如何为 Canvas 添加背景？

如何为 Konva 舞台添加背景？

可以使用两种方法添加背景。

1. 使用 Konva.Rect 图形添加背景

Konva 风格的背景添加方法是在场景底部绘制一个与舞台大小相同的 Konva.Rect 图形。可以使用纯色、渐变或图案图像设置该矩形的样式。

这里唯一需要注意的是矩形的位置和大小。如果通过移动或缩放舞台、图层等背景图形的父节点进行变换，必须“重置”背景图形的位置和大小，使其填满整个 Stage 区域。

2. 使用 CSS 添加背景

另一种方法是为舞台容器 DOM 元素设置 CSS 样式。这比第一种方法简单，因为无需跟踪位置或大小的变化。性能也会稍微好一些，因为无需绘制额外图形。

但这种方法有一个缺点。使用 stage.toImage() 和 stage.toDataURL() 等方法导出时，CSS 背景不可见。

操作说明： 在以下示例中，绿色纯色背景由 CSS 创建，黄蓝渐变由 Konva.Rect 实例创建。尝试拖动舞台。你会看到渐变保持在原位。

Vanilla

import Konva from 'konva';

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

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

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

// there are two ways to add background to the stage.
// the simplest solution is to just using CSS
stage.container().style.backgroundColor = 'green';

// another solution is to use rectangle shape
const background = new Konva.Rect({
  x: 0,
  y: 0,
  width: stage.width(),
  height: stage.height(),
  fillLinearGradientStartPoint: { x: 0, y: 0 },
  fillLinearGradientEndPoint: { x: stage.width(), y: stage.height() },
  // gradient into transparent color, so we can see CSS styles
  fillLinearGradientColorStops: [
    0,
    'yellow',
    0.5,
    'blue',
    0.6,
    'rgba(0, 0, 0, 0)',
  ],
  // remove background from hit graph for better perf
  // because we don't need any events on the background
  listening: false,
});
layer.add(background);

// the stage is draggable
// that means absolute position of background may change
// so we need to reset it back to {0, 0}
stage.on('dragmove', () => {
  background.absolutePosition({ x: 0, y: 0 });
});

// add demo shape
const circle = new Konva.Circle({
  x: stage.width() / 2,
  y: stage.height() / 2,
  radius: 100,
  fill: 'red',
});
layer.add(circle);

React

import { useState, useRef, useEffect } from 'react';
import { Stage, Layer, Rect, Circle } from 'react-konva';

const App = () => {
  const width = window.innerWidth;
  const height = window.innerHeight;
  
  const stageRef = useRef(null);
  const backgroundRef = useRef(null);
  
  // Set CSS background when component mounts
  useEffect(() => {
    if (stageRef.current) {
      // Apply CSS background to stage container
      const container = stageRef.current.container();
      container.style.backgroundColor = 'green';
    }
  }, []);
  
  // Handler to reset background position on stage drag
  const handleDragMove = () => {
    if (backgroundRef.current) {
      backgroundRef.current.absolutePosition({ x: 0, y: 0 });
    }
  };
  
  return (
    <Stage 
      width={width} 
      height={height}
      draggable
      ref={stageRef}
      onDragMove={handleDragMove}
    >
      <Layer>
        {/* Gradient background */}
        <Rect
          ref={backgroundRef}
          x={0}
          y={0}
          width={width}
          height={height}
          fillLinearGradientStartPoint={{ x: 0, y: 0 }}
          fillLinearGradientEndPoint={{ x: width, y: height }}
          fillLinearGradientColorStops={[
            0, 'yellow',
            0.5, 'blue', 
            0.6, 'rgba(0, 0, 0, 0)'
          ]}
          listening={false}
        />
        
        {/* Demo shape */}
        <Circle
          x={width / 2}
          y={height / 2}
          radius={100}
          fill="red"
        />
      </Layer>
    </Stage>
  );
};

export default App;

Vue

<template>
  <v-stage 
    ref="stageRef" 
    :config="stageConfig"
    @dragmove="handleDragMove"
  >
    <v-layer>
      <!-- Gradient background -->
      <v-rect
        ref="backgroundRef"
        :config="backgroundConfig"
      />
      
      <!-- Demo shape -->
      <v-circle
        :config="circleConfig"
      />
    </v-layer>
  </v-stage>
</template>

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

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

const stageRef = ref(null);
const backgroundRef = ref(null);

const stageConfig = {
  width,
  height,
  draggable: true
};

const backgroundConfig = {
  x: 0,
  y: 0,
  width: width,
  height: height,
  fillLinearGradientStartPoint: { x: 0, y: 0 },
  fillLinearGradientEndPoint: { x: width, y: height },
  fillLinearGradientColorStops: [
    0, 'yellow',
    0.5, 'blue',
    0.6, 'rgba(0, 0, 0, 0)'
  ],
  listening: false
};

const circleConfig = {
  x: width / 2,
  y: height / 2,
  radius: 100,
  fill: 'red'
};

// Set CSS background when component mounts
onMounted(() => {
  if (stageRef.value) {
    // Apply CSS background to stage container
    const container = stageRef.value.getNode().container();
    container.style.backgroundColor = 'green';
  }
});

// Handler to reset background position on stage drag
const handleDragMove = () => {
  if (backgroundRef.value) {
    backgroundRef.value.getNode().absolutePosition({ x: 0, y: 0 });
  }
};
</script>