探索PagerView

步骤

在主activity布局添加android.support.v4.view.ViewPager控件

创建几个页面布局

将页面布局与Fragment绑定，将Fragment添加进容器

也可以不绑定，类似于以下PagerAdapter上编写方式，直接获取View

创建适配器，在容器中取出Fragment，然后将其显示在ViewPager中

适配器

FragmentPagerAdapter

该适配器会将生成的每一个Fragment保存在内存中，因此适用于PagerView页面不多的情况

eg：

private class ViewPagerAdapter extends FragmentPagerAdapter{
  private ViewPagerAdapter(FragmentManager fm) {
      super(fm);
  }
  @Override
  public int getCount() {
      return pagers.size();
  }
  @Override
  public Fragment getItem(int position) {
      return pagers.get(position);
  }
}

只需重写

1. 带FragmentManager参数的构造方法

2. getCount

3. getItem

Fragment页面生命周期：

进入第一个页面：

PagerFragmentOne: onAttach
    		      onCreate
PagerFragmentTwo: onAttach
PagerFragmentTwo: onCreate
PagerFragmentOne: onCreateView
PagerFragmentOne: onActivityCreated
    		      onStart
    	 	     onResume
PagerFragmentTwo: onCreateView
PagerFragmentTwo: onActivityCreated
    		      onStart
    		      onResume

进入第二个页面：

PagerFragmentThird: onAttach
                    onCreate
PagerFragmentThird: onCreateView
PagerFragmentThird: onActivityCreated
PagerFragmentThird: onStart
                    onResume

进入第三个页面：

PagerFragmentOne: onPause
PagerFragmentOne: onStop
                  onDestroyView

由上可知ViewPagerAdapter会将左右两个页面预先加载（执行到onResume），当翻页到某页不在该页左右，则该页视图会被销毁（执行到onDestroyView），但不会销毁Fragment对象

FragmentStatePagerAdapter

该适配器不会将Fragment保存在内存中，适合较多页面的PagerView

eg:

private class ViewPagerAdapterr extends android.support.v4.app.FragmentStatePagerAdapter{
  public ViewPagerAdapterr(FragmentManager fm) {
      super(fm);
  }
  @Override
  public int getCount() {
      return pagers.size();
  }
  @Override
  public Fragment getItem(int position) {
      return pagers.get(position);
  }
}

与FragmentPagerAdapter一样需要重写三个方法

Fragment的生命周期：

与FragmentPagerAdapter不同之处在于当翻页到某页不在该页左右，则该页Fragment销毁（执行到onDetch）

ViewPagerAdapter

至少需要重写

1. instantiateItem

2. isViewFromObjec

3. getCount

   eg:

     private class PagerAdapterr extends PagerAdapter{
       public PagerAdapterr() {
           super();
       }
       @Override
       public int getCount() {
           return pagers.size();
       }
       @NonNull
       @Override
       public Object instantiateItem(@NonNull ViewGroup container, int position) {
           container.addView(views.get(position));
           Log.i(TAG,"instantiateItem"+position);
           return views.get(position);
       }
       @Override
       public boolean isViewFromObject(@NonNull View view, @NonNull Object object) {
           return view==object;
       }
   
       @Override
       public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
           container.removeView(views.get(position));
           Log.i(TAG,"destroyItem"+position);
       }
   }

   instantiateItem创建显示的视图

   isViewFromObject判断当前视图是否与object对象相同

   destroyItem销毁指定视图

   生命周期：

   位于第一页：

   ViewPagerActivity: instantiateItem0
                         instantiateItem1

   滑向第二页：

   ViewPagerActivity: instantiateItem2

   滑向第三页：

   ViewPagerActivity: destroyItem0

ViewPager的优化和封装

见lller库的使用文章中的ViewPageRer的使用

疑问

List<Fragment> fragments=new ArrayList<>();
fragments.add(new FirstFragment());
fragments.add(new SecondFragment());

public void setPages(List<Fragment> fragments, List<View> views){
  this.fragments=fragments;
  this.views=views;
}
private class FragmentPagerAdapterr extends FragmentPagerAdapter {
  private FragmentPagerAdapterr(FragmentManager fm) {
      super(fm);
  }
  @Override
  public int getCount() {
      Log.i(TAG,"fragments.size()："+fragments.size());
      return fragments.size();
  }
  @Override
  public Fragment getItem(int position) {
      Log.i(TAG,"position："+position);
      return fragments.get(position);
  }
}

//FirstFragment.class
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
  Log.i(TAG,"onCreateView");
  //若传入container会出错
  return inflater.inflate(R.layout.fragment_first,null);
}

07-31 15:00:13.732 13036-13036/? E/AndroidRuntime: at android.view.ViewGroup.jumpDrawablesToCurrentState(ViewGroup.java:6505)
at android.view.ViewGroup.jumpDrawablesToCurrentState(ViewGroup.java:6505)
at android.view.ViewGroup.jumpDrawablesToCurrentState(ViewGroup.java:6505)
at android.view.ViewGroup.jumpDrawablesToCurrentState(ViewGroup.java:6505)
at android.view.ViewGroup.jumpDrawablesToCurrentState(ViewGroup.java:6505)
at android.view.ViewGroup.jumpDrawablesToCurrentState(ViewGroup.java:6505)
…………..