FragmentStatePagerAdapter与Fragme

FragmentStatePagerAdapter

• FragmentStatePagerAdapter会销毁不需要的fragment。

@Override
public void destroyItem(ViewGroup container, int position, Object object) {
    Fragment fragment = (Fragment)object;
    if (mCurTransaction == null) {
        mCurTransaction = mFragmentManager.beginTransaction(); 
    }
    if (DEBUG) Log.v(TAG, "Removing item #" + position + ": f=" + object + " v=" + ((Fragment)object).getView()); 
    while (mSavedState.size() <= position) {
        mSavedState.add(null);    
    }
    mSavedState.set(position, mFragmentManager.saveFragmentInstanceState(fragment));
    mFragments.set(position, null);
    mCurTransaction.remove(fragment);
}

上面是FragmentStatePagerAdapter中destroyItem方法的源码，其中调用remove(Fragment)将fragment从FragmentManager中彻底移除。

• FragmentStatePagerAdapter类名中的“state”表明：在销毁fragment时，可在fragment的onSaveInstanceState(Bundle)方法中保存fragment的状态信息。在用户切换回来时，保存的实例状态可用来恢复生成新的fragment。

//在销毁fragment之前，FragmentManager会保存fragment的Bundle信息。
mSavedState.set(position, mFragmentManager.saveFragmentInstanceState(fragment));

FragmentPagerAdapter

• 对于不再需要的fragment，FragmentPagerAdapter会调用detach(Fragment)方法来处理，而不是remove(Fragment)。所以FragmentPagerAdapter只是销毁了fragment的视图，fragment实例还保留在FragmentManager中。因此，FragmentPagerAdapter创建的fragment永远不会被销毁。

//FragmentPagerAdapter中的源码
@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());
    mCurTransaction.detach((Fragment)object);
}