我要创建一个 Canvas ,让用户可以在 Canvas 上绘制一些矩形。它可以在用户拖动鼠标时显示矩形。此外,它还允许用户在 Canvas 中绘制一个或多个矩形。我找到了这样的解决方案:
// get references to the canvas and context
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
// style the context
ctx.strokeStyle = "blue";
ctx.lineWidth = 3;
// calculate where the canvas is on the window
// (used to help calculate mouseX/mouseY)
var $canvas = $("#canvas");
var canvasOffset = $canvas.offset();
var offsetX = canvasOffset.left;
var offsetY = canvasOffset.top;
var scrollX = $canvas.scrollLeft();
var scrollY = $canvas.scrollTop();
// this flage is true when the user is dragging the mouse
var isDown = false;
// these vars will hold the starting mouse position
var startX;
var startY;
function handleMouseDown(e) {
e.preventDefault();
e.stopPropagation();
// save the starting x/y of the rectangle
startX = parseInt(e.clientX - offsetX);
startY = parseInt(e.clientY - offsetY);
// set a flag indicating the drag has begun
isDown = true;
}
function handleMouseUp(e) {
e.preventDefault();
e.stopPropagation();
// the drag is over, clear the dragging flag
isDown = false;
}
function handleMouseOut(e) {
e.preventDefault();
e.stopPropagation();
// the drag is over, clear the dragging flag
isDown = false;
}
function handleMouseMove(e) {
e.preventDefault();
e.stopPropagation();
// if we're not dragging, just return
if (!isDown) {
return;
}
// get the current mouse position
mouseX = parseInt(e.clientX - offsetX);
mouseY = parseInt(e.clientY - offsetY);
// Put your mousemove stuff here
// clear the canvas
ctx.clearRect(0, 0, canvas.width, canvas.height);
// calculate the rectangle width/height based
// on starting vs current mouse position
var width = mouseX - startX;
var height = mouseY - startY;
// draw a new rect from the start position
// to the current mouse position
ctx.strokeRect(startX, startY, width, height);
}
// 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);
});
http://jsfiddle.net/m1erickson/6E2yd/
但是,该解决方案只能允许用户绘制一个矩形。如果用户绘制第二个矩形,则之前的矩形将被删除,因为它会在每次拖动鼠标时调用 ctx.clearRect()。
最佳答案
编辑:对不起我的错误。我错过了容器的 position:relative
属性。现在它应该可以工作了。
您的 jsfiddle 中的代码反复重绘以指示矩形。我认为最好将这个指示 Canvas 分离到一个新层,并使用重叠层来显示绘制的矩形。 JS:
// get references to the canvas and context
var canvas = document.getElementById("canvas");
var overlay = document.getElementById("overlay");
var ctx = canvas.getContext("2d");
var ctxo = overlay.getContext("2d");
// style the context
ctx.strokeStyle = "blue";
ctx.lineWidth = 3;
ctxo.strokeStyle = "blue";
ctxo.lineWidth = 3;
// calculate where the canvas is on the window
// (used to help calculate mouseX/mouseY)
var $canvas = $("#canvas");
var canvasOffset = $canvas.offset();
var offsetX = canvasOffset.left;
var offsetY = canvasOffset.top;
var scrollX = $canvas.scrollLeft();
var scrollY = $canvas.scrollTop();
// this flage is true when the user is dragging the mouse
var isDown = false;
// these vars will hold the starting mouse position
var startX;
var startY;
var prevStartX = 0;
var prevStartY = 0;
var prevWidth = 0;
var prevHeight = 0;
function handleMouseDown(e) {
e.preventDefault();
e.stopPropagation();
// save the starting x/y of the rectangle
startX = parseInt(e.clientX - offsetX);
startY = parseInt(e.clientY - offsetY);
// set a flag indicating the drag has begun
isDown = true;
}
function handleMouseUp(e) {
e.preventDefault();
e.stopPropagation();
// the drag is over, clear the dragging flag
isDown = false;
ctxo.strokeRect(prevStartX, prevStartY, prevWidth, prevHeight);
}
function handleMouseOut(e) {
e.preventDefault();
e.stopPropagation();
// the drag is over, clear the dragging flag
isDown = false;
}
function handleMouseMove(e) {
e.preventDefault();
e.stopPropagation();
// if we're not dragging, just return
if (!isDown) {
return;
}
// get the current mouse position
mouseX = parseInt(e.clientX - offsetX);
mouseY = parseInt(e.clientY - offsetY);
// Put your mousemove stuff here
// calculate the rectangle width/height based
// on starting vs current mouse position
var width = mouseX - startX;
var height = mouseY - startY;
// clear the canvas
ctx.clearRect(0, 0, canvas.width, canvas.height);
// draw a new rect from the start position
// to the current mouse position
ctx.strokeRect(startX, startY, width, height);
prevStartX = startX;
prevStartY = startY;
prevWidth = width;
prevHeight = height;
}
// 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);
});
HTML:
<h4>Drag the mouse to create a rectangle</h4>
<div id = "canvasWrapper">
<canvas id="overlay" width=300 height=300></canvas>
<canvas id="canvas" width=300 height=300></canvas>
</div>
CSS:
body{ background-color: ivory; }
canvas{
border: 1px solid red;
position: absolute;
}
#canvasWrapper{
position:relative;
}
关于javascript - 让用户使用 Javascript 在 canvas 中用鼠标绘制矩形,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/65376480/