某学姐

Android Female Developer,Technology Fan,Reader。

FragmentPagerAdapter和FragmentStatePagerAdapter分析

2015-02-02 | Comments

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.

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