title: Some confusion on the method instantiateItem(ViewGroup container, int position) of PagerAdapter
body: <pre><code>public Object instantiateItem(ViewGroup container, int position) {
        ImageView view = new ImageView();
        container.addView(view);
        return view;
  }
  </code></pre>
  
  <p>I read some example code of PagerAdapter, and they all write the addview method. This above is some simple code, And I know 'return view' is used for return the view for display, But what is the 
      container.addView(view) 
  do?</p>