android实现ViewPager懒加载的三种方法

public class FragmentSample extends Fragment{
  ... 
  @Override
  public void setUserVisibleHint(boolean isVisibleToUser) {
    super.setUserVisibleHint(isVisibleToUser);
    if (isVisibleToUser) {
      requestData(); // 在此请求数据
    }
  }
  ...
}

// 注意，这是直接拷贝的ViewPager的源码，只修改了注释处的代码
public class LazyViewPager extends ViewGroup {
 private static final String TAG = "LazyViewPager";
 private static final boolean DEBUG = false;
 private static final boolean USE_CACHE = false;
   // 默认为1，即前后各预加载一个页面，设置为0去掉预加载
   private static final int DEFAULT_OFFSCREEN_PAGES = 0;
 private static final int MAX_SETTLE_DURATION = 600; // ms
 static class ItemInfo {
 Object object;
 int position;
 boolean scrolling;
 }
 private static final Comparator COMPARATOR = new Comparator() {
 @Override
 public int compare(ItemInfo lhs, ItemInfo rhs) {
  return lhs.position - rhs.position;
 }
 };
   ............
}

public class LazyViewPager extends ViewPager {
 private static final float DEFAULT_OFFSET = 0.5f;
 private LazyPagerAdapter mLazyPagerAdapter;
 private float mInitLazyItemOffset = DEFAULT_OFFSET;
 public LazyViewPager(Context context) {
 super(context);
 }
 public LazyViewPager(Context context, AttributeSet attrs) {
 super(context, attrs);
 TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.LazyViewPager);
 setInitLazyItemOffset(a.getFloat(R.styleable.LazyViewPager_init_lazy_item_offset, DEFAULT_OFFSET));
 a.recycle();
 }
  /**
   * change the initLazyItemOffset
   * @param initLazyItemOffset set mInitLazyItemOffset if {@code 0  0 && initLazyItemOffset <= 1) {
   mInitLazyItemOffset = initLazyItemOffset;
    }
 }
 @Override
 public void setAdapter(PagerAdapter adapter) {
 super.setAdapter(adapter);
    mLazyPagerAdapter = adapter != null && adapter instanceof LazyPagerAdapter &＃63; (LazyPagerAdapter) adapter : null;
 }
 @Override
 protected void onPageScrolled(int position, float offset, int offsetPixels) {
 if (mLazyPagerAdapter != null) {
  if (getCurrentItem() == position) {
  int lazyPosition = position + 1;
  if (offset >= mInitLazyItemOffset && mLazyPagerAdapter.isLazyItem(lazyPosition)) {
          mLazyPagerAdapter.startUpdate(this);
          mLazyPagerAdapter.addLazyItem(this, lazyPosition);
          mLazyPagerAdapter.finishUpdate(this);
  }
  } else if (getCurrentItem() > position) {
  int lazyPosition = position;
  if (1 - offset >= mInitLazyItemOffset && mLazyPagerAdapter.isLazyItem(lazyPosition)) {
          mLazyPagerAdapter.startUpdate(this);
          mLazyPagerAdapter.addLazyItem(this, lazyPosition);
          mLazyPagerAdapter.finishUpdate(this);
  }
  }
 }
 super.onPageScrolled(position, offset, offsetPixels);
 }
}

public abstract class LazyFragmentPagerAdapter extends LazyPagerAdapter {
 private static final String TAG = "LazyFragmentPagerAdapter";
 private static final boolean DEBUG = false;
 private final FragmentManager mFragmentManager;
 private FragmentTransaction mCurTransaction = null;
 public LazyFragmentPagerAdapter(FragmentManager fm) {
 mFragmentManager = fm;
 }
 @Override
 public void startUpdate(ViewGroup container) {
 }
 @Override
 public Object instantiateItem(ViewGroup container, int position) {
 if (mCurTransaction == null) {
  mCurTransaction = mFragmentManager.beginTransaction();
 }
 final long itemId = getItemId(position);
 // Do we already have this fragment&＃63;
 String name = makeFragmentName(container.getId(), itemId);
 Fragment fragment = mFragmentManager.findFragmentByTag(name);
 if (fragment != null) {
  if (DEBUG)
  Log.v(TAG, "Attaching item #" + itemId + ": f=" + fragment);
  mCurTransaction.attach(fragment);
 } else {
  fragment = getItem(container, position);
  if (fragment instanceof Laziable) {
  mLazyItems.put(position, fragment);
  } else {
  mCurTransaction.add(container.getId(), fragment, name);
  }
 }
 if (fragment != getCurrentItem()) {
  fragment.setMenuVisibility(false);
  fragment.setUserVisibleHint(false);
 }
 return fragment;
 }
 @Override
 public void destroyItem(ViewGroup container, int position, Object object) {
 if (mCurTransaction == null) {
  mCurTransaction = mFragmentManager.beginTransaction();
 }
 if (DEBUG)
  Log.v(TAG, "Detaching item #" + getItemId(position) + ": f=" + object + " v=" + ((Fragment) object).getView());
 final long itemId = getItemId(position);
 String name = makeFragmentName(container.getId(), itemId);
 if (mFragmentManager.findFragmentByTag(name) == null) {
  mCurTransaction.detach((Fragment) object);
 } else {
      mLazyItems.remove(position);
 }
 }
  @Override
 public Fragment addLazyItem(ViewGroup container, int position) {
 Fragment fragment = mLazyItems.get(position);
 if (fragment == null)
  return null;
 final long itemId = getItemId(position);
 String name = makeFragmentName(container.getId(), itemId);
 if (mFragmentManager.findFragmentByTag(name) == null) {
  if (mCurTransaction == null) {
  mCurTransaction = mFragmentManager.beginTransaction();
  }
  mCurTransaction.add(container.getId(), fragment, name);
      mLazyItems.remove(position);
 }
    return fragment;
 }
 @Override
 public void finishUpdate(ViewGroup container) {
 if (mCurTransaction != null) {
  mCurTransaction.commitAllowingStateLoss();
  mCurTransaction = null;
  mFragmentManager.executePendingTransactions();
 }
 }
  @Override
 public boolean isViewFromObject(View view, Object object) {
 return ((Fragment) object).getView() == view;
 }
 public long getItemId(int position) {
 return position;
 }
 private static String makeFragmentName(int viewId, long id) {
 return "android:switcher:" + viewId + ":" + id;
 }
  /**
   * mark the fragment can be added lazily
   */
  public interface Laziable {
  }
}