ViewPager使用详解(三):FragmentStatePagerAdapter

ViewPager与Fragment的配合使用，除了FragmentPagerAdapter之外，还有另外一个选择FragmentStatePagerAdapter。

从名称上来看，FragmentStatePagerAdapter多了个状态标志，这表明其与FragmentPagerAdapter的差别应该就是状态的问题了。我们可以通过源代码来了解。

FragmentStatePagerAdapter相对于FragmentPagerAdapter多了两个全局变量：

    private ArrayList mSavedState = new ArrayList();
    private ArrayList mFragments = new ArrayList();

mSavedState 保存每个Fragment的状态信息的列表
mFragments 保存每个Fragment实例对象的列表

FragmentStatePagerAdapter添加的方法与FragmentPagerAdapter基本一致，重载的方法则进行了部分调整：

/**
* 获取给定位置对应的Fragment。
*
* @param position 给定的位置
* @return 对应的Fragment
*/
public abstract Fragment getItem(int position);

@Override
public void startUpdate(ViewGroup container) {
}

@Override
public Object instantiateItem(ViewGroup container, int position) {
    // 获取给定位置的已存在Fragment
    if (mFragments.size() > position) {
        Fragment f = mFragments.get(position);
        if (f != null) {
            return f;
        }
    }

    // 创建新事务
    if (mCurTransaction == null) {
        mCurTransaction = mFragmentManager.beginTransaction();
    }

    // 创建给定位置的Fragment
    Fragment fragment = getItem(position);
    if (DEBUG) Log.v(TAG, "Adding item #" + position + ": f=" + fragment);
    // 读取给定位置的Fragment保存的状态，并用该状态对Fragment进行初始化
    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);
    fragment.setUserVisibleHint(false);
    mFragments.set(position, fragment);
    // 添加Fragment到FragmentManager中
    mCurTransaction.add(container.getId(), fragment);

    return fragment;
}

@Override
public void destroyItem(ViewGroup container, int position, Object object) {
    Fragment fragment = (Fragment) object;

    // 创建新事务
    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);
    }
    // 保存当前销毁的Fragment的状态
    mSavedState.set(position, fragment.isAdded()
            ? mFragmentManager.saveFragmentInstanceState(fragment) : null);
    mFragments.set(position, null);
    // 移除销毁的Fragment
    mCurTransaction.remove(fragment);
}

@Override
public void setPrimaryItem(ViewGroup container, int position, Object object) {
    Fragment fragment = (Fragment)object;
    if (fragment != mCurrentPrimaryItem) {
        // 主要项切换,相关菜单及信息进行切换
        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) {
        // 提交事务
        mCurTransaction.commitAllowingStateLoss();
        mCurTransaction = null;
        // 立即运行等待中事务
        mFragmentManager.executePendingTransactions();
    }
}

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

@Override
public Parcelable saveState() {
    Bundle state = null;
    if (mSavedState.size() > 0) {
        // 保存Fragment状态列表
        state = new Bundle();
        Fragment.SavedState[] fss = new Fragment.SavedState[mSavedState.size()];
        mSavedState.toArray(fss);
        state.putParcelableArray("states", fss);
    }
    // 保存Fragment列表
    for (int i=0; i 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);
                }
            }
        }
    }
}

从源代码中，我们可以清楚了解到FragmentStatePagerAdapter与FragmentPagerAdapter的不同：

FragmentStatePagerAdapter对Fragment实例和Fragment状态进行引用保留。
FragmentStatePagerAdapter在instantiateItem方法中，创建新Fragment后，读取对应Fragment状态对其进行初始化设置，并且只使用到add方法。
FragmentStatePagerAdapter在destroyItem方法中，销毁Fragment时，保存其Fragment状态，并且使用remove方法移除Fragment。
FragmentStatePagerAdapter重载了saveState方法和restoreState方法，在其中对于Fragment实例列表和Fragment状态列表进行保存和读取。

我们可以创建个简单应用，并且按照以下逻辑进行操作，查看相关LOG，来进一步了解：

①首次进入默认Fragment1:

    I: StatePagerActivity onCreate null
    I: StatePagerActivity onStart
    I: StatePagerActivity onResume
    I: Fragment1 setUserVisibleHint false
    I: Fragment2 setUserVisibleHint false
    I: Fragment1 onAttach
    I: Fragment1 onCreate null
    I: Fragment1 setUserVisibleHint true
    I: Fragment2 onAttach
    I: Fragment2 onCreate null
    I: Fragment2 onCreateView null
    I: Fragment2 onViewCreated
    I: Fragment2 onActivityCreated null
    I: Fragment2 onViewStateRestored null
    I: Fragment1 onCreateView null
    I: Fragment1 onViewCreated
    I: Fragment1 onActivityCreated null
    I: Fragment1 onViewStateRestored null
    I: Fragment1 onStart
    I: Fragment1 onResume
    I: Fragment2 onStart
    I: Fragment2 onResume

②点击Fragment2:

    I: Fragment3 setUserVisibleHint false
    I: Fragment1 setUserVisibleHint false
    I: Fragment2 setUserVisibleHint true
    I: Fragment3 onAttach
    I: Fragment3 onCreate null
    I: Fragment3 onCreateView null
    I: Fragment3 onViewCreated
    I: Fragment3 onActivityCreated null
    I: Fragment3 onViewStateRestored null
    I: Fragment3 onStart
    I: Fragment3 onResume

③点击Fragment3:

    I: Fragment1 onSaveInstanceState Bundle[{}]
    I: Fragment4 setUserVisibleHint false
    I: Fragment2 setUserVisibleHint false
    I: Fragment3 setUserVisibleHint true
    I: Fragment4 onAttach
    I: Fragment4 onCreate null
    I: Fragment1 onPause
    I: Fragment1 onStop
    I: Fragment1 onDestroyView
    I: Fragment1 onDestroy
    I: Fragment1 onDetach
    I: Fragment4 onCreateView null
    I: Fragment4 onViewCreated
    I: Fragment4 onActivityCreated null
    I: Fragment4 onViewStateRestored null
    I: Fragment4 onStart
    I: Fragment4 onResume

④点击Fragment4:

    I: Fragment4 onResume
    I: Fragment2 onSaveInstanceState Bundle[{}]
    I: Fragment5 setUserVisibleHint false
    I: Fragment3 setUserVisibleHint false
    I: Fragment4 setUserVisibleHint true
    I: Fragment5 onAttach
    I: Fragment5 onCreate null
    I: Fragment2 onPause
    I: Fragment2 onStop
    I: Fragment2 onDestroyView
    I: Fragment2 onDestroy
    I: Fragment2 onDetach
    I: Fragment5 onCreateView null
    I: Fragment5 onViewCreated
    I: Fragment5 onActivityCreated null
    I: Fragment5 onViewStateRestored null
    I: Fragment5 onStart
    I: Fragment5 onResume

⑤点击Fragment5:

    I: Fragment3 onSaveInstanceState Bundle[{}]
    I: Fragment4 setUserVisibleHint false
    I: Fragment5 setUserVisibleHint true
    I: Fragment3 onPause
    I: Fragment3 onStop
    I: Fragment3 onDestroyView
    I: Fragment3 onDestroy
    I: Fragment3 onDetach

⑥进入后台:

    I: Fragment5 onPause
    I: Fragment4 onPause
    I: StatePagerActivity onPause
    I: Fragment5 onSaveInstanceState Bundle[{}]
    I: Fragment4 onSaveInstanceState Bundle[{}]
    I: StatePagerActivity onSaveInstanceState Bundle[{android:viewHierarchyState=Bundle[{android:views={16908290=android.view.AbsSavedState$1@878ae08, 2131492956=android.view.AbsSavedState$1@878ae08, 2131492957=android.view.AbsSavedState$1@878ae08, 2131492958=android.support.v7.widget.Toolbar$SavedState@298b9a1, 2131492959=android.view.AbsSavedState$1@878ae08, 2131492976=FragmentPager.SavedState{71c47c6 position=4}, 2131492977=HorizontalScrollView.SavedState{e0d1387 scrollPosition=0}}}], android:support:fragments=android.support.v4.app.FragmentManagerState@8eea2b4}]
    I: Fragment5 onStop
    I: Fragment4 onStop
    I: StatePagerActivity onStop

⑦返回前台:

    I: Fragment5 onStart
    I: Fragment4 onStart
    I: StatePagerActivity onStart
    I: StatePagerActivity onResume
    I: Fragment5 onResume
    I: Fragment4 onResume

⑧点击Fragment4:

    I: Fragment3 setUserVisibleHint false
    I: Fragment5 setUserVisibleHint false
    I: Fragment4 setUserVisibleHint true
    I: Fragment3 onAttach
    I: Fragment3 onCreate Bundle[{android:user_visible_hint=false}]
    I: Fragment3 onCreateView Bundle[{android:user_visible_hint=false}]
    I: Fragment3 onViewCreated
    I: Fragment3 onActivityCreated Bundle[{android:user_visible_hint=false}]
    I: Fragment3 onViewStateRestored Bundle[{android:user_visible_hint=false}]
    I: Fragment3 onStart
    I: Fragment3 onResume

通过LOG，可以进一步了解到FragmentStatePagerAdapter与FragmentPagerAdapter的不同：

Fragment在FragmentStatePagerAdapter类的destroyItem方法中被remove时，Fragment的onDestroy方法和onDetach方法都被调用到。
当Fragment重新被add时，Fragment的生命周期全部重新调用，但是savedInstanceState参数保留着之前存储的数据。

ViewPager使用详解(三):FragmentStatePagerAdapter

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