Code example for implementing parabolic animation in JS

In a small program project, you need to add a parabolic ball animation when adding a shopping cart.

Analysis

This kind of animation with no fixed starting position naturally cannot use gif images, so it can only be implemented with native code

So what tools do we have to implement animation? Woolen cloth?

• The applet provides JS API createAnimation to create animations

• CSS transition

Tools include Now, let’s take a look at what a parabola is.

Here we only discuss the horizontal parabola. From the mathematical principle, the horizontal parabola is [movement with constant horizontal speed and vertical acceleration]. The conversion to the code level is in the animation effect timingFunction. The horizontal animation uses linear and the vertical animation uses ease-in

So we need to decompose this parabolic animation into two animations that are performed at the same time but with different animation effects.

Implementation

(1) Implementation of small program

JS:

cartAnimation(x, y) { // x y 为手指点击的坐标，即球的起始坐标
    let self = this,
        cartY = app.globalData.winHeight - 50, // 结束位置（购物车图片）纵坐标
        cartX = 50, // 结束位置（购物车图片）的横坐标
        animationX = flyX(cartX, x), // 创建球的横向动画
        animationY = flyY(cartY, y) // 创建球的纵向动画
    this.setData({          ballX: x,          ballY: y,          showBall: true
    })
    setTimeoutES6(100).then(() => { // 100 ms 延时，确保球已经到位并显示
        self.setData({            animationX: animationX.export(),            animationY: animationY.export(),
        })        return setTimeoutES6(400) // 400 ms 是球的抛物线动画时长
    }).then(() => { // 400 ms 延时后隐藏球
        this.setData({            showBall: false,
        })
    })
}function setTimeoutES6(sec) { // Promise 化 setTimeout
    return new Promise((resolve, reject) => {
        setTimeout(() => {resolve()}, sec)
    })
}function flyX(cartX, oriX) { // 水平动画
    let animation = wx.createAnimation({        duration: 400,        timingFunction: 'linear',
    })
    animation.left(cartX).step()    return animation
}function flyY(cartY, oriY) { // 垂直动画
    let animation = wx.createAnimation({        duration: 400,        timingFunction: 'ease-in',
    })
    animation.top(cartY).step()    return animation
}

HTML:

<view animation="{{animationY}}" style="position:fixed;top:{{ballY}}px;" hidden="{{!showBall}}">
    <view class="ball" animation="{{animationX}}" style="position:fixed;left:{{ballX}}px;"></view></view>

translate optimization

As far as I know, the animation implemented using transform: translate() will have better performance than top & left, but it is not that easy to implement.

After much research, I found that the method of translate is one step more than the method of top & left, that is, the translate displacement of the ball needs to be restored (otherwise translate will always have a value) to ensure that the next displacement will change from click The position of the beginning

cartAnimation(x, y) {    let self = this,
        cartY = app.globalData.winHeight - 50,
        cartX = 50,
        animationX = flyX(cartX, x),
        animationY = flyY(cartY, y)    this.setData({        leftNum: x,        topNum: y,        showBall: true
    })
    setTimeoutES6(100).then(() => {
        self.setData({            animationDataX: animationX.export(),            animationDataY: animationY.export(),
        })        return setTimeoutES6(400)
    }).then(() => {        this.setData({            showBall: false,            animationX: flyX(0, 0, 0).export(), // 还原小球位置，即 translate 恢复默认值
            animationY: flyY(0, 0, 0).export(),
        })
    })
}function flyX(cartX,oriX,duration) {    let animation = wx.createAnimation({        duration: duration||400,        timingFunction: 'linear',
    })
    animation.translateX(cartX-oriX).step()    return animation
}function flyY(cartY,oriY,duration) {    let animation = wx.createAnimation({        duration: duration||400,        timingFunction: 'ease-in',
    })
    animation.translateY(cartY-oriY).step()    return animation
}

HTML part remains unchanged

(2) Implementation of H5

In addition to small programs, of course, H5 is more commonly used in daily front-end development. Below I Will use CSS3 transition method to achieve

<!DOCTYPE html><html lang="en" style="width:100%;height:100%;"><head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width">
    <style>
        * {            padding: 0;            margin: 0;
        }        #ball {            width:12px;            height:12px;            background: #5EA345;            border-radius: 50%;            position: fixed;            transition: left 1s linear, top 1s ease-in;
        }    </style>
    <title>CSS3 水平抛物线动画</title></head><body style="width:100%;height:100%;">
    <p id="ball"></p></body><script>
    var $ball = document.getElementById('ball');    document.body.onclick = function (evt) {        console.log(evt.pageX,evt.pageY)
        $ball.style.top = evt.pageY+'px';
        $ball.style.left = evt.pageX+'px';
        $ball.style.transition = 'left 0s, top 0s';
        setTimeout(()=>{
            $ball.style.top = window.innerHeight+'px';
            $ball.style.left = '0px';
            $ball.style.transition = 'left 1s linear, top 1s ease-in';
        }, 20)
    }</script></html>

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