fragment+viewpager懒加载

网上都是通过从Fragment的setUserVisibleHint()和getUserVisibleHint()方法来入手的，但是不行，当你将viewpager滑动到第三个页面的时候，这时候你在滑到第一个页面的时候，你会发现第一个页面又被重新加载了，虽然做到了延迟加载，但是没有做到只加载一次的效果，无意间找到一位大神实现懒加载的方式，是从适配器方面入手实现的懒加载，并且实验后，发现既可以实现延迟加载，也能实现只加载一次的效果。

部分代码如下

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) {
            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();
        }

        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 {
    }

}

最后记得将自己写的Fragment实现LazyFragmentPagerAdapter.Laziable接口

github地址如下：
https://github.com/liang1075963999/lanjiazai