使用ViewPager切换Fragment时，防止频繁调用OnCreatView

Posted 2020-08-20 掌握当下

使用ViewPager切换Fragment，我原先使用系统自带的适配器FragmentPagerAdapter。

切换fragment时，频繁调用oncreatview()。

查看FragmentPagerAdapter的源码，发现两个关键的地方

 1 @Override
 2     public Object instantiateItem(ViewGroup container, int position) {
 3         if (mCurTransaction == null) {
 4             mCurTransaction = mFragmentManager.beginTransaction();
 5         }
 6 
 7         final long itemId = getItemId(position);
 8 
 9         // Do we already have this fragment?
10         String name = makeFragmentName(container.getId(), itemId);
11         Fragment fragment = mFragmentManager.findFragmentByTag(name);
12         if (fragment != null) {
13             if (DEBUG) Log.v(TAG, "Attaching item #" + itemId + ": f=" + fragment);
14            //该处使用attach导致频繁调用oncreatview
15             mCurTransaction.attach(fragment);
16 
17         } else {
18             fragment = getItem(position);
19             if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment);
20             mCurTransaction.add(container.getId(), fragment,
21                     makeFragmentName(container.getId(), itemId));
22         }
23         if (fragment != mCurrentPrimaryItem) {
24             fragment.setMenuVisibility(false);
25             fragment.setUserVisibleHint(false);
26         }
27 
28         return fragment;
29     }
30 
31     @Override
32     public void destroyItem(ViewGroup container, int position, Object object) {
33         if (mCurTransaction == null) {
34             mCurTransaction = mFragmentManager.beginTransaction();
35         }
36         if (DEBUG) Log.v(TAG, "Detaching item #" + getItemId(position) + ": f=" + object
37                 + " v=" + ((Fragment)object).getView());
38         //该处使用detach导致频繁调用oncreatview
39         mCurTransaction.detach((Fragment)object);
40     }

attach和detach的频繁使用导致了fragment频繁调用oncreatview。

找到元凶了，接下来就好办了。

自定义一个适配器，将attach改为show，将detach改为hide。

完美解决问题。

@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);
            //用show而不用attach，防止频繁调用oncreatview
            mCurTransaction.show(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());
        //用hide而不用detach，防止频繁调用oncreatview
        mCurTransaction.hide((Fragment) object);
    }

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