I am going to create a canvas to let users can draw some rectangles in canvas. It can show the rectangle when the user is dragging the mouse. Also, it allows the user to draw one or more rectangles in canvas. I've found a solution like this:

// 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);
});

/

However, the solution can only allow users to draw one rectangle. If users draw the second rectangle, the previous rectangle will be wiped out because it will call ctx.clearRect() every time during dragging the mouse.

I am going to create a canvas to let users can draw some rectangles in canvas. It can show the rectangle when the user is dragging the mouse. Also, it allows the user to draw one or more rectangles in canvas. I've found a solution like this:

// 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/m1erickson/6E2yd/

However, the solution can only allow users to draw one rectangle. If users draw the second rectangle, the previous rectangle will be wiped out because it will call ctx.clearRect() every time during dragging the mouse.

• javascript
• html
• canvas

2 Answers 2

Edit: Sorry for my mistake. I missed the position:relative property for the container. Now it should work.

The code in your jsfiddle redraw repeatedly to give indications of the rectangle. I think it would be better to separate this indication canvas to a new layer, and use an overlapped layer to show the drawn rectangles. 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/xkmqz9ho/

I've never tried anything on canvas, but you could try adding ctx.save() to the mouseout function and replacing ctx.clear(...) with ctx.restore().

more information about this on: https://developer.mozilla/es/docs/Web/API/CanvasRenderingContext2D/save