跳到主要内容

Canvas 绘图——使用 JavaScript 在 HTML5 Canvas 上自由绘制和涂画

构建一个 Canvas 绘图工具，让用户直接在 HTML5 Canvas 上绘制自由笔画。这是 Canvas 应用最常见的功能之一，包括白板、标注工具、设计编辑器和签名板。

使用 Konva 进行 Canvas 绘图有两种常见方法：

  1. 基于 Konva 的矢量图形——每条笔画都是一个 Konva.Line 对象，以后可以选择、移动和删除（简单，适合大多数场景）
  2. 手动 2D Canvas 绘图——直接绘制到 Canvas 像素缓冲区，在包含大量笔画时获得最高性能（高级）

使用 Konva 节点自由绘制​

第一种方法可能也是最简单的方法：

  1. 创建新的 Konva.Line，并在 mousedown/touchstart 时开始绘制
  2. 在 mousemove/touchmove 时向线条添加新点

此方法适用于许多应用。也可以在某处以矢量形式轻松存储绘图 state，例如存储在 React store 中或以 JSON 格式保存到数据库。

import Konva from 'konva';

// create tool select
const select = document.createElement('select');
select.innerHTML = `
  <option value="brush">Brush</option>
  <option value="eraser">Eraser</option>
`;
document.body.appendChild(select);

const width = window.innerWidth;
const height = window.innerHeight - 25;

// first we need Konva core things: stage and layer
const stage = new Konva.Stage({
  container: 'container',
  width: width,
  height: height,
});

const layer = new Konva.Layer();
stage.add(layer);

let isPaint = false;
let mode = 'brush';
let lastLine;

stage.on('mousedown touchstart', function (e) {
  isPaint = true;
  const pos = stage.getPointerPosition();
  lastLine = new Konva.Line({
    stroke: '#df4b26',
    strokeWidth: 5,
    globalCompositeOperation:
      mode === 'brush' ? 'source-over' : 'destination-out',
    // round cap for smoother lines
    lineCap: 'round',
    lineJoin: 'round',
    // add point twice, so we have some drawings even on a simple click
    points: [pos.x, pos.y, pos.x, pos.y],
  });
  layer.add(lastLine);
});

stage.on('mouseup touchend', function () {
  isPaint = false;
});

// and core function - drawing
stage.on('mousemove touchmove', function (e) {
  if (!isPaint) {
    return;
  }

  // prevent scrolling on touch devices
  e.evt.preventDefault();

  const pos = stage.getPointerPosition();
  const newPoints = lastLine.points().concat([pos.x, pos.y]);
  lastLine.points(newPoints);
});

select.addEventListener('change', function () {
  mode = select.value;
});
Open on CodeSandboxOpen Sandbox

手动自由绘制​

如果要直接使用底层 2D Canvas API，第一种方法存在限制。如果需要对 Canvas 进行高级访问，最好使用原生上下文访问。

我们将创建一个特殊的离屏 Canvas，并在其中添加所有绘图。 通过原生 Canvas 访问，可以使用底层 2D 上下文函数。 要在舞台上显示此 Canvas，将使用 Konva.Image。

import Konva from 'konva';

// create tool select
const select = document.createElement('select');
select.innerHTML = `
  <option value="brush">Brush</option>
  <option value="eraser">Eraser</option>
`;
document.body.appendChild(select);

const width = window.innerWidth;
const height = window.innerHeight - 25;

// first we need Konva core things: stage and layer
const stage = new Konva.Stage({
  container: 'container',
  width: width,
  height: height,
});

const layer = new Konva.Layer();
stage.add(layer);

// then we are going to draw into special canvas element
const canvas = document.createElement('canvas');
canvas.width = stage.width();
canvas.height = stage.height();

// created canvas we can add to layer as "Konva.Image" element
const image = new Konva.Image({
  image: canvas,
  x: 0,
  y: 0,
});
layer.add(image);

// Good. Now we need to get access to context element
const context = canvas.getContext('2d');
context.strokeStyle = '#df4b26';
context.lineJoin = 'round';
context.lineWidth = 5;

let isPaint = false;
let lastPointerPosition;
let mode = 'brush';

// now we need to bind some events
// we need to start drawing on mousedown
// and stop drawing on mouseup
image.on('mousedown touchstart', function () {
  isPaint = true;
  lastPointerPosition = stage.getPointerPosition();
});

stage.on('mouseup touchend', function () {
  isPaint = false;
});

// and core function - drawing
stage.on('mousemove touchmove', function () {
  if (!isPaint) {
    return;
  }

  if (mode === 'brush') {
    context.globalCompositeOperation = 'source-over';
  }
  if (mode === 'eraser') {
    context.globalCompositeOperation = 'destination-out';
  }
  context.beginPath();

  const localPos = {
    x: lastPointerPosition.x - image.x(),
    y: lastPointerPosition.y - image.y(),
  };
  context.moveTo(localPos.x, localPos.y);
  const pos = stage.getPointerPosition();
  const newLocalPos = {
    x: pos.x - image.x(),
    y: pos.y - image.y(),
  };
  context.lineTo(newLocalPos.x, newLocalPos.y);
  context.closePath();
  context.stroke();

  lastPointerPosition = pos;
  // redraw manually
  layer.batchDraw();
});

select.addEventListener('change', function () {
  mode = select.value;
});
Open on CodeSandboxOpen Sandbox