UniApp Design and Development Guide for Implementing Drawing Functions and Drawing Board Effects

UniApp Design and Development Guide for Implementing Drawing Functions and Drawing Board Effects

Introduction:
In the era of mobile Internet, drawing functions and drawing board effects have a wide range of application scenarios in various applications. UniApp, as a cross-platform development framework based on Vue.js, can enable a set of codes to run on multiple platforms at the same time, providing convenience to developers. This article will introduce how to use UniApp to realize drawing functions and drawing board effects, as well as some common development techniques and precautions in actual projects.

1. Design and development of drawing function

1. Determine requirements and functional design
   Before designing and developing drawing function, we first need to determine our needs and functions design. For example, we may need to implement functions such as drawing lines, drawing graphics, filling colors, and erasing. According to specific needs, we can design our drawing function.
2. Implementation of drawing graphics
   In UniApp, we can use the canvas component to implement the function of drawing graphics. Canvas is an important feature of HTML5 that can draw graphics on the page. We can use the API provided by canvas to implement various drawing functions. The following is a simple code example for drawing lines:

<template>
  <view>
    <canvas :canvas-id="canvasId" style="width:100%;height:100%;" @touchstart="touchStart" @touchmove="touchMove">
    </canvas>
  </view>
</template>

<script>
export default {
  data () {
    return {
      canvasId: 'myCanvas',
      startX: 0,
      startY: 0,
      endX: 0,
      endY: 0,
      ctx: null
    }
  },
  mounted () {
    this.ctx = uni.createCanvasContext(this.canvasId)
  },
  methods: {
    touchStart (e) {
      const touches = e.touches[0]
      this.startX = touches.x
      this.startY = touches.y
      this.ctx.moveTo(this.startX, this.startY)
    },
    touchMove (e) {
      const touches = e.touches[0]
      this.endX = touches.x
      this.endY = touches.y
      this.ctx.lineTo(this.endX, this.endY)
      this.ctx.stroke()
      this.ctx.draw(true)
      this.startX = this.endX
      this.startY = this.endY
    }
  }
}
</script>

In the above code, we use the canvas component and pass canvas-id A unique identifier is determined. We also define some state and event handling methods.

When the user starts touching the screen, the touchStart method will be triggered. In this method, we record the starting point of the user's touch and set the starting point to the current point. When the user slides his finger, the touchMove method will be triggered. In this method, we record the end point during the sliding process and draw the line. The function of drawing lines is realized by calling the ctx.lineTo method and the ctx.stroke method. Finally, we update the drawn content to the canvas through ctx.draw(true).

2. Design and development of the drawing board effect

1. Realizing the paint bucket effect of the drawing board
   The paint bucket effect of the drawing board is a key step in realizing the drawing board function. When implementing the paint bucket effect, we need to first determine the area where the user clicked and replace the color of that area with the new color. The following is a simple implementation code example:

<template>
  <view>
    <canvas :canvas-id="canvasId" style="width:100%;height:100%;" @touchstart="touchStart" @touchmove="touchMove">
    </canvas>
  </view>
</template>

<script>
export default {
  data () {
    return {
      canvasId: 'myCanvas',
      x: 0,
      y: 0,
      ctx: null
    }
  },
  mounted () {
    this.ctx = uni.createCanvasContext(this.canvasId)
  },
  methods: {
    touchStart (e) {
      const touches = e.touches[0]
      this.x = touches.x
      this.y = touches.y
      this.bucketFill(this.x, this.y)
    },
    touchMove (e) {
      const touches = e.touches[0]
      this.x = touches.x
      this.y = touches.y
      this.bucketFill(this.x, this.y)
    },
    bucketFill (x, y) {
      const ctx = this.ctx
      const imageData = ctx.getImageData(0, 0, uni.upx2px(750), uni.upx2px(750))
      const width = imageData.width
      const height = imageData.height
      const data = imageData.data
      const index = (y * width + x) * 4
      const r = data[index]
      const g = data[index + 1]
      const b = data[index + 2]
      const color = [r, g, b, 255]
      const fillColor = [255, 0, 0, 255]
      if (this.isSameColor(color, fillColor)) {
        return
      }
      this.fill(x, y, width, height, data, color, fillColor)
      ctx.putImageData(imageData, 0, 0)
      ctx.draw(true)
    },
    isSameColor (color1, color2) {
      return color1[0] === color2[0] && color1[1] === color2[1] && color1[2] === color2[2] && color1[3] === color2[3]
    },
    fill (x, y, width, height, data, color, fillColor) {
      if (x < 0 || x >= width || y < 0 || y >= height) {
        return
      }
      const index = (y * width + x) * 4
      if (!this.isSameColor([data[index], data[index + 1], data[index + 2], data[index + 3]], color)) {
        return
      }
      data[index] = fillColor[0]
      data[index + 1] = fillColor[1]
      data[index + 2] = fillColor[2]
      data[index + 3] = fillColor[3]

      this.fill(x - 1, y, width, height, data, color, fillColor)
      this.fill(x + 1, y, width, height, data, color, fillColor)
      this.fill(x, y - 1, width, height, data, color, fillColor)
      this.fill(x, y + 1, width, height, data, color, fillColor)
    }
  }
}
</script>

In the above code, we mainly use the getImageData method and the putImageData method to implement the paint bucket Effect.

In the touchStart method, we obtain the coordinates of the user's click and achieve the paint bucket effect by calling the bucketFill method.

In the bucketFill method, we first obtain the pixel data on the canvas by calling the ctx.getImageData method, and then compare the color and fill color of the pixels one by one, Return if identical. Then the actual filling operation is implemented by calling the fill method. In the fill method, we use recursion to implement the filling operation. Stop filling when the colors are different, otherwise continue filling adjacent pixels until all adjacent pixels are filled.

Summary:
This article introduces how to use UniApp to realize drawing functions and drawing board effects, and gives specific code examples. By using the canvas component and related APIs provided by UniApp, we can easily implement various drawing functions and drawing board effects. In actual development, we can also expand more complex and rich functions according to specific needs and scenarios. Hope this article can be helpful to you!

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