【转】实现ViewPager懒加载的三种方法

方法一

在Fragment可见时请求数据。此方案仍预加载了前后的页面，但是没有请求数据，只有进入到当前Framgent时才请求数据。

优点：实现了数据的懒加载
缺点：一次仍是三个Framgment对象，不是完全意义的懒加载

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

方法二

直接修改ViewPager源码。通过查看ViewPager源码可知，控制其预加载的是一个常量 DEFAULT_OFFSCREEN_PAGES，其默认值为1，表示当前页面前后各预加载一个页面，在这里我们直接将其设置为0即可，即去掉预加载。但是，这样有一个问题，那就是在使用其他控件时需要传入ViewPager时，这个就不能用了。

优点：完全屏蔽掉了预加载
缺点：应用太受限制，比如使用ViewPagerIndicator时需要传入ViewPager对象，这时傻眼了。

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// 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<ItemInfo> COMPARATOR = new Comparator<ItemInfo>() { @Override public int compare(ItemInfo lhs, ItemInfo rhs) { return lhs.position - rhs.position; } }; ............ }

方法三

直接继承ViewPager，结合PagerAdapter实现懒加载。该方案是我用到的最完善的方法，完全的懒加载，每次只会建立一个Fragment对象。

优点：完全屏蔽预加载
缺点：稍微复杂，需要自定义，开源库地址

这个库就4个类，作者通过继承ViewPager（保证其普适性）、自定义ViewPagerAdapter和 LazyFragmentPagerAdapter以及设置懒加载的标记接口，很好的实现了懒加载。感谢作者。

在此贴出关键代码，有兴趣的同学可以学习下。

LazyViewPager:

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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 < initLazyItemOffset <= 1} */ public void setInitLazyItemOffset(float initLazyItemOffset) { if (initLazyItemOffset > 0 && initLazyItemOffset <= 1) { mInitLazyItemOffset = initLazyItemOffset; } } @Override public void setAdapter(PagerAdapter adapter) { super.setAdapter(adapter); mLazyPagerAdapter = adapter != null && adapter instanceof LazyPagerAdapter ? (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<Fragment> { 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? 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 { } }

友情提示：填充LazyViewPager的Fragment一定要实现接口LazyFragmentPagerAdapter.Laziable，其实就是一个标记。

原文链接：

http://answerzhao.github.io/2016/06/06/%E5%AE%9E%E7%8E%B0ViewPager%E6%87%92%E5%8A%A0%E8%BD%BD%E7%9A%84%E4%B8%89%E7%A7%8D%E6%96%B9%E6%B3%95/#

其他参考地址：

http://www.jianshu.com/p/104be7cd72b6

http://www.cnblogs.com/dasusu/p/5926731.html