WeChat Small Program Canvas

Canvas canvas

All drawings in canvas must be done with JavaScript:

WXML: (We'll use this WXML as a template in the next example if there's no special declaration, don't repeat it)

<canvas canvas-id="myCanvas" style="border: 1px solid;"/>

JS: (We'll put JS in onLoad in the next example)

const ctx = wx.createCanvasContext('myCanvas')
ctx.setFillStyle('red')
ctx.fillRect(10, 10, 150, 75)
ctx.draw()

Step 1: Create a Canvas drawing context

First, we need to create a Canvas drawing context for CanvasContext.

CanvasContext is an object built into a small program and has some drawing methods:

const ctx = wx.createCanvasContext('myCanvas')

Step 2: Use the Canvas drawing context to describe the drawing

Next, let's describe what to draw in Canvas.

Set the fill color of the drawing context to red:

ctx.setFillStyle('red')

Draw a rectangle with the fillRect (x, y, width, height) method, filled with the red just set:

ctx.fillRect(10, 10, 150, 75)

Step 3: Draw

Tell the canvas component that you want to draw the description you just made:

ctx.draw()

Results:

Coordinates

canvas is in a two-dimensional grid. The coordinates in the upper left corner are (0, 0).

In the last section, we used this method fillRect (0, 0, 150, 75).

It means to draw a 150 x 75px rectangle starting at the upper left corner (0, 0).

Code example

We can add some events to canvas to observe its coordinate system

<canvas canvas-id="myCanvas"
  style="margin: 5px; border:1px solid #d3d3d3;"
  bindtouchstart="start"
  bindtouchmove="move"
  bindtouchend="end"/>

<view hidden="{{hidden}}">
  Coordinates: ({{x}}, {{y}})
</view>

Page({
  data: {
    x: 0,
    y: 0,
    hidden: true
  },
  start (e) {
    this.setData({
      hidden: false,
      x: e.touches[0].x,
      y: e.touches[0].y
    })
  },
  move (e) {
    this.setData({
      x: e.touches[0].x,
      y: e.touches[0].y
    })
  },
  end (e) {
    this.setData({
      hidden: true
    })
  }
})

When you put your finger in canvas, the coordinates of the touch point are displayed between the lower parts:

Gradient

Gradients can be used to fill a rectangle, circle, line, text, etc. Fill colors can be not fixed to a fixed color.

We offer two ways to color gradients:

• CreateLinearGradient (x, y, x1, y1) creates a linear gradient
• CreateCircularGradient (x, y, r) creates a gradient that starts at the center of the circle

Once we have created a gradient object, we must add two color gradient points.

The addColorStop (position, color) method specifies the position and color of the color gradient point, which must be between 0 and 1.

You can set the gradient using the setFillStyle and setStyle methods, and then paint a picture description.

Use createLinearGradient()

const ctx = wx.createCanvasContext('myCanvas')

// Create linear gradient
const grd = ctx.createLinearGradient(0, 0, 200, 0)
grd.addColorStop(0, 'red')
grd.addColorStop(1, 'white')

// Fill with gradient
ctx.setFillStyle(grd)
ctx.fillRect(10, 10, 150, 80)
ctx.draw()

Use createCircularGradient()

const ctx = wx.createCanvasContext('myCanvas')

// Create circular gradient
const grd = ctx.createCircularGradient(75, 50, 50)
grd.addColorStop(0, 'red')
grd.addColorStop(1, 'white')

// Fill with gradient
ctx.setFillStyle(grd)
ctx.fillRect(10, 10, 150, 80)
ctx.draw()