Published by orzz.org(). (https://orzz.org/android-unlimited-viewpager/)
ViewPager是google SDK里用来做滑动式的视图翻页的控件。我们可以新建一个默认的Android Application，指定Navigation Type为“Swipe Views (ViewPager)”，可以很直观的看到ViewPager的用法。
首先，我们新建一个测试项目，Activity的选项如下：
向导会自动帮我们在activity_main.xml里写入如下内容：
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<android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:id="@+id/pager" android:layout_width="match_parent" android:layout_height="match_parent" tools:context="com.example.testviewpager.MainActivity" /> |
另外fragment_main.xml里会自动写上一个TextView：
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<TextView android:id="@+id/section_label" android:layout_width="wrap_content" android:layout_height="wrap_content" /> |
之后，MainActivity则是非常简单的，ViewPager里的每一个视图都是一个PlaceholderFragment：
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/** * A placeholder fragment containing a simple view. */ public static class PlaceholderFragment extends Fragment { /** * The fragment argument representing the section number for this * fragment. */ private static final String ARG_SECTION_NUMBER = "section_number"; /** * Returns a new instance of this fragment for the given section * number. */ public static PlaceholderFragment newInstance(int sectionNumber) { PlaceholderFragment fragment = new PlaceholderFragment(); Bundle args = new Bundle(); args.putInt(ARG_SECTION_NUMBER, sectionNumber); fragment.setArguments(args); return fragment; } public PlaceholderFragment() { } @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View rootView = inflater.inflate(R.layout.fragment_main, container, false); TextView textView = (TextView) rootView.findViewById(R.id.section_label); textView.setText(Integer.toString(getArguments().getInt(ARG_SECTION_NUMBER))); return rootView; } } |
然后通过一个自定义的FragmentPagerAdapter，为ViewPager提供视图：
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/** * A {@link FragmentPagerAdapter} that returns a fragment corresponding to * one of the sections/tabs/pages. */ public class SectionsPagerAdapter extends FragmentPagerAdapter { public SectionsPagerAdapter(FragmentManager fm) { super(fm); } @Override public Fragment getItem(int position) { // getItem is called to instantiate the fragment for the given page. // Return a PlaceholderFragment (defined as a static inner class below). return PlaceholderFragment.newInstance(position + 1); } @Override public int getCount() { // Show 3 total pages. return 3; } @Override public CharSequence getPageTitle(int position) { Locale l = Locale.getDefault(); switch (position) { case 0: return getString(R.string.title_section1).toUpperCase(l); case 1: return getString(R.string.title_section2).toUpperCase(l); case 2: return getString(R.string.title_section3).toUpperCase(l); } return null; } } |
最后，在MainActivity的onCreate函数中，如下代码负责把上面的PagerAdapter放到PagerAdapter里去：
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/** * The {@link android.support.v4.view.PagerAdapter} that will provide * fragments for each of the sections. We use a * {@link FragmentPagerAdapter} derivative, which will keep every * loaded fragment in memory. If this becomes too memory intensive, it * may be best to switch to a * {@link android.support.v4.app.FragmentStatePagerAdapter}. */ SectionsPagerAdapter mSectionsPagerAdapter; /** * The {@link ViewPager} that will host the section contents. */ ViewPager mViewPager; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); // Create the adapter that will return a fragment for each of the three // primary sections of the activity. mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager()); // Set up the ViewPager with the sections adapter. mViewPager = (ViewPager) findViewById(R.id.pager); mViewPager.setAdapter(mSectionsPagerAdapter); } |
最终实现出来的效果如下：
当然，上面是可以翻页的，一共3页，每一页上显示的都是当前的序号。
默认情况下，ViewPager可以显示的视图数目，和PagerAdapter的getCount方法的返回值有关，如果我们把getCount改成这样：
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@Override public int getCount() { // Show 3 total pages. return 5; } |
选项卡也会变成5个。虽然在这个例子里，暂时用不到PageTitle，不过后续若需要Title显示文字，getPageTitle是必须要显示正确的。
对于本例，我们可以简单的把strings.xml改成这样：
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<?xml version="1.0" encoding="utf-8"?> <resources> <string name="app_name">TestViewPager</string> <string name="title_section">Section</string> <string name="action_settings">Settings</string> </resources> |
接下来，改一改getPageTitle：
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@Override public CharSequence getPageTitle(int position) { Locale l = Locale.getDefault(); return getString(R.string.title_section).toUpperCase(l) + " " + (position + 1); } |
这样，通过一个position总能得到一个有效的PageTitle。为了看到修改的效果，我们可以让PlaceholderFragment显示PageTitle，而不止是一个数字：
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/** * A {@link FragmentPagerAdapter} that returns a fragment corresponding to * one of the sections/tabs/pages. */ public class SectionsPagerAdapter extends FragmentPagerAdapter { public SectionsPagerAdapter(FragmentManager fm) { super(fm); } @Override public Fragment getItem(int position) { // getItem is called to instantiate the fragment for the given page. // Return a PlaceholderFragment (defined as a static inner class below). return PlaceholderFragment.newInstance(getPageTitle(position).toString()); } @Override public int getCount() { // Show 3 total pages. return 5; } @Override public CharSequence getPageTitle(int position) { Locale l = Locale.getDefault(); return getString(R.string.title_section).toUpperCase(l) + " " + (position + 1); } } /** * A placeholder fragment containing a simple view. */ public static class PlaceholderFragment extends Fragment { /** * The fragment argument representing the section number for this * fragment. */ private static final String ARG_SECTION_NUMBER = "section_number"; /** * Returns a new instance of this fragment for the given section * number. */ public static PlaceholderFragment newInstance(String section) { PlaceholderFragment fragment = new PlaceholderFragment(); Bundle args = new Bundle(); args.putString(ARG_SECTION_NUMBER, section); fragment.setArguments(args); return fragment; } public PlaceholderFragment() { } @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View rootView = inflater.inflate(R.layout.fragment_main, container, false); TextView textView = (TextView) rootView.findViewById(R.id.section_label); textView.setText(getArguments().getString(ARG_SECTION_NUMBER)); return rootView; } } |
这样我们就有了5页的ViewPager，每一页显示当然页的PageTitle：
下面我们来实现无限多的页面。
可以考虑这样一种方法：getCount永远返回3，当我们进行翻页操作时，先全部更新所有页面上应当显示的数据，之后PagerAdapter自动调用setCurrentItem(1, false)将页面重新固定回中间那一页。这样可以让我们看起来好像是翻页了，但实际上仅仅是数据发生了变化而已。
首先，我们来实现这样一个UnlimitedPagerAdapter：
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public class UnlimitedPagerAdapter extends FragmentPagerAdapter { public interface UnlimitedPager { public void onRefresh(); public void onChanged(int offset); public Fragment getItem(int position); } private ViewPager mViewPager = null; private UnlimitedPager mPager = null; private boolean mIsChanged = false; public UnlimitedPagerAdapter(ViewPager viewPager, FragmentManager fm) { super(fm); mViewPager = viewPager; } public void setPage(UnlimitedPager pager) { mPager = pager; if (mPager != null) { mPager.onRefresh(); } } @Override public Fragment getItem(int position) { return mPager.getItem(position); } @Override public int getCount() { return 3; } @Override public void setPrimaryItem(ViewGroup container, int position, Object object) { super.setPrimaryItem(container, position, object); if (position == 1) { if (mIsChanged) { if (mPager != null) { mPager.onRefresh(); } mIsChanged = false; } } else { if (mPager != null) { mPager.onChanged(position - 1); mIsChanged = true; } mViewPager.setCurrentItem(1, false); } } } |
通过setPrimaryItem，在当前页面变化的时候更新数据，刷新显示内容，重新定位页面。同样的，PlaceholderFragment需要稍微调整一下：
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/** * A placeholder fragment containing a simple view. */ public static class PlaceholderFragment extends Fragment { /** * The fragment argument representing the section number for this * fragment. */ private static final String ARG_SECTION_NUMBER = "section_number"; /** * Returns a new instance of this fragment for the given section * number. */ public static PlaceholderFragment newInstance(int sectionNumber) { PlaceholderFragment fragment = new PlaceholderFragment(); Bundle args = new Bundle(); args.putInt(ARG_SECTION_NUMBER, sectionNumber); fragment.setArguments(args); return fragment; } public PlaceholderFragment() { } private View mRootView = null; public void setText(int sectionNumber) { getArguments().putInt(ARG_SECTION_NUMBER, sectionNumber); if (mRootView != null) { TextView textView = (TextView) mRootView.findViewById(R.id.section_label); textView.setText(getString(R.string.title_section).toUpperCase(Locale.getDefault()) + " " + sectionNumber); } } @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { mRootView = inflater.inflate(R.layout.fragment_main, container, false); setText(getArguments().getInt(ARG_SECTION_NUMBER)); return mRootView; } } |
这样我们就可以完成UnlimitedPagerAdapter.UnlimitedPager接口，通过PlaceholderFragment提供的setText方法修改页面上显示的数据：
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/** * A {@link FragmentPagerAdapter} that returns a fragment corresponding to * one of the sections/tabs/pages. */ public class SectionsPager implements UnlimitedPagerAdapter.UnlimitedPager { private final PlaceholderFragment[] sFragments = new PlaceholderFragment[] { PlaceholderFragment.newInstance(0), PlaceholderFragment.newInstance(1), PlaceholderFragment.newInstance(2) }; private int mData = 1; @Override public void onRefresh() { sFragments[0].setText(mData); sFragments[1].setText(mData + 1); sFragments[2].setText(mData + 2); } @Override public void onChanged(int offset) { mData += offset; } @Override public Fragment getItem(int position) { return sFragments[position]; } } |
如上，在需要的时候（每次ViewPager被翻页时），onChanged就会被调用，mData会被刷新掉。之后每次onRefresh被调用时，sFragments会根据当前的mData调整每个PlaceholderFragment视图应当如何显示。经过这样设计的类关系图如下所示：
之后，在MainActivity的onCreate里把SectionsPager、UnlimitedPagerAdapter和ViewPager连起来：
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/** * The {@link ViewPager} that will host the section contents. */ ViewPager mViewPager; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); // Set up the ViewPager with the sections adapter. mViewPager = (ViewPager) findViewById(R.id.pager); UnlimitedPagerAdapter adapter = new UnlimitedPagerAdapter(mViewPager, getSupportFragmentManager()); adapter.setPage(new SectionsPager()); mViewPager.setAdapter(adapter); } |
好了，大功告成了。现在我们得到了一个可以“无限”翻页的ViewPager。不过需要注意的是，这样的ViewPager和Action Bar Tabs是不能直接联动的，除非再设计一个“无限”的Action Bar Tabs。
使用上面的UnlimitedPager接口配合UnlimitedPagerAdapter，实际上也可以很轻松的实现循环翻页的ViewPager，只需要修改派生出来的SectionsPager，让数据模型形成环状就可以了。
参考文章：
Published by orzz.org(). (https://orzz.org/android-unlimited-viewpager/)