我将一个元素作为字符串绘制到画布上。然后，我希望能够单击该元素（或点击移动设备）并将其拖动。我现在拥有的是：
<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="bootstrap.min.css" />
<link type="text/css" rel="stylesheet" href="stylesheet.css"/>
<script src="jquery-1.11.0.js"></script>
</head>
<body>
<canvas id="canvas1" width="662" height="983">
<script src="jquery-1.11.0.js"></script>
</canvas>
<script>
var chosenChord=""; //this gets updated at various times by excluded code, but that code works, because the chord does always print correctly below
` $("button").click(function() {
canvas = document.getElementById('canvas1');
canvas1.addEventListener('click', on_canvas_click, false);
function on_canvas_click(ev) {
x = ev.clientX - canvas1.offsetLeft-40;
y = ev.clientY - canvas1.offsetTop;
//add to canvas
var canvas = document.getElementById("canvas1");
var context = canvas.getContext("2d");
context.fillStyle = "blue";
context.font = "bold 16px Arial";
context.fillText([theString], [x], [y]);
});
});
</script>
</body>
</html>
为了启用此拖动功能，我需要添加什么？
答案 0 :(得分：1)
以下是如何在画布上拖动元素的大纲
演示：http://jsfiddle.net/m1erickson/AGd6u/
侦听鼠标事件：mousedown，mousemove，mouseup，mouseout
// listen for mouse events
$("#canvas").mousedown(function(e){handleMouseDown(e);});
$("#canvas").mousemove(function(e){handleMouseMove(e);});
$("#canvas").mouseup(function(e){handleMouseUp(e);});
$("#canvas").mouseout(function(e){handleMouseOut(e);});
将您的文本片段定义为数组中的对象
// some text objects
var texts=[];
// some test texts
texts.push({text:"Hello",x:20,y:20});
texts.push({text:"World",x:20,y:70});
在mousedown：
如果是，请选择要拖动的文字
// handle mousedown events
// iterate through texts[] and see if the user
// mousedown'ed on one of them
// If yes, set the selectedText to the index of that text
function handleMouseDown(e){
e.preventDefault();
startX=parseInt(e.clientX-offsetX);
startY=parseInt(e.clientY-offsetY);
// Put your mousedown stuff here
for(var i=0;i<texts.length;i++){
if(textHittest(startX,startY,i)){
selectedText=i;
}
}
}
在mousemove中：
重绘画布
// handle mousemove events
// calc how far the mouse has been dragged since
// the last mousemove event and move the selected text
// by that distance
function handleMouseMove(e){
if(selectedText<0){return;}
e.preventDefault();
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mousemove stuff here
var dx=mouseX-startX;
var dy=mouseY-startY;
startX=mouseX;
startY=mouseY;
var text=texts[selectedText];
text.x+=dx;
text.y+=dy;
draw();
}
在mouseup中：
拖动结束，因此取消选择文本
// done dragging
function handleMouseUp(e){
e.preventDefault();
selectedText=-1;
}
示例代码：
<!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; }
#canvas{border:1px solid red;}
</style>
<script>
$(function(){
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
// variables used to get mouse position on the canvas
var $canvas=$("#canvas");
var canvasOffset=$canvas.offset();
var offsetX=canvasOffset.left;
var offsetY=canvasOffset.top;
var scrollX=$canvas.scrollLeft();
var scrollY=$canvas.scrollTop();
// variables to save last mouse position
// used to see how far the user dragged the mouse
// and then move the text by that distance
var startX;
var startY;
// some text objects
var texts=[];
// some test texts
texts.push({text:"Hello",x:20,y:20});
texts.push({text:"World",x:20,y:70});
// calculate width of each text for hit-testing purposes
ctx.font="16px verdana";
for(var i=0;i<texts.length;i++){
var text=texts[i];
text.width=ctx.measureText(text.text).width;
text.height=16;
}
// this var will hold the index of the selected text
var selectedText=-1;
// START: draw all texts to the canvas
draw();
// clear the canvas draw all texts
function draw(){
ctx.clearRect(0,0,canvas.width,canvas.height);
for(var i=0;i<texts.length;i++){
var text=texts[i];
ctx.fillText(text.text,text.x,text.y);
}
}
// test if x,y is inside the bounding box of texts[textIndex]
function textHittest(x,y,textIndex){
var text=texts[textIndex];
return(x>=text.x &&
x<=text.x+text.width &&
y>=text.y-text.height &&
y<=text.y);
}
// handle mousedown events
// iterate through texts[] and see if the user
// mousedown'ed on one of them
// If yes, set the selectedText to the index of that text
function handleMouseDown(e){
e.preventDefault();
startX=parseInt(e.clientX-offsetX);
startY=parseInt(e.clientY-offsetY);
// Put your mousedown stuff here
for(var i=0;i<texts.length;i++){
if(textHittest(startX,startY,i)){
selectedText=i;
}
}
}
// done dragging
function handleMouseUp(e){
e.preventDefault();
selectedText=-1;
}
// also done dragging
function handleMouseOut(e){
e.preventDefault();
selectedText=-1;
}
// handle mousemove events
// calc how far the mouse has been dragged since
// the last mousemove event and move the selected text
// by that distance
function handleMouseMove(e){
if(selectedText<0){return;}
e.preventDefault();
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mousemove stuff here
var dx=mouseX-startX;
var dy=mouseY-startY;
startX=mouseX;
startY=mouseY;
var text=texts[selectedText];
text.x+=dx;
text.y+=dy;
draw();
}
// listen for mouse events
$("#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>
<canvas id="canvas" width=300 height=300></canvas>
</body>
</html>
答案 1 :(得分：0)
我认为@markE的答案非常好。我只想添加我的2c。
直接使用画布可能会很快变得令人生畏，因为您必须自己进行各种状态管理。
我认为，当你想做这样的事情时，更明智地利用其中许多令人惊叹的框架之一，旨在使画布上的矢量绘图更容易。
仅举几例：