ViewPager实现Recycle机制和响应notifyDataSetChanged
1.目标 主界面要求水平移动翻页效果，每次只能翻一页，可以翻无数页。 2.实现思路 针对“每次只能翻一页”这个要求，简单使用SDK的话只有用ViewPager。ViewPager的PageAdapter是没有实现RecyclerView的ViewHolder.itemView回收机制的。即使是子类FragmentStatePagerAdapter，也只是保存状态后销毁Fragment，Fragment本身是在不断地创建和销毁，没有重复利用。 正确地使用PageAdapter，必须先理解Object instantiateItem (ViewGroup container, int position)函数中返回的Object的意义。如果View能重复利用，则表明它仅用于展示数据，没有业务逻辑。数据的部分，我们通常用Model表示。instantiateItem函数的返回值Object，正是要求返回在position位置的Model数据对象，与实现这个函数时创建的View形成映射关系。 3.自定义PageAdapter 直接在代码注释中讲解： // 转载请注明出处：http://blog.csdn.net/hursing private class ViewPageAdapter extends PagerAdapter { // 用List保存本来要销毁的View，需要的时候再取出来 private List<ItemView> mViewRecycler = new LinkedList<>(); // 用Map保存Model Object（这里是OnePiece）和View的映射关系 private Map<OnePiece, ItemView> mOnePieceItemViewMap = new HashMap<>(); @Override public int getCount() { // Model保存着OnePiece的对象数组 return mPieceModel.pieces.size(); } @Override public boolean isViewFromObject(View view, Object object) { // 如果不需要回收利用View，都是用`return view == object`实现的， // 与此对应`instantiateItem()`返回View， // `destroyItem()`要销毁View。 OnePiece onePiece = (OnePiece) object; return view == mOnePieceItemViewMap.get(onePiece); } @Override public Object instantiateItem (ViewGroup container, int position) { ItemView itemView; // 这个if else是实现回收利用的关键处之一。如果有缓存则取缓存， // 没有则创建对象 if (mViewRecycler.size() > 0) { itemView = mViewRecycler.remove(0); } else { itemView = (ItemView) LayoutInflater.from(MainActivity.this) .inflate(R.layout.item_view, container, false); } // container实际是ViewPager对象。从这个函数的实现看， // ViewPager并不知道对它add的View是做什么用的，所以 // 需要`isViewFromObject()`函数来询问这个View是否有 // 意义（是ItemView），如果是，后面会对它做正确layout container.addView(itemView); OnePiece onePiece = mPieceModel.pieces.get(position); // 这里是itemView根据Model object来重新设置界面元素的时机。 itemView.setOnePiece(onePiece); mOnePieceItemViewMap.put(onePiece, itemView); // 如果不需要回收利用View，这里的实现是`return itemView`。 // 要回收，就要建立Model object和View的映射关系，因此 // 这里返回Model object return onePiece; } @Override public void destroyItem (ViewGroup container, int position, Object object) { OnePiece onePiece = (OnePiece) object; ItemView itemView = mOnePieceItemViewMap.remove(onePiece); // 必须自行remove itemView container.removeView(itemView); // itemView要被回收放入缓存了，这里提供一个时机做清理。 itemView.reset(); // 实现回收的关键处，把itemView保存到List里。而不是任由GC销毁。 mViewRecycler.add(itemView); } // 必须重写此函数。super固定地`return POSITION_UNCHANGED`， // 这就会导致`notifyDataSetChanged()`没有效果，因为它的意义 // 是说这个Model object的位置没变化，自然不需要刷新。 @Override public int getItemPosition(Object object) { OnePiece onePiece = (OnePiece) object; if (mPieceModel.pieces.contains(onePiece)) { return mPieceModel.pieces.indexOf(onePiece); } else { // 返回POSITION_NONE说明这个model object已经被废弃了， // 接下来就会由`destroyItem()`回调来销毁它对应的View。 return POSITION_NONE; } } } 4.完整代码 代码放在 https://github.com/hursing/ViewPagerDemo 界面结构：5个按钮，分别是增、删、重置Model object。下面是ViewPager，按完按钮滚动区域会有变化。每一页有三个TextView，其中最上面的是显示ItemView本身的引用信息，留意它的引用地址（如截图中的12f46ad2），会发现最多创建4个ItemView（缓存了左、右、当前，还有一个预备），再怎么滑都会重复利用。 截图： 下面是贴些代码，可不看了： OnePiece.java public class OnePiece { public char letter; public int number; public OnePiece(char letter, int number) { this.letter = letter; this.number = number; } } item_view.xml <com.example.liuhx.viewpagerdemo.ItemView xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:background="#ccc" android:orientation="vertical" android:paddingTop="40dp" android:paddingBottom="40dp" android:paddingLeft="20dp" android:paddingRight="20dp"> <TextView android:id="@+id/this_info" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="reference info" /> <TextView android:id="@+id/text1" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="sample1" /> <TextView android:id="@+id/text2" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="sample2" /> </com.example.liuhx.viewpagerdemo.ItemView> ItemView.java public class ItemView extends LinearLayout { private TextView mTextView1; private TextView mTextView2; private OnePiece mOnePiece; public ItemView(Context context, AttributeSet attrs) { super(context, attrs); } public void setOnePiece(OnePiece onePiece) { mOnePiece = onePiece; mTextView1.setText(String.valueOf(onePiece.letter)); mTextView2.setText(String.valueOf(onePiece.number)); } public void reset() { // Release resource here: network connection, database cursor, ... mOnePiece = null; } @Override public void onFinishInflate() { // To differentiate View objects. ((TextView) findViewById(R.id.this_info)).setText(this.toString()); mTextView1 = (TextView) findViewById(R.id.text1); mTextView2 = (TextView) findViewById(R.id.text2); } } PieceModel.java public class PieceModel { public interface OnPiecesChangedListener { void onPiecesChanged(); } public List<OnePiece> pieces = new LinkedList<>(); private OnPiecesChangedListener mOnPiecesChangedListener; private char mLetterSeed = 'A'; private int mNumberSeed = 0; public void setOnPiecesChangedListener(OnPiecesChangedListener listener) { mOnPiecesChangedListener = listener; } public void addPiece(boolean toTail) { OnePiece onePiece = new OnePiece(mLetterSeed, mNumberSeed++); if (toTail) pieces.add(onePiece); else pieces.add(0, onePiece); mOnPiecesChangedListener.onPiecesChanged(); } public void removePiece(boolean fromTail) { if (fromTail) pieces.remove(pieces.size() - 1); else pieces.remove(0); mOnPiecesChangedListener.onPiecesChanged(); } public void changePieces() { ++mLetterSeed; mNumberSeed = 0; int size = pieces.size(); pieces.clear(); for (; mNumberSeed < size; ++mNumberSeed) { OnePiece onePiece = new OnePiece(mLetterSeed, mNumberSeed); pieces.add(onePiece); } mOnPiecesChangedListener.onPiecesChanged(); } } activity_main.xml <?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:id="@+id/activity_main" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical" tools:context="com.example.liuhx.viewpagerdemo.MainActivity"> <LinearLayout android:layout_width="match_parent" android:layout_height="wrap_content"> <Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_weight="1" android:id="@+id/add_head" android:text="add head"/> <Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_weight="1" android:id="@+id/add_tail" android:text="add tail"/> <Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_weight="1" android:id="@+id/remove_head" android:text="remove head"/> <Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_weight="1" android:id="@+id/remove_tail" android:text="remove tail"/> <Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_weight="1" android:id="@+id/change" android:text="change"/> </LinearLayout> <android.support.v4.view.ViewPager android:layout_width="match_parent" android:layout_height="match_parent" android:id="@+id/view_pager" /> </LinearLayout> MainActivity.java public class MainActivity extends AppCompatActivity implements View.OnClickListener { private ViewPager mViewPager; private ViewPageAdapter mViewPageAdapter; private PieceModel mPieceModel; private class ViewPageAdapter extends PagerAdapter { private List<ItemView> mViewRecycler = new LinkedList<>(); private Map<OnePiece, ItemView> mOnePieceItemViewMap = new HashMap<>(); @Override public int getCount() { return mPieceModel.pieces.size(); } @Override public boolean isViewFromObject(View view, Object object) { OnePiece onePiece = (OnePiece) object; return view == mOnePieceItemViewMap.get(onePiece); } @Override public Object instantiateItem (ViewGroup container, int position) { ItemView itemView; if (mViewRecycler.size() > 0) { itemView = mViewRecycler.remove(0); } else { itemView = (ItemView) LayoutInflater.from(MainActivity.this) .inflate(R.layout.item_view, container, false); } container.addView(itemView); OnePiece onePiece = mPieceModel.pieces.get(position); itemView.setOnePiece(onePiece); mOnePieceItemViewMap.put(onePiece, itemView); return onePiece; } @Override public void destroyItem (ViewGroup container, int position, Object object) { OnePiece onePiece = (OnePiece) object; ItemView itemView = mOnePieceItemViewMap.remove(onePiece); container.removeView(itemView); itemView.reset(); mViewRecycler.add(itemView); } // Must override this method, or else notifyDataSetChanged() has no effect. @Override public int getItemPosition(Object object) { OnePiece onePiece = (OnePiece) object; if (mPieceModel.pieces.contains(onePiece)) { return mPieceModel.pieces.indexOf(onePiece); } else { return POSITION_NONE; } } } @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); mPieceModel = new PieceModel(); mPieceModel.setOnPiecesChangedListener(new PieceModel.OnPiecesChangedListener() { @Override public void onPiecesChanged() { mViewPageAdapter.notifyDataSetChanged(); } }); mViewPager = (ViewPager) findViewById(R.id.view_pager); mViewPageAdapter = new ViewPageAdapter(); mViewPager.setAdapter(mViewPageAdapter); mViewPager.setOffscreenPageLimit(1); findViewById(R.id.add_head).setOnClickListener(this); findViewById(R.id.add_tail).setOnClickListener(this); findViewById(R.id.remove_head).setOnClickListener(this); findViewById(R.id.remove_tail).setOnClickListener(this); findViewById(R.id.change).setOnClickListener(this); } @Override public void onClick(View view) { switch (view.getId()) { case R.id.add_head: mPieceModel.addPiece(false); break; case R.id.add_tail: mPieceModel.addPiece(true); break; case R.id.remove_head: if (mPieceModel.pieces.size() == 0) { Toast.makeText(this, "Please press ADD first", Toast.LENGTH_SHORT).show(); } else { mPieceModel.removePiece(false); } break; case R.id.remove_tail: if (mPieceModel.pieces.size() == 0) { Toast.makeText(this, "Please press ADD first", Toast.LENGTH_SHORT).show(); } else { mPieceModel.removePiece(true); } break; case R.id.change: if (mPieceModel.pieces.size() == 0) { Toast.makeText(this, "Please press ADD first", Toast.LENGTH_SHORT).show(); } else { mPieceModel.changePieces(); } break; default: break; } } } 转载请注明出处：http://blog.csdn.net/hursing
