Element: mousedown イベント

mousedown イベントは、要素上でポインティングデバイスのボタンが押下されたときに Element に発生します。

例

mousedown, mousemove, mouseup イベントを使用して、ユーザーがキャンバスにお絵かきができるようにする例です。

The following example uses the mousedown, mousemove, and mouseup events to allow the user to draw on an HTML5 canvas. Its functionality is simple: the thickness of the line is set to 1, and the color is always black.

When the page loads, constants myPics and context are created to store a reference to the canvas and the 2d context we will use to draw. Finally, the constant rect is created to store the coordinates of the canvas relative to the page.

Drawing begins when the mousedown event fires. First we store the x and y coordinates of the mouse pointer in the variables x and y, and then set isDrawing to true.

As the mouse moves over the page, the mousemove event fires. If isDrawing is true, the event handler calls the drawLine function to draw a line from the stored x and y values to the current location. (Remember, the current x and y values are modified using the rect constant to adjust the coordinates so they are relative to the upper-left of the canvas.)

When the drawLine() function returns, we adjust the coordinates and then save them in x and y.

The mouseup event draws the final line segment, sets x and y to 0, and stops further drawing by setting isDrawing to false.

HTML

<h1>Drawing with mouse events</h1>
<canvas id="myPics" width="560" height="360"></canvas>

CSS

canvas {
  border: 1px solid black;
  width: 560px;
  height: 360px;
}

JavaScript

// When true, moving the mouse draws on the canvas
let isDrawing = false;
let x = 0;
let y = 0;

const myPics = document.getElementById('myPics');
const context = myPics.getContext('2d');

// The x and y offset of the canvas from the edge of the page
const rect = myPics.getBoundingClientRect();

// Add the event listeners for mousedown, mousemove, and mouseup
myPics.addEventListener('mousedown', e => {
  x = e.clientX - rect.left;
  y = e.clientY - rect.top;
  isDrawing = true;
});

myPics.addEventListener('mousemove', e => {
  if (isDrawing === true) {
    drawLine(context, x, y, e.clientX - rect.left, e.clientY - rect.top);
    x = e.clientX - rect.left;
    y = e.clientY - rect.top;
  }
});

window.addEventListener('mouseup', e => {
  if (isDrawing === true) {
    drawLine(context, x, y, e.clientX - rect.left, e.clientY - rect.top);
    x = 0;
    y = 0;
    isDrawing = false;
  }
});

function drawLine(context, x1, y1, x2, y2) {
  context.beginPath();
  context.strokeStyle = 'black';
  context.lineWidth = 1;
  context.moveTo(x1, y1);
  context.lineTo(x2, y2);
  context.stroke();
  context.closePath();
}

Result

仕様書

ブラウザーの対応

関連情報

• イベントの紹介
• mouseup
• mousemove
• click
• dblclick
• mouseover
• mouseout
• mouseenter
• mouseleave
• contextmenu