ViewPager+Fragment实现懒加载

主要实现是复写Fragment下面这个方法，isVisibleToUser这个参数下面有解释：

/** * Set a hint to the system about whether this fragment‘s UI is currently visible * to the user. This hint defaults to true and is persistent across fragment instance * state save and restore. * * <p>An app may set this to false to indicate that the fragment‘s UI is * scrolled out of visibility or is otherwise not directly visible to the user. * This may be used by the system to prioritize operations such as fragment lifecycle updates * or loader ordering behavior.</p> * * @param isVisibleToUser true if this fragment‘s UI is currently visible to the user (default), *                        false if it is not. */

public void setUserVisibleHint(boolean isVisibleToUser) {
    if (!mUserVisibleHint && isVisibleToUser && mState < STARTED) {
        mFragmentManager.performPendingDeferredStart(this);
    }
    mUserVisibleHint = isVisibleToUser;
    mDeferStart = !isVisibleToUser;
}

注意，这个方法并不是fragment生命周期的一部分，而是Fragment与Viewpager一起使用时，ViewPager设置适配器--FragmentPagerAdapter调用的:

@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(position);
            if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment);
            mCurTransaction.add(container.getId(), fragment,
                    makeFragmentName(container.getId(), itemId));
        }
        if (fragment != mCurrentPrimaryItem) {
            fragment.setMenuVisibility(false);
            fragment.setUserVisibleHint(false);
        }

        return fragment;
    }

　实现适配器的时候，注意记得调用super就OK，不然这个方法不能触发。