如何访问原生 2D 上下文

如何从 Konva 访问原生 2D Canvas 上下文

Konva 提供了在 Canvas 上绘制图形的对象模型。应用从舞台（div 包装器）开始，舞台内部包含一个或多个图层（Canvas DOM 元素）。

可以访问 Konva 内部（或 DOM 内部）并直接在 Canvas 上绘制，而不创建任何图形。但不建议这样做。Konva 完全控制绘制过程，因此可能重置手动绘制的内容，或在执行 stage.toDataURL() 等导出操作时丢失这些内容。

如果想手动绘制内容，请使用以下两种方式之一：

1. 使用自定义图形
2. 手动创建新的 Canvas 元素，然后将其用于 Konva.Image。

这两种方式都能让绘制内容进入 Konva 的场景图和导出过程。

Vanilla

import Konva from 'konva';

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

const stage = new Konva.Stage({
  container: 'container',
  width: width,
  height: height,
});

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

// if you want to make something with native 2d canvas
// we can create it and use it for Konva.Image
const canvas = document.createElement('canvas');
canvas.width = 200;
canvas.height = 150;
const ctx = canvas.getContext('2d');

const image = new Konva.Image({
  x: 50,
  y: 50,
  image: canvas,
  draggable: true,
});
layer.add(image);

// make manual drawings
ctx.fillStyle = 'blue';
ctx.fillRect(5, 5, canvas.width - 10, canvas.height / 2);
ctx.fillStyle = 'red';
ctx.beginPath();
ctx.arc(100, 75, 50, 0, 2 * Math.PI);
ctx.fill();

// such as canvas is updated we need to redraw the layer
layer.batchDraw();

React

import { Stage, Layer, Image } from 'react-konva';
import { useMemo, useState } from 'react';

const App = () => {
  const [position, setPosition] = useState({ x: 50, y: 50 });

  const canvas = useMemo(() => {
    const canvas = document.createElement('canvas');
    canvas.width = 200;
    canvas.height = 150;
    const ctx = canvas.getContext('2d');

    // make manual drawings
    ctx.fillStyle = 'blue';
    ctx.fillRect(5, 5, canvas.width - 10, canvas.height / 2);
    ctx.fillStyle = 'red';
    ctx.beginPath();
    ctx.arc(100, 75, 50, 0, 2 * Math.PI);
    ctx.fill();

    return canvas;
  }, []);

  return (
    <Stage width={window.innerWidth} height={window.innerHeight}>
      <Layer>
        <Image
          x={position.x}
          y={position.y}
          image={canvas}
          draggable
          onDragEnd={(e) => {
            setPosition({
              x: e.target.x(),
              y: e.target.y(),
            });
          }}
        />
      </Layer>
    </Stage>
  );
};

export default App;

Vue

<template>
  <v-stage :config="stageSize">
    <v-layer>
      <v-image
        :config="{
          x: position.x,
          y: position.y,
          image: canvas,
          draggable: true,
        }"
        @dragend="handleDragEnd"
      />
    </v-layer>
  </v-stage>
</template>

<script setup>
import { ref, onMounted } from 'vue';

const stageSize = {
  width: window.innerWidth,
  height: window.innerHeight
};

const position = ref({ x: 50, y: 50 });
const canvas = ref(null);

onMounted(() => {
  const canvasEl = document.createElement('canvas');
  canvasEl.width = 200;
  canvasEl.height = 150;
  const ctx = canvasEl.getContext('2d');

  // make manual drawings
  ctx.fillStyle = 'blue';
  ctx.fillRect(5, 5, canvasEl.width - 10, canvasEl.height / 2);
  ctx.fillStyle = 'red';
  ctx.beginPath();
  ctx.arc(100, 75, 50, 0, 2 * Math.PI);
  ctx.fill();

  canvas.value = canvasEl;
});

const handleDragEnd = (e) => {
  position.value = {
    x: e.target.x(),
    y: e.target.y(),
  };
};
</script>