javascript - 为什么此代码在 Android 上需要视口(viewport)元标记？

Question

更新 2 - 18-03-2017:显然我需要使用 window.devicePixelRatio。我还不知 Prop 体如何。但这正是我现在需要弄清楚的。

我正在尝试将 300px 的 CSS 大小(每个彩色 block 都设置了该高度)除以 window.devicePixelRatio 但我的计算似乎仍然不对.

更新 1 - 18-03-2017:我想我已经在一定程度上指出了问题所在:

最大 scrollTop 值远低于使用例如Chrome for Android 而不是仅使用 Chrome(用于桌面)。问题一定出在#scrolldistract 的paddingTop 上。我需要以这种方式填充空的 div，以便此高度或填充在 Chrome for Android 中注册，因为它会增加最大 scrollTop 值。现在，无论我使用 min-height 或 line-height 还是具有一定像素高度的 table，它都保持在 540 左右等

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在我的代码中，我试图动态获取一些高度和坐标。如果我使用 HTML 代码注释中指定的视口(viewport)元标记，一切都适用于 Android 版 Chrome 和 Android 版 Firefox，就像它使用例如Chrome(桌面)。

但是当我不插入视口(viewport)元标记时，即使我动态请求它们，它也会有错误的测量值(在 Chrome for Android 和 Firefox for Android 上)。

我知道移动设备上有虚拟像素这样的东西:https://developer.mozilla.org/en-US/docs/Mozilla/Mobile/Viewport_meta_tag

但我不明白为什么浏览器没有返回正确的值(我需要使用的实际值)。我不太明白我在这里做错了什么？

为什么不使用视口(viewport)元标记？因为我想启用双击 - 由于 300 毫秒的延迟，这显然对我来说非常顽皮。我真的很想使用双击，因为需要缩放一些文本(以可视形式)。

好吧，当然还有双指缩放手势，它仍然可以通过视口(viewport)元标记启用(并且在有或没有视口(viewport)元标记的情况下似乎也存在一些问题)。但两次点击更适合此目的。

var $window = $(window);
var $document = $(document);
// Element which needs to fade in and out.
var $fadingblack = $("#fadingblack");
var $scrolldistract = $("#scrolldistract");
var $scrollsviascrolldistract = $("#scrollsviascrolldistract");
// Pulls up the child divs of #scrollsviascrolldistract, under it.
var $puller = $("#puller");


// Start of fading area (Y-value).
var scrollTopStart = $fadingblack.position().top;
// And of course the Y-value of the end of the fading area.
var scrollTopEnd = scrollTopStart + $fadingblack.height();

// Maximum scrollTop-value (when scrollbar is at 100%).
var lastScrollTop = $document.height() - $window.height();

// Amount of scrolled pixels (vertically) including amount scrolled while
// the fading element is fading.
var scrollAmountWithFadeAmount = $document.height + $fadingblack.height();
// Setting height does not quite work for an empty div,
// so we are using some padding.
$scrolldistract.css("paddingTop", scrollAmountWithFadeAmount);
// Percentage of which we have scrolled (1 = 100%).
var currentScrollTopP;
// Current scrollTop value.
var realCurY;

$(function() {
  // Off you go code...

  function doScrollOrFade() {
    currentScrollTopP = Math.ceil($window.scrollTop() / lastScrollTop * 100) / 100;
    realCurY = currentScrollTopP * lastScrollTop;

    if (realCurY >= scrollTopStart && realCurY <= scrollTopEnd) {
      // Current realCurY dictates we are in fade area.
      // So scroll the fading area into view at top of browser viewport.
      $puller.css("marginTop", -scrollTopStart);
      // Determine opacity percentage.
      var fadePercent = (realCurY - scrollTopStart) / (scrollTopEnd - scrollTopStart);
      // Fade to current opacity immediately.
      $fadingblack.fadeTo(0, fadePercent);
    } else {
      // We are outside of the fading area and in scroll-mode.
      if (realCurY < scrollTopStart) {
        // We are somewhere before the fading area, so set opacity to 0.
        $fadingblack.fadeTo(0, 0);
      } else {
        // We are somewhere after the fading area, so set opacity to 1.
        $fadingblack.fadeTo(0, 1);
      }

      if (realCurY > scrollTopEnd) {
        // We have passed the fading area. So we have an amount
        // of pixels we wasted on the opacity changes.
        // Correct it here.
        $puller.css("marginTop", -realCurY + $fadingblack.height());
      } else {
        $puller.css("marginTop", -realCurY);
      }
    }
    window.requestAnimationFrame(doScrollOrFade);
  }

  window.requestAnimationFrame(doScrollOrFade);

  $window.on('resize orientationchange', function(e) {
    // On resize or orientation change recalculate some stuff.
    lastScrollTop = $document.height() - $window.height();
    scrollAmountWithFadeAmount = $document.height + $fadingblack.height();
    $scrolldistract.css("paddingTop", scrollAmountWithFadeAmount);
    window.requestAnimationFrame(doScrollOrFade);
  });
});

body {
  background-color: whitesmoke;
}

#scrollsviascrolldistract {
  position: fixed;
  left: 0px;
  top: 0px;
  width: 100%;
}

#scrolldistract {
  position: absolute;
  left: 0px;
  top: 0px;
  width: 100%;
  padding-top: 2100px;
  height: 0px;
}

#puller {
  position: relative;
  margin-top: 0px;
  left: 0px;
  top: 0px;
}

img {
  display: block;
}

.black,
.red,
.blue {
  border: solid 1px yellow;
  font-size: 32pt;
  position: relative;
  width: 100%;
  height: 300px;
}

.red {
  background-color: red;
}

.blue {
  background-color: blue;
}

.black {
  background-color: black;
}

<!--
For mobile support use viewport meta-tag inside <head>:
<meta name="viewport" content="width=device-width,initial-scale=1,user-scalable=yes">
-->

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="scrolldistract"></div>

<div id="scrollsviascrolldistract">
  <!-- For pulling up the red, blue and fading area -->
  <div id="puller"></div>
  <div class="red">BEGIN</div>
  <div class="blue">Fading black area is ahead...</div>
  <div id="fadingblack" class="black">&nbsp;</div>
  <div class="blue">&nbsp;</div>
  <div class="red">&nbsp;</div>
  <div class="blue">END</div>
</div>

Answer 1

视口(viewport)元标记用于将页面缩放到设备物理高度和显示宽度的大小；没有它，浏览器加载页面并将其缩小以适合屏幕。因此理论上没有视口(viewport)的页面的高度和宽度与桌面相同