让用户使用 JavaScript 在画布上用鼠标绘制矩形

Question

我将创建一个画布，让用户可以在画布上绘制一些矩形。当用户拖动鼠标时，它可以显示矩形。此外，它还允许用户在画布上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/

然而，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，则之前的矩形将被擦除，因为每次拖动鼠标时都会调用 ctx.clearRect()。

Answer 1

编辑：很抱歉我的错误。我错过了position:relative容器的属性。现在应该可以了。

jsfiddle 中的代码会重复重绘以指示矩形。我认为最好将这个指示画布分离到一个新层，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;
}

http://jsfiddle.net/xkmqz9ho/