1.背景

在內(nèi)存不足的手機(jī)上，某些非前臺(tái)頁(yè)面會(huì)因?yàn)閮?nèi)存不足而銷毀魂挂，此時(shí)再次進(jìn)入會(huì)執(zhí)行reconstruct 的邏輯，也就是 save 马昨、restore 邏輯，此時(shí)界面展示異常

用戶正在瀏覽大眾點(diǎn)評(píng)的團(tuán)購(gòu)詳情頁(yè)，然后微信來了一條消息，此時(shí)打開微信，可能點(diǎn)評(píng)的團(tuán)購(gòu)詳情頁(yè)就被銷毀了

目的：不想要當(dāng)前 activity 保留狀態(tài)仓坞，銷毀后和重新進(jìn)入頁(yè)面保持一致

2.問題探索

//偽代碼如下
 for (int i = 0, len = Math.min(xx.size(), TAB_MAX_COUNT); i < len; i++) {
            Fragment fragment = new Fragment()
            fragment.setListener(listener);
            mFragments.add(fragment);
        }

走一遍主流程后，發(fā)現(xiàn) listener 在回調(diào)的時(shí)候?yàn)榭?/p>

此時(shí)幸好使用了kotlin腰吟，避免了一次明顯的空指針異常无埃，線上沒有 crash

@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();
        }
        //回調(diào)adapter的getItem方法
        Fragment fragment = getItem(position);
        //下面代碼省略
    }

在幾個(gè)關(guān)鍵節(jié)點(diǎn)(隨緣)打斷點(diǎn)后發(fā)現(xiàn)界面上展示的 fragment 不是生成的fragment，然后 debug adapter 的 getItem 方法毛雇，發(fā)現(xiàn)根本就沒有調(diào)用 getItem 方法嫉称，因?yàn)?mFragments 中有fragment
其實(shí)吧，上述代碼里面的注釋已經(jīng)寫得很清楚了

此時(shí)便基本確定不是代碼的問題灵疮，而是 FragmentStatePagerAdapter 源碼實(shí)現(xiàn)的問題了

3. 源碼查看

//Viewpager.java
@Override
    public Parcelable onSaveInstanceState() {
        Parcelable superState = super.onSaveInstanceState();
        SavedState ss = new SavedState(superState);
        ss.position = mCurItem;
        if (mAdapter != null) {
            ss.adapterState = mAdapter.saveState();
        }
        return ss;
    }
    
     @Override
    public void onRestoreInstanceState(Parcelable state) {
        if (!(state instanceof SavedState)) {
            super.onRestoreInstanceState(state);
            return;
        }

        SavedState ss = (SavedState) state;
        super.onRestoreInstanceState(ss.getSuperState());

        if (mAdapter != null) {
            mAdapter.restoreState(ss.adapterState, ss.loader);
            setCurrentItemInternal(ss.position, false, true);
        } else {
            mRestoredCurItem = ss.position;
            mRestoredAdapterState = ss.adapterState;
            mRestoredClassLoader = ss.loader;
        }
    }

由代碼可知织阅，Viewpager 在 save 時(shí)主要存儲(chǔ)了 mAdapter.saveState();
敲黑板 此時(shí)可以復(fù)習(xí)下 save 和 restore 的調(diào)用時(shí)機(jī)和注意點(diǎn)，具體查看 onSaveInstanceState執(zhí)行時(shí)機(jī)

//FragmentStatePagerAdapter.java
   @Override
    public Parcelable saveState() {
        Bundle state = null;
        if (mSavedState.size() > 0) {
            state = new Bundle();
            Fragment.SavedState[] fss = new Fragment.SavedState[mSavedState.size()];
            mSavedState.toArray(fss);
            state.putParcelableArray("states", fss);
        }
        for (int i=0; i<mFragments.size(); i++) {
            Fragment f = mFragments.get(i);
            if (f != null && f.isAdded()) {
                if (state == null) {
                    state = new Bundle();
                }
                String key = "f" + i;
                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);
            Parcelable[] fss = bundle.getParcelableArray("states");
            mSavedState.clear();
            mFragments.clear();
            if (fss != null) {
                for (int i=0; i<fss.length; i++) {
                    mSavedState.add((Fragment.SavedState)fss[i]);
                }
            }
            Iterable<String> 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) {
                        while (mFragments.size() <= index) {
                            mFragments.add(null);
                        }
                        f.setMenuVisibility(false);
                        mFragments.set(index, f);
                    } else {
                        Log.w(TAG, "Bad fragment at key " + key);
                    }
                }
            }
        }
    }

saveState 主要是將 fragment.mIndex 存儲(chǔ)到 bundle 中震捣，然后通過 mFragmentManager.getFragment(bundle, key);來進(jìn)行 fragment 的恢復(fù)

如果有興趣荔棉，可以繼續(xù)看 Fragment.java-->void restoreChildFragmentState(@Nullable Bundle savedInstanceState) {

4 .解決方式

復(fù)寫 FragmentStatePagerAdapter.saveState()

public class TestAdapter extends FragmentStatePagerAdapter {
    @Override
    public Parcelable saveState() {//空實(shí)現(xiàn)，不傳數(shù)據(jù)
        return null;
    }
}