title: How are the views for this view pager being created?
body: <p>I'm working my way through the tutorials for android development and <a href="http://developer.android.com/training/animation/screen-slide.html#views" rel="nofollow">I'm currently trying to understand how the viewpager class works</a>. I've downloaded the demo app <a href="http://developer.android.com/shareables/training/Animations.zip" rel="nofollow">here</a> and I'm trying to wrap my head around how the collection of views is created.</p>
  
  <p>I understand that there's a constant, NUM_PAGES which sets the number of views generated. I would expect to find a loop or something creating NUM_PAGES worth of views at the entry point to the application but I can't find anything - it's driving me mad.</p>
  
  <p>All I'm trying to work out is how 5 views are being added to the ViewPager. How is it being done?</p>
  
  <p>Here is the <strong>MainActivity.java</strong> file:</p>
  
  <pre><code>/*
   * Copyright 2012 The Android Open Source Project
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  package com.example.android.animationsdemo;
  
  import android.app.Activity;
  import android.app.ListActivity;
  import android.content.Intent;
  import android.os.Bundle;
  import android.view.View;
  import android.widget.ArrayAdapter;
  import android.widget.ListView;
  
  /**
   * The launchpad activity for this sample project. This activity launches other activities that
   * demonstrate implementations of common animations.
   */
  public class MainActivity extends ListActivity {
      /**
       * This class describes an individual sample (the sample title, and the activity class that
       * demonstrates this sample).
       */
      private class Sample {
          private CharSequence title;
          private Class&lt;? extends Activity&gt; activityClass;
  
          public Sample(int titleResId, Class&lt;? extends Activity&gt; activityClass) {
              this.activityClass = activityClass;
              this.title = getResources().getString(titleResId);
          }
  
          @Override
          public String toString() {
              return title.toString();
          }
      }
  
      /**
       * The collection of all samples in the app. This gets instantiated in {@link
       * #onCreate(android.os.Bundle)} because the {@link Sample} constructor needs access to {@link
       * android.content.res.Resources}.
       */
      private static Sample[] mSamples;
  
      public void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
  
          // Instantiate the list of samples.
          mSamples = new Sample[]{
                  new Sample(R.string.title_crossfade, CrossfadeActivity.class),
                  new Sample(R.string.title_card_flip, CardFlipActivity.class),
                  new Sample(R.string.title_screen_slide, ScreenSlideActivity.class),
                  new Sample(R.string.title_zoom, ZoomActivity.class),
                  new Sample(R.string.title_layout_changes, LayoutChangesActivity.class),
          };
  
          setListAdapter(new ArrayAdapter&lt;Sample&gt;(this,
                  android.R.layout.simple_list_item_1,
                  android.R.id.text1,
                  mSamples));
      }
  
      @Override
      protected void onListItemClick(ListView listView, View view, int position, long id) {
          // Launch the sample associated with this list position.
          startActivity(new Intent(MainActivity.this, mSamples[position].activityClass));
      }
  }
  </code></pre>
  
  <p>Here is the <strong>ScreenSlideActivity.java</strong> file:</p>
  
  <pre><code>/*
   * Copyright 2012 The Android Open Source Project
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  package com.example.android.animationsdemo;
  
  import android.app.Fragment;
  import android.app.FragmentManager;
  import android.content.Intent;
  import android.os.Bundle;
  import android.support.v13.app.FragmentStatePagerAdapter;
  import android.support.v4.app.FragmentActivity;
  import android.support.v4.app.NavUtils;
  import android.support.v4.view.PagerAdapter;
  import android.support.v4.view.ViewPager;
  import android.view.Menu;
  import android.view.MenuItem;
  
  /**
   * Demonstrates a "screen-slide" animation using a {@link ViewPager}. Because {@link ViewPager}
   * automatically plays such an animation when calling {@link ViewPager#setCurrentItem(int)}, there
   * isn't any animation-specific code in this sample.
   *
   * &lt;p&gt;This sample shows a "next" button that advances the user to the next step in a wizard,
   * animating the current screen out (to the left) and the next screen in (from the right). The
   * reverse animation is played when the user presses the "previous" button.&lt;/p&gt;
   *
   * @see ScreenSlidePageFragment
   */
  public class ScreenSlideActivity extends FragmentActivity {
      /**
       * The number of pages (wizard steps) to show in this demo.
       */
      private static final int NUM_PAGES = 5;
  
      /**
       * The pager widget, which handles animation and allows swiping horizontally to access previous
       * and next wizard steps.
       */
      private ViewPager mPager;
  
      /**
       * The pager adapter, which provides the pages to the view pager widget.
       */
      private PagerAdapter mPagerAdapter;
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_screen_slide);
  
          // Instantiate a ViewPager and a PagerAdapter.
          mPager = (ViewPager) findViewById(R.id.pager);
          mPagerAdapter = new ScreenSlidePagerAdapter(getFragmentManager());
          mPager.setAdapter(mPagerAdapter);
          mPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
              @Override
              public void onPageSelected(int position) {
                  // When changing pages, reset the action bar actions since they are dependent
                  // on which page is currently active. An alternative approach is to have each
                  // fragment expose actions itself (rather than the activity exposing actions),
                  // but for simplicity, the activity provides the actions in this sample.
                  invalidateOptionsMenu();
              }
          });
      }
  
      @Override
      public boolean onCreateOptionsMenu(Menu menu) {
          super.onCreateOptionsMenu(menu);
          getMenuInflater().inflate(R.menu.activity_screen_slide, menu);
  
          menu.findItem(R.id.action_previous).setEnabled(mPager.getCurrentItem() &gt; 0);
  
          // Add either a "next" or "finish" button to the action bar, depending on which page
          // is currently selected.
          MenuItem item = menu.add(Menu.NONE, R.id.action_next, Menu.NONE,
                  (mPager.getCurrentItem() == mPagerAdapter.getCount() - 1)
                          ? R.string.action_finish
                          : R.string.action_next);
          item.setShowAsAction(MenuItem.SHOW_AS_ACTION_IF_ROOM | MenuItem.SHOW_AS_ACTION_WITH_TEXT);
          return true;
      }
  
      @Override
      public boolean onOptionsItemSelected(MenuItem item) {
          switch (item.getItemId()) {
              case android.R.id.home:
                  // Navigate "up" the demo structure to the launchpad activity.
                  // See http://developer.android.com/design/patterns/navigation.html for more.
                  NavUtils.navigateUpTo(this, new Intent(this, MainActivity.class));
                  return true;
  
              case R.id.action_previous:
                  // Go to the previous step in the wizard. If there is no previous step,
                  // setCurrentItem will do nothing.
                  mPager.setCurrentItem(mPager.getCurrentItem() - 1);
                  return true;
  
              case R.id.action_next:
                  // Advance to the next step in the wizard. If there is no next step, setCurrentItem
                  // will do nothing.
                  mPager.setCurrentItem(mPager.getCurrentItem() + 1);
                  return true;
          }
  
          return super.onOptionsItemSelected(item);
      }
  
      /**
       * A simple pager adapter that represents 5 {@link ScreenSlidePageFragment} objects, in
       * sequence.
       */
      private class ScreenSlidePagerAdapter extends FragmentStatePagerAdapter {
          public ScreenSlidePagerAdapter(FragmentManager fm) {
              super(fm);
          }
  
          @Override
          public Fragment getItem(int position) {
              return ScreenSlidePageFragment.create(position);
          }
  
          @Override
          public int getCount() {
              return NUM_PAGES;
          }
      }
  }
  </code></pre>
  
  <p>Finally, here is the ScreenSlidePageFragment.java file:</p>
  
  <pre><code>/*
   * Copyright 2012 The Android Open Source Project
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  package com.example.android.animationsdemo;
  
  import android.app.Fragment;
  import android.os.Bundle;
  import android.view.LayoutInflater;
  import android.view.View;
  import android.view.ViewGroup;
  import android.widget.TextView;
  
  /**
   * A fragment representing a single step in a wizard. The fragment shows a dummy title indicating
   * the page number, along with some dummy text.
   *
   * &lt;p&gt;This class is used by the {@link CardFlipActivity} and {@link
   * ScreenSlideActivity} samples.&lt;/p&gt;
   */
  public class ScreenSlidePageFragment extends Fragment {
      /**
       * The argument key for the page number this fragment represents.
       */
      public static final String ARG_PAGE = "page";
  
      /**
       * The fragment's page number, which is set to the argument value for {@link #ARG_PAGE}.
       */
      private int mPageNumber;
  
      /**
       * Factory method for this fragment class. Constructs a new fragment for the given page number.
       */
      public static ScreenSlidePageFragment create(int pageNumber) {
          ScreenSlidePageFragment fragment = new ScreenSlidePageFragment();
          Bundle args = new Bundle();
          args.putInt(ARG_PAGE, pageNumber);
          fragment.setArguments(args);
          return fragment;
      }
  
      public ScreenSlidePageFragment() {
      }
  
      @Override
      public void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          mPageNumber = getArguments().getInt(ARG_PAGE);
      }
  
      @Override
      public View onCreateView(LayoutInflater inflater, ViewGroup container,
              Bundle savedInstanceState) {
          // Inflate the layout containing a title and body text.
          ViewGroup rootView = (ViewGroup) inflater
                  .inflate(R.layout.fragment_screen_slide_page, container, false);
  
          // Set the title view to show the page number.
          ((TextView) rootView.findViewById(android.R.id.text1)).setText(
                  getString(R.string.title_template_step, mPageNumber + 1));
  
          return rootView;
      }
  
      /**
       * Returns the page number represented by this fragment object.
       */
      public int getPageNumber() {
          return mPageNumber;
      }
  }
  </code></pre>
  
  <p>Thanks</p>