Canvas 仅在按下键盘时绘制矩形

问题描述 投票：0回答：1

我正在使用鼠标在画布上绘制矩形。但是，我想仅当按下键盘上的 

d
键时才激活鼠标操作来绘制矩形。

如果我使用

if (event.ctrlKey) {
 //draw rectangles
}

我可以通过按住键盘上的 ctrl 键来绘图。 但是，如果我使用

if (e.which == 88) {
 //draw rectangles
}

这不起作用。我无法将键盘按键事件绑定到画布上的鼠标事件。我知道 canvas 元素没有 onKeyPress 事件，并且根据一些博客中的建议，我尝试了以下代码，但它不起作用。我该如何开始？

我有很多事件，我必须在画布上绑定键盘按下事件。

<!doctype html>
<html>

<head>
    <meta charset="utf-8">
    <title>parallelogram</title>
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
</head>

<body>
    <canvas id="canvas" width="800" height="600" style="border:solid 1px;margin:0;padding:0;;z-index:1"></canvas>
</body>

</html>

<script type="text/javascript">
var canvas, context;
var dragging = false;
var startLocation;
var dragStartLocation;
var dragStopLocation;
var dragThirdLocation;
var snapshot;
var coords = [];
var pointsNum = 0;
var d = {
    x: 0,
    y: 0
};

//Get mouse click coordinates
function getCanvasCoordinates(event)
{
    var x = event.clientX - canvas.getBoundingClientRect().left;
    var y = event.clientY - canvas.getBoundingClientRect().top;
    return {
        x: x,
        y: y
    };
}

//save the canvas original state
function takeSnapShot()
{
    snapshot = context.getImageData(0, 0, canvas.width, canvas.height);
}

//restore the canvas original state
function restoreSnapShot()
{
    context.putImageData(snapshot, 0, 0);
}

//draw a point on mouse click
function drawPoint(position)
{
    context.beginPath();
    context.arc(position.x, position.y, 5.5, 0, Math.PI * 2, false);
    context.stroke();
}

//draw a line on mouse move
function drawLine(start, end)
{
    context.beginPath();
    context.moveTo(start.x, start.y);
    context.lineTo(end.x, end.y);
    context.stroke();
}

//start the event with first mouse click
function dragStart(event)
{
    dragging = true;
    dragStartLocation = getCanvasCoordinates(event);
    coords['imageClicked'] = localStorage.getItem('clickedImage');
    console.log(coords);
    var brandNode = localStorage.getItem('brandNode');
    var locationNode = localStorage.getItem('locationNode');
    drawPoint(dragStartLocation);
    pointsNum++;
    takeSnapShot();
    if (pointsNum == 1) startLocation = dragStartLocation;
}

//draw a line along with the mouse move from the first click
function drag(event)
{
    var position;
    if (snapshot && pointsNum && pointsNum < 3)
    {
        restoreSnapShot();
        position = getCanvasCoordinates(event);
        drawLine(dragStartLocation, position);
        drawPoint(position);
        if (pointsNum == 2) drawFourthCoord(position)

    }
}

//stop the mouse movement and drawing line.
function dragStop(event)
{
    dragging = false;
    restoreSnapShot();
    var position = getCanvasCoordinates(event);
    dragStopLocation = position;
    drawPoint(dragStopLocation);
    pointsNum++;
    drawLine(dragStartLocation, dragStopLocation);
    takeSnapShot();
    d = {
        x: dragStartLocation.x - dragStopLocation.x,
        y: dragStartLocation.y - dragStopLocation.y
    };

    dragStartLocation = position;
    console.log(dragStartLocation);
    if (pointsNum > 3) pointsNum = 0;
}

//draw the fourth coordinate
function drawFourthCoord(position)
{
    var p = {
        x: position.x + d.x,
        y: position.y + d.y
    };
    drawLine(position, p);
    drawPoint(p);
    drawLine(startLocation, p);
}

/*
  This code works...but does not have binding to keyCode x
  function init() {
  canvas = document.getElementById('canvas');
  context = canvas.getContext('2d');
  context.lineCap = "round";
  context.lineWidth = 3;
  context.strokeStyle = '#f4d03f';
  canvas.addEventListener('mousedown', dragStart, false);
  canvas.addEventListener('mousemove', drag, false);
  canvas.addEventListener('mouseup', dragStop, false);  

}

window.addEventListener('load', init, false);
*/

var canvaskey = document.getElementById('canvas');
canvaskey.addEventListener('keydown', doKeyDown, true);

function doKeyDown(e)
{
    if (e.which == 88)
    {
        canvas = document.getElementById('canvas');
        context = canvas.getContext('2d');
        context.lineCap = "round";
        context.lineWidth = 3;
        context.strokeStyle = '#f4d03f';
        canvas.addEventListener('mousedown', dragStart, false);
        canvas.addEventListener('mousemove', drag, false);
        canvas.addEventListener('mouseup', dragStop, false);
    }
}
</script>

javascript canvas keycode
1个回答
1
投票

只需在绘图函数有权访问的范围内保存一个 semaphor 变量即可。

按下所需按键时，将其设置为 

true
，按下按键时设置为 
false
。
在draw函数中，检查信号量是否设置为
true
，否则，立即返回。

// our keydown semaphor
var d_down = false;

function doKeyDown(e)
{
    if (e.key == "d")
    {
      d_down = true;
    }
}
function doKeyUp(e)
{
    if (e.key == "d")
    {
      d_down = false;
    }
}


var canvas, context;
var dragging = false;
var startLocation;
var dragStartLocation;
var dragStopLocation;
var dragThirdLocation;
var snapshot;
var coords = [];
var pointsNum = 0;
var d = {
    x: 0,
    y: 0
};

//Get mouse click coordinates
function getCanvasCoordinates(event)
{
    var x = event.clientX - canvas.getBoundingClientRect().left;
    var y = event.clientY - canvas.getBoundingClientRect().top;
    return {
        x: x,
        y: y
    };
}

//save the canvas original state
function takeSnapShot()
{
    snapshot = context.getImageData(0, 0, canvas.width, canvas.height);
}

//restore the canvas original state
function restoreSnapShot()
{
    context.putImageData(snapshot, 0, 0);
}

//draw a point on mouse click
function drawPoint(position)
{
    context.beginPath();
    context.arc(position.x, position.y, 5.5, 0, Math.PI * 2, false);
    context.stroke();
}

//draw a line on mouse move
function drawLine(start, end)
{
    context.beginPath();
    context.moveTo(start.x, start.y);
    context.lineTo(end.x, end.y);
    context.stroke();
}

//start the event with first mouse click
function dragStart(event)
{
  // check our semaphor
  if(!d_down)
    return; // we're not allowed to go farther
    
    dragging = true;
    dragStartLocation = getCanvasCoordinates(event);
/*    coords['imageClicked'] = localStorage.getItem('clickedImage');
    console.log(coords);
    var brandNode = localStorage.getItem('brandNode');
    var locationNode = localStorage.getItem('locationNode');*/
    drawPoint(dragStartLocation);
    pointsNum++;
    takeSnapShot();
    if (pointsNum == 1) startLocation = dragStartLocation;
}

//draw a line along with the mouse move from the first click
function drag(event)
{
  // check our semaphor
  if(!d_down)
    return; // we're not allowed to go farther
    var position;
    if (snapshot && pointsNum && pointsNum < 3)
    {
        restoreSnapShot();
        position = getCanvasCoordinates(event);
        drawLine(dragStartLocation, position);
        drawPoint(position);
        if (pointsNum == 2) drawFourthCoord(position)

    }
}

//stop the mouse movement and drawing line.
function dragStop(event)
{
    dragging = false;
  // check our semaphor
  if(!d_down)
    return; // we're not allowed to go farther
    restoreSnapShot();
    var position = getCanvasCoordinates(event);
    dragStopLocation = position;
    drawPoint(dragStopLocation);
    pointsNum++;
    drawLine(dragStartLocation, dragStopLocation);
    takeSnapShot();
    d = {
        x: dragStartLocation.x - dragStopLocation.x,
        y: dragStartLocation.y - dragStopLocation.y
    };

    dragStartLocation = position;
//    console.log(dragStartLocation);
    if (pointsNum > 3) pointsNum = 0;
}

//draw the fourth coordinate
function drawFourthCoord(position)
{
    var p = {
        x: position.x + d.x,
        y: position.y + d.y
    };
    drawLine(position, p);
    drawPoint(p);
    drawLine(startLocation, p);
}

  function init() {
  canvas = document.getElementById('canvas');
  context = canvas.getContext('2d');
  context.lineCap = "round";
  context.lineWidth = 3;
  context.strokeStyle = '#f4d03f';
  canvas.addEventListener('mousedown', dragStart, false);
  canvas.addEventListener('mousemove', drag, false);
  canvas.addEventListener('mouseup', dragStop, false); 
  snapshot = context.createImageData(1,1)

}

window.addEventListener('load', init, false);

var canvaskey = document.getElementById('canvas');
canvaskey.addEventListener('keydown', doKeyDown);
canvaskey.addEventListener('keyup', doKeyUp);
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<canvas id="canvas" width="800" height="600" style="border:solid 1px;margin:0;padding:0;;z-index:1" tabindex="1"></canvas>

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