Android ViewPager Adapter 内存管理的坑
好久没有记录bug了的，因为最近一直在解bug... 之所以写这个文章，确实是出于无奈，毕竟同一个问题，项目上遇到了两次。这里主要是提一下 ViewPager + Fragment。
ViewPager + Fragment 有 '两种' Adapter
- FragmentPagerAdapter
https://developer.android.com/reference/android/support/v4/app/FragmentPagerAdapter
Implementation of[PagerAdapter](https://developer.android.com/reference/android/support/v4/view/PagerAdapter.html)that represents each page as a[Fragment](https://developer.android.com/reference/android/support/v4/app/Fragment.html)that is persistently kept in the fragment manager as long as the user can return to the page.
示例：
public static class MyAdapter extends FragmentPagerAdapter {
public MyAdapter(FragmentManager fm) {
super(fm);
}
@Override
public int getCount() {
return NUM_ITEMS;
}
@Override
public Fragment getItem(int position) {
return ArrayListFragment.newInstance(position);
}
}
实现上的核心代码：
@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;
}
@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);
}
- FragmentStatePagerAdapter
https://developer.android.com/reference/android/support/v4/app/FragmentStatePagerAdapter
Implementation of[PagerAdapter](https://developer.android.com/reference/android/support/v4/view/PagerAdapter.html)that uses a[Fragment](https://developer.android.com/reference/android/support/v4/app/Fragment.html)to manage each page. This class also handles saving and restoring of fragment's state.
示例：
public static class MyAdapter extends FragmentStatePagerAdapter {
public MyAdapter(FragmentManager fm) {
super(fm);
}
@Override
public int getCount() {
return NUM_ITEMS;
}
@Override
public Fragment getItem(int position) {
return ArrayListFragment.newInstance(position);
}
}
实现上的核心代码：
@Override
public Object instantiateItem(ViewGroup container, int position) {
// If we already have this item instantiated, there is nothing
// to do. This can happen when we are restoring the entire pager
// from its saved state, where the fragment manager has already
// taken care of restoring the fragments we previously had instantiated.
if (mFragments.size() > position) {
Fragment f = mFragments.get(position);
if (f != null) {
return f;
}
}
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
Fragment fragment = getItem(position);
if (DEBUG) Log.v(TAG, "Adding item #" + position + ": f=" + fragment);
if (mSavedState.size() > position) {
Fragment.SavedState fss = mSavedState.get(position);
if (fss != null) {
fragment.setInitialSavedState(fss);
}
}
while (mFragments.size() <= position) {
mFragments.add(null);
}
fragment.setMenuVisibility(false);
mFragments.set(position, fragment);
mCurTransaction.add(container.getId(), fragment);
return 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);
}
相同点： 二者在Adapter初始化上看上去没什么差别，都是基于 PagerAdapter 实现，只需要重写 getItem 返回每一页对应的 fragment 即可。
重点说一下不同点： 就像官方描述那样，我就不做翻译了的，从内部实现也确实印证了这一点.
前者 FragmentPagerAdapter 通过 mFragmentManager.findFragmentByTag(name) 从视图上把之前的视图找出来，然后执行 attach, 找不到就回调 getItem 问实现方要 fragment， 然而这个attach 有一个很隐藏的坑，我们都知道viewpager可以设置offsetLimit, 也就是说会销毁limit之外的page，然而在使用 FragmentPagerAdapter 你会发现，当你再次滑动回来的时候，因为之前 destroyItem 仅仅走的是 detach方法，所以通过 findFragmentByTag(name) 依旧能找到上次使用的 fragment，但此时由于 ViewPager 本身实现层面上已经把它踢出了的，所以当走 mCurTransaction.attach(fragment) 的时候会走 fragment的 onCreateView方法重新loadView, 但是由于这个'fragment'实例还是上次初始化的那个实例，意味着其实数据(成员变量都在)， 此时如果我们不重置一下数据的话就会出现一些逻辑上的bug. 所以网上就有人说可以这么搞：
override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?, savedInstanceState: Bundle?): View? {
//if (rootView == null) {
rootView = inflater?.inflate(getRootViewLayoutResId(), container, false)
/*} else run {
val p = rootView!!.getParent() as? ViewGroup
if (p != null) {
p.removeAllViewsInLayout()
}
}*/
简单的说就是缓存一下上一次使用的view, 下次重新加载的时候就把缓存的rootView 重新添加到屏幕上，然而这种方案会不会让我们的内存占比比较大呢？
后者 FragmentStatePagerAdapter 是用一个数组ArrayList<Fragment>去管理fragment，当走到destroyItem的时候fragment 会从这个ArrayList<Fragment>中踢出掉:
mFragments.set(position, null);
所以每次确实是重新创建一个fragment 实例。但这句话也不完全正确，因为还有一个关键信息是State. 我们可以看到 FragmentStatePagerAdapter中有一个ArrayList<Fragment.SavedState> mSavedState来管理页面的状态，当走到 destroyItem 的时候会保留即将销毁的这个fragment的状态：
mSavedState.set(position, mFragmentManager.saveFragmentInstanceState(fragment));
总结
当我们在使用两个adapter切记要管理好fragment的状态。。。。
