Detailed explanation of mobile REM layout adaptation method for H5 activity page

After getting the design draft, how to restore the layout? If you only need to do non-exact responsive design, then using media queries to achieve it is OK. If it is necessary to accurately restore the design draft, this is usually achieved through zooming. Common solutions include viewport-based and rem-based scaling solutions. This article mainly introduces relevant information that explains in detail the mobile REM layout adaptation method of H5 activity pages. I hope it can help everyone.

1 viewport scaling solution

On the mobile side, the viewport scaling page size ratio can be used to achieve the purpose.

Simply put, all the width and height pixels are the same as the visual draft output, and then the viewport is dynamically set through the ratio of the page width to the visual draft width. Core code reference of the scaling solution:

(function () {
    var docEl = document.documentElement;
    var isMobile = window.isMobile /Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini|Mobi/i.test(navigator.userAgent);

    function setScale() {
        var pageScale = 1;

        if (window.top !== window) {
            return pageScale;
        }

        var width = docEl.clientWidth || 360;
        var height = docEl.clientHeight || 640;
        if (width / height >= 360 / 640) {
            // 高度优先
            pageScale = height / 640;
        } else {
            pageScale = width / 360;
        }

        var content = 'width=' + 360 + ', initial-scale=' + pageScale 
          + ', maximum-scale=' + pageScale + ', user-scalable=no';
        document.getElementById('viewport').setAttribute('content', content);
        window.pageScale = pageScale;
    }
    if (isMobile) {
        setScale();
    } else {
        docEl.className += ' pc';
    }
})()

This solution was practiced in our H5 activity page design last year.

But if you want it to be displayed on the PC, since there is no zoom concept of viewport, it can only be set with a fixed value, which is not ideal.

2 rem Layout Adaptation Solution

rem Layout adaptation solution has been mentioned a lot and is widely used in the products of major Internet companies.

Simply speaking, the method is:

1. According to the ratio of the design draft to the width of the device, dynamically calculate and set the font-size of the HTML root tag;

2. In css, the width, height, relative position, etc. of the design draft elements are converted into values ​​in rem units according to the same proportion;

3. Design The fonts in the manuscript use px as the unit and are slightly adjusted through media queries.

Let’s give an example to illustrate.

2.1 Dynamically set the font-size of the html tag

The first problem is the dynamic calculation of the font-size of the html tag. This depends on how to agree on the conversion ratio. Taking ten equal parts of the page width as an example, the core code reference:

(function(WIN) {
    var  setFontSize = WIN.setFontSize = function (_width) {
        var  docEl = document.documentElement; 
        // 获取当前窗口的宽度
        var  width = _width || docEl.clientWidth; // docEl.getBoundingClientRect().width;
        // 大于 1080px 按 1080
        if (width > 1080) { 
            width = 1080;
        }
        var  rem = width / 10;
        console.log(rem);
        docEl.style.fontSize = rem + 'px';
       // 部分机型上的误差、兼容性处理
        var  actualSize = win.getComputedStyle && parseFloat(win.getComputedStyle(docEl)["font-size"]);
        if (actualSize !== rem && actualSize > 0 && Math.abs(actualSize - rem) > 1) {
            var remScaled = rem * rem / actualSize;
            docEl.style.fontSize = remScaled + 'px';
        }
    }
    var timer;
    //函数节流
    function dbcRefresh() {
        clearTimeout(timer);
        timer = setTimeout(setFontSize, 100);
    }
    //窗口更新动态改变 font-size
    WIN.addEventListener('resize', dbcRefresh, false);
    //页面显示时计算一次
    WIN.addEventListener('pageshow', function(e) {
        if (e.persisted) { 
            dbcRefresh() 
        }
    }, false);
    setFontSize();
})(window)

In addition, for full screen The displayed H5 activity page has requirements for the width-to-height ratio, and adjustments should be made at this time. You can do it like this:

function adjustWarp(warpId = '#warp') {
    // if (window.isMobile) return;
    const $win = $(window);
    const height = $win.height();
    let width = $win.width();
    // 考虑导航栏情况
    if (width / height < 360 / 600) {
        return;
    }
    width = Math.ceil(height * 360 / 640);
    $(warpId).css({
        height,
        width,
        postion: &#39;relative&#39;,
        top: 0,
        left: &#39;auto&#39;,
        margin: &#39;0 auto&#39;
    });
    // 重新计算 rem
    window.setFontSize(width);
}

According to this scaling method, precise layout with equal proportions can be achieved on almost any device. .

2.2 Element size value method

The second problem is the value of the element size.

Taking the width of the design draft as 1080px as an example, we divide the width into 10 equal parts for easy conversion, then 1rem = 1080 / 10 = 108px. The conversion method is:

const px2rem = function(px, rem = 108) {
    let remVal = parseFloat(px) / rem;
    if (typeof px === "string" && px.match(/px$/)) { 
        remVal += 'rem';
    }

    return remVal;
}

For example, there is a picture in the design draft with a size of 460x210 and a relative page position of top: 321px; left : 70; . According to the above conversion method, the final css style of the element should be:

.img_demo {
    position: absolute;
    background-size: cover;
    background-image: url('demo.png');
    top: 2.97222rem;
    left: 0.64814rem;
    width: 4.25926rem;
    height: 1.94444rem;
}

##2.3 Development method of rem layout scheme

Through the above method, the rem layout scheme is realized. But manually calculating the value of rem is obviously unrealistic.

With the less/sass preprocessing tool, we only need to set the mixins method, and then set the value according to the actual size of the design draft. Taking less as an example, the mixins reference is as follows:

// px 转 rem
.px2rem(@px, @attr: 'width', @rem: 108rem) {
    @{attr}: (@px / @rem);
}

.px2remTLWH(@top, @left, @width, @height, @rem: 108rem) {
    .px2rem(@top, top, @rem);
    .px2rem(@left, left, @rem);
    .px2rem(@width, width, @rem);
    .px2rem(@height, height, @rem);
}

##For the example element in the previous article, the css style can be written like this:

.img_demo {
    position: absolute;
    background-size: cover;
    background-image: url('demo.png');
    .px2remTLWH(321, 70, 460, 210);
}

Here, the width and height can be read directly through the size of the picture element output by the design draft; the value of top/left can be read through Move guides to position elements quickly in Photoshop.

2.4 Use px as the unit for fonts

Using rem scaling for fonts will cause display problems. You only need to use media queries to set several sizes. .

Example reference:

// 字体响应式
@media screen and (max-width: 321px) {
    body {
        font-size: 13px;
    }
}

@media screen and (min-width: 321px) and (max-width: 400px) {
    body {
        font-size: 14px;
    }
}

@media screen and (min-width: 400px) {
    body {
        font-size: 16px;
    }
}

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