Android学习笔记——FragmentPagerAdapter/FragmentStatePagerAdapter

1、ViewPager
（1）页面填充populate
（1）状态保存和恢复onSaveInstanceState/onRestoreInstanceState
2、FragmentPagerAdapter／FragmentStatePagerAdapter差别
3、参考文档

1、ViewPager

ViewPager搭配PagerAdapter一起使用，实现页面的左右切换。
PagerAdapter里的item可以是View（填充View list），也可以是fragment（使用FragmentPagerAdapter／FragmentStatePagerAdapter）

/**属性**/
private PagerAdapter mAdapter;
//以mCurItem为中心的左右位置偏移
private int mOffscreenPageLimit = DEFAULT_OFFSCREEN_PAGES;
//当前显示的item位置
private int mCurItem;

/**方法**/
setOffscreenPageLimit(int limit)
public void setAdapter(PagerAdapter adapter)
public void setCurrentItem(int item)
//填充页面
populate();
//状态保存和恢复
public Parcelable onSaveInstanceState()
public void onRestoreInstanceState(Parcelable state)

（1）页面填充populate

ViewPager状态发生变化时会调到populate()去填充页面，如setCurrentItem(item)或setAdapter()或dataSetChanged()时。

填充规则是：
对于mCurItem-mOffscreenPageLimit和mCurItem＋mOffscreenPageLimit范围内的页面，会调用mAdapter.instantiateItem()初始化（如果没初始化的话）
对于超出这个范围的页面，会调用mAdapter.destroyItem()进行销毁操作。

（2）状态保存和恢复onSaveInstanceState/onRestoreInstanceState

会在View的生命周期调用mAdapter.saveState()和mAdapter.restoreState()实现状态的保存和恢复。

当然前提还是要看PagerAdapter在回调中有没有保存和恢复逻辑。

//ViewPager

@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;
    }
}

2、FragmentPagerAdapter／FragmentStatePagerAdapter差别

差别主要体现在左右页面切换时对mCurItem-mOffscreenPageLimit和mCurItem＋mOffscreenPageLimit范围外的页面的处理上。

（1）fragments对象的处理：FragmentPagerAdapter范围外fragments会保存在内存中(detach)，但是fragment对应的View会被销毁；FragmentStatePagerAdapter范围外fragments不会保存在内存中(remove)，View也会被销毁。
（2）状态的处理：FragmentPagerAdapter范围外fragments对应的SavedState会保存；FragmentStatePagerAdapter范围外fragments对应的SavedState不会保存。这个SavedState在Fragment的生命周期回调中供外部传参数，和Activity类似。
（3）适用场景：相同数量的fragments，FragmentPagerAdapter内存较大，但页面切换更友好；FragmentStatePagerAdapter内存占用少，页面切换稍差。因此FragmentPagerAdapter适用于Fragment数量少的情况，FragmentStatePagerAdapter适用于Fragment数量多的情况。

关于fragments对象的处理：

（1）FragmentPagerAdapter：
destroyItem时，调用detach()，对应的生命周期方法onPause() -> onStop() -> onDestroyView()
instantiateItem时，调用attach()，onCreateView() -> onStart() -> onResume()

（2）FragmentStatePagerAdapter：
destroyItem时，调用remove()，对应的生命周期方法onPause() -> onStop() -> onDestroyView() -> onDestroy() -> onDetach()
instantiateItem时，调用add()，onAttach() -> onCreate() -> onCreateView() -> onStart() -> onResume()

//FragmentPagerAdapter
@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
@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);
    fragment.setUserVisibleHint(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);
}

关于状态保存和恢复：

//FragmentPagerAdapter
@Override
public Parcelable saveState() {
    return null;
}

@Override
public void restoreState(Parcelable state, ClassLoader loader) {
}


//FragmentStatePagerAdapter
@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);
                }
            }
        }
    }
}

3、参考文档

（1）FragmentPagerAdapter和FragmentStatePagerAdapter源码中的三宝

——————————————我是分割线——————————————–

一、FragmentPagerAdapter

适合于 Fragment数量不多的情况。当某个页面不可见时，该页面对应的View可能会被销毁，但是所有的Fragment都会一直存在于内存中。如果Fragment需要保存的状态较多时，会导致占用内存较大，因此对于Fragment数量较多的情况，建议使用FragmentStatePagerAdapter。

当使用FragmentPagerAdapter时，ViewPager必须有有效id。

@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);
}

private static String makeFragmentName(int viewId, long id) {
    return "android:switcher:" + viewId + ":" + id;
}

通过源码，可以看出FragmentPagerAdapter填充每一个页面的流程：
（1）根据containerId和position得到fragment的tag
（2）FragmentManager根据tag查找对应的Fragment，若存在则直接attach
（3）若不存在，则调用getItem(position)，并设置tag，调用add方法添加

结论：
（1） FragmentPagerAdapter中Fragment的tag 为”android:switcher:” + viewId + “:” + id;
（2） getItem(position)中new Fragment()，并且不需要设置 tag，Fragment.instantiate()即可

二、FragmentStatePagerAdapter

适合于Fragment数量较多的情况。当页面不可见时，对应的Fragment实例可能会被销毁，但是Fragment的状态会被保存。因此每个Fragment占用的内存会更少，但是页面切换会引起较大开销。
当使用FragmentStatePagerAdapter时，ViewPager必须有有效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.
    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);
    fragment.setUserVisibleHint(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);
}

根据源码，FragmentStatePagerAdapter填充每一个页面的流程：
（1）先试着找mFragments的list中index为position是否有Fragment，若有则直接返回；
（2）若没有则调用getItem(position)实例化Fragment并设置初始状态，调用add
销毁流程：
（1）保存 Fragment对应的状态（2）remove Fragment

结论：
（1）Fragment没有指定格式的tag
（2）FragmentStatePagerAdapter会保持和恢复 Fragment的状态

本文原文发自某学姐, 转载请保留出处, 谢谢.

某学姐

Android Female Developer, Technology Fan, Reader.