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问题:

Here is the problem. I took default navbar example from bootstrap side(https://v4-alpha.getbootstrap.com/components/navbar/). They show full size navbar in result, that can collapse, when screen gets smaller. But when I've inserted the code into my project, it shows this:

https://ibb.co/fjBNhQ

As you can see it is always collapsed. I tried a lot of other examples, but nothing workes. Here is the code:

<!DOCTYPE html>
<html>

<head>
  <title>Test area</title>
  <link rel="stylesheet" href="media/css/bootstrap.min.css">
</head>

<body>
  <div class="header">
    <div class="top-header">
      <div class="container">
        <nav class="navbar navbar-toggleable-md navbar-light bg-faded">
          <button class="navbar-toggler navbar-toggler-right" type="button" data-toggle="collapse" data-target="#navbarNavAltMarkup" aria-controls="navbarNavAltMarkup" aria-expanded="false" aria-label="Toggle navigation">
                              <span class="navbar-toggler-icon"></span>
                          </button>
          <a class="navbar-brand" href="#">Navbar</a>
          <div class="collapse navbar-collapse" id="navbarNavAltMarkup">
            <div class="navbar-nav">
              <a class="nav-item nav-link active" href="#">Home <span class="sr-only">(current)</span></a>
              <a class="nav-item nav-link" href="#">Features</a>
              <a class="nav-item nav-link" href="#">Pricing</a>
              <a class="nav-item nav-link disabled" href="#">Disabled</a>
            </div>
          </div>
        </nav>
      </div>
    </div>
  </div>
  <script src="media/js/jquery.min.js"></script>
  <script src="media/js/popper.min.js"></script>
  <script src="media/js/bootstrap.min.js"></script>
</body>

</html>

回答1:

The code you posted will only work on Bootstrap alpha 6, not beta. This is because the navbar-toggleable* classes have changed to navbar-expand* as of Bootstrap 4 beta.

Bootstrap 4.0.0: https://www.codeply.com/go/0c0FC1Lrn2

   <nav class="navbar navbar-expand-lg navbar-light bg-faded">
          <a class="navbar-brand" href="#">Navbar</a>
          <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNavAltMarkup" aria-controls="navbarNavAltMarkup" aria-expanded="false" aria-label="Toggle navigation">
              <span class="navbar-toggler-icon"></span>
          </button>
          <div class="collapse navbar-collapse" id="navbarNavAltMarkup">
            <div class="navbar-nav">
              <a class="nav-item nav-link active" href="#">Home <span class="sr-only">(current)</span></a>
              <a class="nav-item nav-link" href="#">Features</a>
              <a class="nav-item nav-link" href="#">Pricing</a>
              <a class="nav-item nav-link disabled" href="#">Disabled</a>
            </div>
          </div>
   </nav>

Also see: Bootstrap 4 navbar layout works using CDN, breaks using downloaded code

回答2:

You need to add the required meta tags as described here:

 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

And use the correct css classes as described here

<nav class="navbar navbar-expand-lg navbar-light bg-light">

The navbar will collapse at the lg breakpoint

<!DOCTYPE html>
<html>

<head>
  <!-- Required meta tags -->
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
  <title>Test area</title>
  <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/css/bootstrap.min.css" integrity="sha384-/Y6pD6FV/Vv2HJnA6t+vslU6fwYXjCFtcEpHbNJ0lyAFsXTsjBbfaDjzALeQsN6M" crossorigin="anonymous">
</head>

<body>
  <div class="header">
    <div class="top-header">
      <div class="container">
        <nav class="navbar navbar-expand-lg navbar-light bg-faded">
          <button class="navbar-toggler navbar-toggler-right" type="button" data-toggle="collapse" data-target="#navbarNavAltMarkup" aria-controls="navbarNavAltMarkup" aria-expanded="false" aria-label="Toggle navigation">
                              <span class="navbar-toggler-icon"></span>
                          </button>
          <a class="navbar-brand" href="#">Navbar</a>
          <div class="collapse navbar-collapse" id="navbarNavAltMarkup">
            <div class="navbar-nav">
              <a class="nav-item nav-link active" href="#">Home <span class="sr-only">(current)</span></a>
              <a class="nav-item nav-link" href="#">Features</a>
              <a class="nav-item nav-link" href="#">Pricing</a>
              <a class="nav-item nav-link disabled" href="#">Disabled</a>
            </div>
          </div>
        </nav>
      </div>
    </div>
  </div>
  <script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.11.0/umd/popper.min.js" integrity="sha384-b/U6ypiBEHpOf/4+1nzFpr53nxSS+GLCkfwBdFNTxtclqqenISfwAzpKaMNFNmj4" crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/js/bootstrap.min.js" integrity="sha384-h0AbiXch4ZDo7tp9hKZ4TsHbi047NrKGLO3SEJAg45jXxnGIfYzk4Si90RDIqNm1" crossorigin="anonymous"></script>
</body>

</html>