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FragmentPagerAdapter和FragmentStatePagerAdapter 使用場景及源碼分析

        FragmentPagerAdapter 和 FragmentStatePagerAdapter  内部都封裝有 FragmentManager和 FragmentTransaction,用于管理Fragment;

使用Fragment 來表示一頁,顯得更加簡單和直覺,Fragment  本身提供的一些特性可以讓我們友善的對每一頁進行管理,使用FragmentManager可以根據ID或TAG來查找Fragment , 動态添加、删除、替換,Fragment  可以管理自己的生命周期,像Activity一樣提供了一些生命周期回調方法

       讓Fragment 成為ViewPager的一頁時,FragmentManager會一直儲存管理建立好了的Fragment,即使目前不是顯示的這一頁,Fragment對象也不會被銷毀,在背景默默等待重新顯示。但如果Fragment不再可見時,它的視圖層次會被銷毀掉,下次顯示時視圖會重新建立

首先來分析一下 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);
             mCurTransaction.add(container.getId(), fragment,
                makeFragmentName(container.getId(), itemId));
    }
     return fragment;
}      

在FragmentPagerAdapter 裡的instantiateItem方法執行個體化Fragment,首先通過TAG去找Fragment是否存在,如果不存在調用getItem()添加,子類需要複寫getItem()

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);
}      

在destroyItem()方法中僅僅是把Fragment detach掉,Fragment界面被銷毀,但是Fragment還在記憶體中沒有銷毀

 是以在使用FragmentPagerAdapter  時,Fragment對象會一直存留在記憶體中,是以當有大量的顯示頁時,就不适合用FragmentPagerAdapter 了,FragmentPagerAdapter  适用于隻有少數的page情況,像頁籤

接下來看FragmentStatePagerAdapter 源碼:

@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 (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, fragment.isAdded()
            ? mFragmentManager.saveFragmentInstanceState(fragment) : null);
    mFragments.set(position, null);

    mCurTransaction.remove(fragment);
}      

從源碼上看當使用 FragmentStatePagerAdapter   時,如果Fragment不顯示,那麼Fragment對象會被銷毀,但在回調onDestroy()方法之前會回調onSaveInstanceState(Bundle outState)方法來儲存Fragment的狀态,下次Fragment顯示時通過onCreate(Bundle savedInstanceState)把存儲的狀态值取出來, FragmentStatePagerAdapter    比較适合頁面比較多的情況,像一個頁面的ListView    item