How to implement the WeChat mini program 'Santa Hat'

This article mainly shares with you the implementation method of the WeChat applet "Santa Hat", hoping to help everyone. In the past two days, the circle of friends has been flooded with "Santa Hat". Even WeChat officials came out to refute the rumors about this small program. It is another phenomenon-level thing. From a product perspective, it is undoubtedly very successful, but from a technical perspective, it is indeed commonplace, and creativity is very important! Let’s briefly talk about the idea: get the avatar, draw the avatar in Canvas, then draw a hat in Canvas, adjust the parameters of the hat (position, size, rotation), and finally save it as a picture.

Let’s take a look at the effect first

Thoughts

1. Get user avatar

wx.getUserInfo({
    success: function(res) {
        var userInfo = res.userInfo
        var avatarUrl = userInfo.avatarUrl
  }
})

There is a problem to note here. Canvas does not support network images. What is obtained above is only the avatar image address, so you need to download the image here. to WeChat’s temporary directory. The code is as follows:

wx.downloadFile({   
    url: userInfo.avatarUrl, 
    success: function (res) {    
        if (res.statusCode === 200) {      
            avatarUrl = res.tempFilePath //这里的地址是指向本地图片 
        }     
    }  
})

This step of obtaining the avatar uses WeChat’s ready-made API, which is more convenient.

2. Draw user avatar

Commonly used methods are encapsulated here. The avatarImg.w and avatarImg.h below refer to the size of the avatar.

drawAvatar: function (img) {
    ctx.drawImage(img, 0, 0, avatarImg.w, avatarImg.h)
}

Draw the picture using the drawImage function

3. Draw the hat

Before drawing the hat, I defined An object object to save the parameters of the hat

var hat = { 
    url: "../res/hat01.png", 
    w: 40,  
    h: 40,  
    x: 100, 
    y: 100, 
    b: 1,//缩放的倍率
    rotate: 0//旋转的角度
}

Next, start drawing the hat

drawHat: function (hat) { 
    ctx.translate(hat.x, hat.y) 
    ctx.scale(hat.b, hat.b) 
    ctx.rotate(hat.rotate * Math.PI / 180) 
    ctx.drawImage(hat.url, -hat.w / 2, -hat.h / 2, hat.w, hat.h) 
}

Here is a little explanation , using the center point of the hat as the origin for scaling and rotation

ctx.translate(hat.x, hat.y) //translate是将画布的中心点移动到指定坐标处

The origin at this time has moved from (0, 0) to (x, y), that is The center point of the hat, where one-half of the hat's length and one-half of its width meet.

ctx.drawImage(hat.url, -hat.w / 2, -hat.h / 2, hat.w, hat.h)

The key to drawing a hat is to move x and y outside the origin. The schematic diagram is as follows:

4. Change the parameters of the hat

Move the hat:

##

moveHat: function (e) { 
     hat.x = e.touches[0].x
     hat.y = e.touches[0].y 
     that.drawA() 
 }

Rotate the hat:

rotateHat: function (e) {  
     hat.rotate = e.detail.value    //这一块偷懒了，用slider组件 ，滑动取值
     that.drawA() 
 }

Scale the hat:

scaleHat: function (e) {  
     hat.b = e.detail.value   
     hat.w = 40 * hat.b    
     hat.h = 40 * hat.b    
     that.drawA()    ////此处用slider组件 ，滑动取值 
}

Change the hat style:

changeHat: function (e) {
    hat.url = e.currentTarget.dataset.url  //改变帽子的样式   
    that.drawA() 
}

All of these methods There is drawA(), which mainly redraws the canvas every time it moves, rotates, scales, or changes parameters.

5.Canvas export pictures

WeChat official provides corresponding API

saveToPhoto: function () {
    wx.canvasToTempFilePath({  
        x: 0,
        y: 0,
        width: 240,
        height: 240,
        destWidth: 240,
        destHeight: 240,
        canvasId: 'ctx',
        success: function (res) {
            //canvas转图片成功回调
        }
    })
}

Finally save to the album

wx.saveImageToPhotosAlbum({
    filePath: res.tempFilePath,
}) 
wx.showToast({title: '保存成功'})

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