FragmentStatePagerAdapter的刷新问题

众所周知，FragmentStatePagerAdapter是谷歌官方专门为Fragment和ViewPager推出的Adapter，其特点是为Fragment提供缓存，避免重复加载Fragment。这个我们从源代码就可以看得出来：

@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) {
        // 存在缓存表示该Fragment已经加载过了
        // 值得注意的是，这个地方Fragment的缓存是与position绑定在一起的
            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未加载，但是存在缓存的状态信息
                fragment.setInitialSavedState(fss);
            }
        }
        while (mFragments.size() <= position) {
            mFragments.add(null);
        }
        fragment.setMenuVisibility(false);
        fragment.setUserVisibleHint(false);
        mFragments.set(position, fragment);
        mCurTransaction.add(container.getId(), fragment);

        return fragment;
    }

这就是FragmentStatePagerAdapter加载缓存的代码，其中两个关键点，我都加了中文注释。在这里我要讲的是第一个地方，也就是Fragment是和position绑定在一起的，这样的话就会有一个问题，当我们想要隐藏或者显示(插入)其中某一个Fragment，我们就必须要让所有的Fragment重新走一遍生命周期。

这是因为，想要刷新ViewPager的Fragment必须修改getItemPosition(Object object)这个方法，使其返回POSITION_NONE，但是这样的话，所有的Fragment都会先被remove，然后再重新add。

    /**
     * Called when the host view is attempting to determine if an item's position
     * has changed. Returns {@link #POSITION_UNCHANGED} if the position of the given
     * item has not changed or {@link #POSITION_NONE} if the item is no longer present
     * in the adapter.
     */
    public int getItemPosition(Object object) {
        return POSITION_UNCHANGED;
    }

然儿这并不是我们想要，因此就有必要寻找一种替代方案。我们依然可以从getItemPosition这个方法入手，getItemPosition返回值表示的是page的位置，也就是说我们可以用这个方法来调整page的位置，而fragment也不需要重新加载。

既然知道问题在哪，只要对症下药就好了。修改一下FragmentStatePagerAdapter的部分逻辑即可，具体代码如下：

  /**
 * Description: Copied from android.support.v4.app.FragmentStatePagerAdapter, Fix bug for that fragments cache not refreshed when adapter items's position changed.
 * FixBug#修复Fragment的位置更新而缓存未更新导致显示错误以及crash的问题
 * Author: xuqingqi
 * E-mail: [email protected]
 * Date: 2017/12/8
 */
@Keep
public abstract class FragmentStatePagerAdapterCompat extends PagerAdapter {

    private static final String TAG = FragmentStatePagerAdapterCompat.class.getSimpleName();
    private static final boolean DEBUG = false;

    private final FragmentManager mFragmentManager;
    private FragmentTransaction mCurTransaction = null;

    private SparseArray mSavedState = new SparseArray();
    private SparseArray mFragments = new SparseArray();
    private Fragment mCurrentPrimaryItem = null;

    public FragmentStatePagerAdapterCompat(FragmentManager fm) {
        mFragmentManager = fm;
    }

    /**
     * Return the Fragment associated with a specified position.
     */
    public abstract Fragment getItem(int position);

    /**
     * Return a unique identifier for the item at the given position.
     *
     * 
The default implementation returns the given position.
     * Subclasses should override this method if the positions of items can change.
     *
     * @param position Position within this adapter
     * @return Unique identifier for the item at position
     */
    //添加一个方法，将Fragment和Id绑定在一起而不是position
    public int getItemId(int position) {
        return position;
    }

    @Override
    public void startUpdate(ViewGroup container) {
        if (container.getId() == View.NO_ID) {
            throw new IllegalStateException("ViewPager with adapter " + this
                    + " requires a view id");
        }
    }

    @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.
        int itemId = getItemId(position);
        Fragment f = mFragments.get(itemId);
        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);
        Fragment.SavedState fss = mSavedState.get(itemId);
        if (fss != null) {
            fragment.setInitialSavedState(fss);
        }

        fragment.setMenuVisibility(false);
        fragment.setUserVisibleHint(false);
        mFragments.put(itemId, 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());

        int index = mFragments.indexOfValue(fragment);
        if (index >= 0) {
            int itemId = mFragments.keyAt(index);
            mSavedState.put(itemId, fragment.isAdded()
                    ? mFragmentManager.saveFragmentInstanceState(fragment) : null);
            mFragments.remove(itemId);
        }

        mCurTransaction.remove(fragment);
    }

    @Override
    public void setPrimaryItem(ViewGroup container, int position, Object object) {
        Fragment fragment = (Fragment)object;
        if (fragment != mCurrentPrimaryItem) {
            if (mCurrentPrimaryItem != null) {
                mCurrentPrimaryItem.setMenuVisibility(false);
                mCurrentPrimaryItem.setUserVisibleHint(false);
            }
            if (fragment != null) {
                fragment.setMenuVisibility(true);
                fragment.setUserVisibleHint(true);
            }
            mCurrentPrimaryItem = fragment;
        }
    }

    @Override
    public void finishUpdate(ViewGroup container) {
        if (mCurTransaction != null) {
            mCurTransaction.commitNowAllowingStateLoss();
            mCurTransaction = null;
        }
    }

    @Override
    public boolean isViewFromObject(View view, Object object) {
        return ((Fragment)object).getView() == view;
    }

    @Override
    public Parcelable saveState() {
        Bundle state = null;
        if (mSavedState.size() > 0) {
            state = new Bundle();
            for (int i=0; i< mSavedState.size(); i++) {
                int itemId = mFragments.keyAt(i);
                Fragment.SavedState ss = mSavedState.get(itemId);
                if (ss != null) {
                    String key = "s" + itemId;
                    state.putParcelable(key, ss);
                }
            }
        }
        for (int i=0; i< mFragments.size(); i++) {
            int itemId = mFragments.keyAt(i);
            Fragment f = mFragments.get(itemId);
            if (f != null && f.isAdded()) {
                if (state == null) {
                    state = new Bundle();
                }
                String key = "f" + itemId;
                mFragmentManager.putFragment(state, key, f);
            }
        }
        return state;
    }

    @Override
    public void restoreState(Parcelable state, ClassLoader loader) {
        if (state != null) {
            Bundle bundle = (Bundle)state;
            bundle.setClassLoader(loader);
            mSavedState.clear();
            mFragments.clear();
            Iterable keys = bundle.keySet();
            for (String key: keys) {
                if (key.startsWith("f")) {
                    int index = Integer.parseInt(key.substring(1));
                    Fragment f = mFragmentManager.getFragment(bundle, key);
                    if (f != null) {
                        f.setMenuVisibility(false);
                        mFragments.put(index, f);
                    } else {
                        Log.w(TAG, "Bad fragment at key " + key);
                    }
                }
                else if (key.startsWith("s")) {
                    int index = Integer.parseInt(key.substring(1));
                    Parcelable parcelable = bundle.getParcelable(key);
                    if (parcelable instanceof Fragment.SavedState) {
                        mSavedState.put(index, (Fragment.SavedState) parcelable);
                    }
                }
            }
        }
    }
}