已開始
function handleMouseDown(e){ mouseX=parseInt(e.clientX-offsetX); mouseY=parseInt(e.clientY-offsetY); // Put your mousedown stuff here startX=mouseX; startY=mouseY; ctx2.clearRect(0,0,canvas2.width,canvas2.height); $("#canvas2").css({ left:0, top:0 }); isDown=true; }
鼠標移動:
從起始XY到當前鼠標位置在臨時畫布上繪制矩形
function handleMouseMove(e){ mouseX=parseInt(e.clientX-offsetX); mouseY=parseInt(e.clientY-offsetY); // Put your mousemove stuff here ctx2.clearRect(0,0,canvas2.width,canvas2.height); drawRect(mouseX,mouseY,ctx2); }
鼠標松開:
繪制完成后清除isDown標志
function handleMouseUp(e){ mouseX=parseInt(e.clientX-offsetX); mouseY=parseInt(e.clientY-offsetY); // Put your mouseup stuff here isDown=false; $("#canvas2").css({ left:-500, top:0 }); drawRect(mouseX,mouseY,ctx); }
這是代碼和小提琴: http : //jsfiddle.net/m1erickson/puk8A/
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<style>
body{ background-color: ivory; }
#wrapper{
position:relative;
width:300px;
height:200px;
}
#canvas{
position:absolute; top:0px; left:0px;
border:1px solid green;
width:100%;
height:100%;
}
#canvas2{
position:absolute; top:0px; left:0px;
border:3px solid red;
width:100%;
height:100%;
}
</style>
<script>
$(function(){
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var canvas2=document.getElementById("canvas2");
var ctx2=canvas2.getContext("2d");
var canvasOffset=$("#canvas").offset();
var offsetX=canvasOffset.left;
var offsetY=canvasOffset.top;
var startX;
var startY;
var isDown=false;
$("#canvas2").css({ left:-500, top:0 });
function drawRect(toX,toY,context){
context.beginPath();
context.rect(startX,startY,toX-startX,toY-startY);
context.stroke();
}
function handleMouseDown(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mousedown stuff here
startX=mouseX;
startY=mouseY;
ctx2.clearRect(0,0,canvas2.width,canvas2.height);
$("#canvas2").css({ left:0, top:0 });
isDown=true;
}
function handleMouseUp(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mouseup stuff here
isDown=false;
$("#canvas2").css({ left:-500, top:0 });
drawRect(mouseX,mouseY,ctx);
}
function handleMouseOut(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mouseup stuff here
isDown=false;
$("#canvas2").css({ left:-500, top:0 });
drawRect(mouseX,mouseY,ctx);
}
function handleMouseMove(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mousemove stuff here
ctx2.clearRect(0,0,canvas2.width,canvas2.height);
drawRect(mouseX,mouseY,ctx2);
}
$("#canvas").mousedown(function(e){handleMouseDown(e);});
$("#canvas").mousemove(function(e){handleMouseMove(e);});
$("#canvas").mouseup(function(e){handleMouseUp(e);});
$("#canvas").mouseout(function(e){handleMouseOut(e);});
}); // end $(function(){});
</script>
</head>
<body>
<p>Drag to draw a rect</p>
<div id="wrapper">
<canvas id="canvas2" width=300 height=200></canvas>
<canvas id="canvas" width=300 height=200></canvas>
</div>
</body>
</html>
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