android开发分享详解Android ViewPager2中的缓存和复用机制

目录

• 1. 前言
• 2. 回顾recyclerview缓存机制
• 3. offscreenpagelimit原理
• 4. fragmentstateadapter原理以及缓存机制
• 4.1 简单使用
• 4.2 原理
• 5. 案例讲解回收机制
• 5.1 默认情况
• 5.2 offscreenpagelimit=1
• 总结

  //androidx.viewpager2:viewpager2:1.0.0@aar  //viewpager2.java  public void setoffscreenpagelimit(@offscreenpagelimit int limit) {      if (limit < 1 && limit != offscreen_page_limit_default) {          throw new illegalargumentexception(                  "offscreen page limit must be offscreen_page_limit_default or a number > 0");      }      moffscreenpagelimit = limit;      mrecyclerview.requestlayout();    }  

  //androidx.viewpager2:viewpager2:1.0.0@aar  //viewpager2$linearlayoutmanagerimpl  @override  protected void calculateextralayoutspace(@nonnull recyclerview.state state,          @nonnull int[] extralayoutspace) {      int pagelimit = getoffscreenpagelimit();      if (pagelimit == offscreen_page_limit_default) {          super.calculateextralayoutspace(state, extralayoutspace);          return;      }      final int offscreenspace = getpagesize() * pagelimit;      extralayoutspace[0] = offscreenspace;      extralayoutspace[1] = offscreenspace;  }  

  public abstract class fragmentstateadapter extends          recyclerview.adapter<fragmentviewholder> implements statefuladapter {      public abstract fragment createfragment(int position);  }  

  class viewpager2withfragmentsactivity : appcompatactivity() {      private lateinit var mviewpager2: viewpager2      override fun oncreate(savedinstancestate: bundle?) {          super.oncreate(savedinstancestate)          setcontentview(r.layout.activity_recycler_view_view_pager2)          mviewpager2 = findviewbyid(r.id.viewpager2)          (mviewpager2.getchildat(0) as recyclerview).layoutmanager?.apply {  //            isitemprefetchenabled = false          }          mviewpager2.orientation = viewpager2.orientation_vertical          mviewpager2.adapter = myadapter(this)  //        mviewpager2.offscreenpagelimit = 1      }        inner class myadapter(fragmentactivity: fragmentactivity) :          fragmentstateadapter(fragmentactivity) {          override fun getitemcount(): int {              return 100          }            override fun createfragment(position: int): fragment {              return myfragment("item $position")          }        }        class myfragment(val text: string) : fragment() {          init {              println("myfragment $text")          }          override fun oncreateview(              inflater: layoutinflater,              container: viewgroup?,              savedinstancestate: bundle?          ): view? {              var view = layoutinflater.inflate(r.layout.view_item_view_pager_snap, container)              view.findviewbyid<textview>(r.id.text_view).text = text              return view;          }      }  }    

  public final class fragmentviewholder extends viewholder {      private fragmentviewholder(@nonnull framelayout container) {          super(container);      }        @nonnull static fragmentviewholder create(@nonnull viewgroup parent) {          framelayout container = new framelayout(parent.getcontext());          container.setlayoutparams(                  new viewgroup.layoutparams(viewgroup.layoutparams.match_parent,                          viewgroup.layoutparams.match_parent));          container.setid(viewcompat.generateviewid());          container.setsaveenabled(false);          return new fragmentviewholder(container);      }        @nonnull framelayout getcontainer() {          return (framelayout) itemview;      }  }    

  final longsparsearray<fragment> mfragments = new longsparsearray<>();    private final longsparsearray<integer> mitemidtoviewholder = new longsparsearray<>();    

  @override  public final void onviewrecycled(@nonnull fragmentviewholder holder) {      final int viewholderid = holder.getcontainer().getid();      final long bounditemid = itemforviewholder(viewholderid); // item currently bound to the vh      if (bounditemid != null) {          removefragment(bounditemid);          mitemidtoviewholder.remove(bounditemid);      }  }  

  @override  public final void onbindviewholder(final @nonnull fragmentviewholder holder, int position) {      final long itemid = holder.getitemid();      final int viewholderid = holder.getcontainer().getid();      final long bounditemid = itemforviewholder(viewholderid); // item currently bound to the vh      if (bounditemid != null && bounditemid != itemid) {          removefragment(bounditemid);          mitemidtoviewholder.remove(bounditemid);      }        mitemidtoviewholder.put(itemid, viewholderid); // this might overwrite an existing entry      ensurefragment(position);        /** special case when {@link recyclerview} decides to keep the {@link container}       * attached to the window, but not to the view hierarchy (i.e. parent is null) */      final framelayout container = holder.getcontainer();      if (viewcompat.isattachedtowindow(container)) {          if (container.getparent() != null) {              throw new illegalstateexception("design assumption violated.");          }          container.addonlayoutchangelistener(new view.onlayoutchangelistener() {              @override              public void onlayoutchange(view v, int left, int top, int right, int bottom,                      int oldleft, int oldtop, int oldright, int oldbottom) {                  if (container.getparent() != null) {                      container.removeonlayoutchangelistener(this);                      placefragmentinviewholder(holder);                  }              }          });      }        gcfragments();  }