TabLayout踩坑传奇

今天中午休息的时候，同事研究起了MD，对!就是传说中的Material design。他想要写一个MD主题的app，可是遇到个关于TabLayout的问题。本来想做出的效果是这个样子的。

想象中的效果 好了开工，写完一运行，额，完全不对啊！我的title呢？？！ img title不见了！！

我好奇的过去瞅瞅，看了看。没有问题啊。

 private void initViews() {
        tabLayout.addTab(tabLayout.newTab().setText("ftm1"));
        tabLayout.addTab(tabLayout.newTab().setText("ftm2"));
        viewPager.setAdapter(new FragmentStatePagerAdapter(getSupportFragmentManager()) {
            @Override
            public Fragment getItem(int position) {
                return fragments.get(position);
            }

            @Override
            public int getCount() {
                return fragments == null ? 0 : fragments.size();
            }
        });
        tabLayout.setupWithViewPager(viewPager);
    }

设置了tab也加上了setupWithViewPager方法啊。查阅了一番资料，是调用setupWithViewPager方法将所有tab清除了。不说废话了上代码，看下是怎么移除tab的：

public void setupWithViewPager(@Nullable final ViewPager viewPager, boolean autoRefresh) {
        setupWithViewPager(viewPager, autoRefresh, false);
    }

    private void setupWithViewPager(@Nullable final ViewPager viewPager, boolean autoRefresh,
            boolean implicitSetup) {
        if (mViewPager != null) {
            // If we've already been setup with a ViewPager, remove us from it
            if (mPageChangeListener != null) {
                mViewPager.removeOnPageChangeListener(mPageChangeListener);
            }
            if (mAdapterChangeListener != null) {
                mViewPager.removeOnAdapterChangeListener(mAdapterChangeListener);
            }
        }

        if (mCurrentVpSelectedListener != null) {
            // If we already have a tab selected listener for the ViewPager, remove it
            removeOnTabSelectedListener(mCurrentVpSelectedListener);
            mCurrentVpSelectedListener = null;
        }

        if (viewPager != null) {
            mViewPager = viewPager;

            // Add our custom OnPageChangeListener to the ViewPager
            if (mPageChangeListener == null) {
                mPageChangeListener = new TabLayoutOnPageChangeListener(this);
            }
            mPageChangeListener.reset();
            viewPager.addOnPageChangeListener(mPageChangeListener);

            // Now we'll add a tab selected listener to set ViewPager's current item
            mCurrentVpSelectedListener = new ViewPagerOnTabSelectedListener(viewPager);
            addOnTabSelectedListener(mCurrentVpSelectedListener);

            final PagerAdapter adapter = viewPager.getAdapter();
            if (adapter != null) {
                // Now we'll populate ourselves from the pager adapter, adding an observer if
                // autoRefresh is enabled
                setPagerAdapter(adapter, autoRefresh);
            }

            // Add a listener so that we're notified of any adapter changes
            if (mAdapterChangeListener == null) {
                mAdapterChangeListener = new AdapterChangeListener();
            }
            mAdapterChangeListener.setAutoRefresh(autoRefresh);
            viewPager.addOnAdapterChangeListener(mAdapterChangeListener);

            // Now update the scroll position to match the ViewPager's current item
            setScrollPosition(viewPager.getCurrentItem(), 0f, true);
        } else {
            // We've been given a null ViewPager so we need to clear out the internal state,
            // listeners and observers
            mViewPager = null;
            setPagerAdapter(null, false);
        }

        mSetupViewPagerImplicitly = implicitSetup;
    }

在调用setupWithViewPager会调用到setPagerAdapter(null, false)看下这个方法里发生了什么吧。

void setPagerAdapter(@Nullable final PagerAdapter adapter, final boolean addObserver) {
        if (mPagerAdapter != null && mPagerAdapterObserver != null) {
            // If we already have a PagerAdapter, unregister our observer
            mPagerAdapter.unregisterDataSetObserver(mPagerAdapterObserver);
        }

        mPagerAdapter = adapter;

        if (addObserver && adapter != null) {
            // Register our observer on the new adapter
            if (mPagerAdapterObserver == null) {
                mPagerAdapterObserver = new PagerAdapterObserver();
            }
            adapter.registerDataSetObserver(mPagerAdapterObserver);
        }

        // Finally make sure we reflect the new adapter
        populateFromPagerAdapter();
    }

在setPagerAdapter里又调用了populateFromPagerAdapter（）,点开这个方法：

void populateFromPagerAdapter() {
        removeAllTabs();

        if (mPagerAdapter != null) {
            final int adapterCount = mPagerAdapter.getCount();
            for (int i = 0; i < adapterCount; i++) {
                addTab(newTab().setText(mPagerAdapter.getPageTitle(i)), false);
            }

            // Make sure we reflect the currently set ViewPager item
            if (mViewPager != null && adapterCount > 0) {
                final int curItem = mViewPager.getCurrentItem();
                if (curItem != getSelectedTabPosition() && curItem < getTabCount()) {
                    selectTab(getTabAt(curItem));
                }
            }
        }
    }

这个populateFromPagerAdapter（）方法里将tabLayout里所有的tab移除了，又重新new了tab。所以原来add的tab没有显示，那怎么解决呢？再仔细看这个方法，对没错，addTab(newTab().setText(mPagerAdapter.getPageTitle(i)), false);在重新new tab的过程中有重新传title，这个值是通过mPagerAdapter.getPageTitle(i))获取的，那么我们在ViewPager的FragmentPagerAdapter重写getPageTitle方法，默认的PagerAdapter返回的是null。

public CharSequence getPageTitle(int position) {
   return null;
}

现在我们把代码修改成下面的样子：

private void initViews() {
        viewPager.setAdapter(new FragmentPagerAdapter(getSupportFragmentManager()) {
            @Override
            public Fragment getItem(int position) {
                return fragments.get(position);
            }

            @Override
            public int getCount() {
                return fragments == null ? 0 : fragments.size();
            }

            @Override
            public CharSequence getPageTitle(int position) {
                return titles.get(position);
            }
        });
        tabLayout.setupWithViewPager(viewPager);
    }

O(∩_∩)O哈哈~现在就是我们想要的样子啦。。。我写的Demo源码

参考资料