使用简单的视图和片段实现选项卡

Question

在使用TabActivity和片段以及viewPager等标签实现常用方法的一周之后，我无法获得活动或标签片段的实例，但无法找到解决问题的方法。所以我决定以不同的方式实现它。首先，我用一个简单的按钮制作一个tabWidget。在android开发者网站中，我找到了一种在运行时替换片段的方法。所以唯一剩下的就是如何访问制表符片段来调用FragmentActivity中的方法。

这是我使用pageViewer的FragmentActivity。当我想在setup()方法中获取我的选项卡的片段对象时，我得到nullpointer：

public class MainActivity extends FragmentActivity implements
    OnTabChangeListener, OnPageChangeListener {
    private TabHost mTabHost;
private ViewPager mViewPager;
private HashMap<String, TabInfo> mapTabInfo = new HashMap<String, MainActivity.TabInfo>();
private PagerAdapter mPagerAdapter;
private TabInfo mLastTab = null;

private class TabInfo {
    private String tag;
    private Class clss;
    private Bundle args;
    private Fragment fragment;

    TabInfo(String tag, Class clazz, Bundle args) {
        this.tag = tag;
        this.clss = clazz;
        this.args = args;
    }

}

class TabFactory implements TabContentFactory {

    private final Context mContext;

    /**
     * @param context
     */
    public TabFactory(Context context) {
        mContext = context;
    }

    /**
     * (non-Javadoc)
     * 
     * @see android.widget.TabHost.TabContentFactory#createTabContent(java.lang.String)
     */
    public View createTabContent(String tag) {
        View v = new View(mContext);
        v.setMinimumWidth(0);
        v.setMinimumHeight(0);
        return v;
    }

}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    this.requestWindowFeature(Window.FEATURE_NO_TITLE);
    Log.d("checking", "setup tabs...");
    setContentView(R.layout.activity_main);
    // //
    initialiseTabHost(savedInstanceState);
    if (savedInstanceState != null) {
        mTabHost.setCurrentTabByTag(savedInstanceState.getString("tab")); 
    }
    intialiseViewPager();
    // //
    setup();
    // ///
}

protected void onSaveInstanceState(Bundle outState) {
    outState.putString("tab", mTabHost.getCurrentTabTag()); // save the tab
                                                            // selected
    super.onSaveInstanceState(outState);
}

private void intialiseViewPager() {

    List<Fragment> fragments = new Vector<Fragment>();
    fragments
            .add(Fragment.instantiate(this, CoachFragment.class.getName()));
    fragments
            .add(Fragment.instantiate(this, LogingFragment.class.getName()));
    fragments.add(Fragment.instantiate(this,
            HistoryFragment.class.getName()));
    this.mPagerAdapter = new PagerAdapter(
            super.getSupportFragmentManager(), fragments);
    //
    this.mViewPager = (ViewPager) super.findViewById(R.id.viewpager);
    this.mViewPager.setAdapter(this.mPagerAdapter);
    this.mViewPager.setOnPageChangeListener(this);
    mViewPager.setOffscreenPageLimit(1000000);
}

private void initialiseTabHost(Bundle args) {
    mTabHost = (TabHost) findViewById(android.R.id.tabhost);
    mTabHost.setup();
    mTabHost.getTabWidget().setDividerDrawable(R.drawable.tab_divider);
    TabInfo tabInfo = null;
    View tabView1 = createTabView(this, coach");
    MainActivity.AddTab(this, this.mTabHost, this.mTabHost
            .newTabSpec("Tab1").setIndicator(tabView1),
            (tabInfo = new TabInfo("Tab1", CoachFragment.class, args)));
    this.mapTabInfo.put(tabInfo.tag, tabInfo);
    View tabView2 = createTabView(this, logbook");
    MainActivity.AddTab(this, this.mTabHost, this.mTabHost
            .newTabSpec("Tab2").setIndicator(tabView2),
            (tabInfo = new TabInfo("Tab2", LogingFragment.class, args)));
    this.mapTabInfo.put(tabInfo.tag, tabInfo);
    View tabView3 = createTabView(this, "history");
    MainActivity.AddTab(this, this.mTabHost, this.mTabHost
            .newTabSpec("Tab3").setIndicator(tabView3),
            (tabInfo = new TabInfo("Tab3", HistoryFragment.class, args)));
    this.mapTabInfo.put(tabInfo.tag, tabInfo);
    mTabHost.setOnTabChangedListener(this);
}

private static void AddTab(MainActivity activity, TabHost tabHost,
        TabHost.TabSpec tabSpec, TabInfo tabInfo) {
    // Attach a Tab view factory to the spec
    tabSpec.setContent(activity.new TabFactory(activity));
    tabHost.addTab(tabSpec);
}

public void setup() {
    ....
                        CoachFragment fragment=(CoachFragment) mPagerAdapter.getRegisteredFragment(mViewPager.getCurrentItem());
                        fragment.setTempView(R.id.welcome);
                        fragment.animate();
                    ....
}

public void onTabChanged(String tag) {
    // TabInfo newTab = this.mapTabInfo.get(tag);
    int pos = this.mTabHost.getCurrentTab();
    this.mViewPager.setCurrentItem(pos);
}

private static View createTabView(final Context context, final String text) {
    View view = LayoutInflater.from(context)
            .inflate(R.layout.tabs_bg, null);
    TextView tv = (TextView) view.findViewById(R.id.tabsText);
    tv.setText(text);
    return view;
}


@Override
public void onPageScrollStateChanged(int arg0) {
    // TODO Auto-generated method stub

}

@Override
public void onPageScrolled(int arg0, float arg1, int arg2) {
    // TODO Auto-generated method stub

}

@Override
public void onPageSelected(int position) {
    // TODO Auto-generated method stub
    this.mTabHost.setCurrentTab(position);
}

}

my pageAdapter：

public class PagerAdapter extends FragmentPagerAdapter {

private List<Fragment> fragments;
private HashMap<Integer, Fragment> registeredFragments=new HashMap<Integer, Fragment>();
/**
 * @param fm
 * @param fragments
 */
public PagerAdapter(FragmentManager fm, List<Fragment> fragments) {
    super(fm);
    this.fragments = fragments;
}
/* (non-Javadoc)
 * @see android.support.v4.app.FragmentPagerAdapter#getItem(int)
 */
@Override
public Fragment getItem(int position) {
    return this.fragments.get(position);
}


@Override
public Object instantiateItem(ViewGroup container, int position) {
    Fragment fragment = (Fragment) super.instantiateItem(container, position);
    registeredFragments.put(position, fragment);
    return fragment;
}

@Override
public void destroyItem(ViewGroup container, int position, Object object) {
    registeredFragments.remove(position);
    super.destroyItem(container, position, object);
}


public Fragment getRegisteredFragment(int position) {
    return registeredFragments.get(position);
}

/* (non-Javadoc)
 * @see android.support.v4.view.PagerAdapter#getCount()
 */
@Override
public int getCount() {
    return this.fragments.size();
}

}

和我的LogingFragment。其他2个片段完全相同：

public class LogingFragment extends Fragment{

public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    if (container == null) {
        // We have different layouts, and in one of them this
        // fragment's containing frame doesn't exist.  The fragment
        // may still be created from its saved state, but there is
        // no reason to try to create its view hierarchy because it
        // won't be displayed.  Note this is not needed -- we could
        // just run the code below, where we would create and return
        // the view hierarchy; it would just never be used.
        return null;
    }
    return (LinearLayout)inflater.inflate(R.layout.coach_activity, container, false);
}

}

每件事都很完美，所有看起来和滑动都很好但是在setup（）方法中我得到了nullPointer。请帮帮我。

Answer 1

我强烈建议运行并测试使用Tabs和ViewPager的样本有效导航

http://developer.android.com/training/implementing-navigation/index.html

Answer 2

最后我想出了nullpointer的原因是什么。寻呼机适配器应该是这样的：

public class PagerAdapter extends FragmentPagerAdapter {

Fragment screens[];
public PagerAdapter(FragmentManager fm, MainActivity context) {
    super(fm);
    screens = new Fragment[3];
    screens[0] = Fragment.instantiate(context,CoachFragment.class.getName() );
    screens[1] = Fragment.instantiate(context,LogingFragment.class.getName() );
    screens[2] = Fragment.instantiate(context,HistoryFragment.class.getName() );
}

@Override
public Fragment getItem(int index) {
    if(index <= screens.length)
    {
        return screens[index];
    }
    return null;
}

@Override
public int getCount() {
    return screens.length;
}

}

如此实例化视图寻呼机的变化如下：

private void intialiseViewPager() {


    this.mPagerAdapter = new PagerAdapter(super.getSupportFragmentManager(),this);
    //
    this.mViewPager = (ViewPager) super.findViewById(R.id.viewpager);
    this.mViewPager.setAdapter(this.mPagerAdapter);
    this.mViewPager.setOnPageChangeListener(this);
    //
    this.mViewPager = (ViewPager) super.findViewById(R.id.viewpager);
    this.mViewPager.setAdapter(this.mPagerAdapter);
    this.mViewPager.setOnPageChangeListener(this);
}

感谢@Bruno Mateus

Answer 3

在我看来，你似乎是Android的新手，所以让我指出你正确的方向（如果你不是Android的新手，我的道歉）：

如果您想在应用中使用标签，最新且最好的方法是使用Action Bar tabs。

这只涉及修改menu.xml文件并在主要活动中添加onCreateOptionsMenu()方法。

另外，要回答你的问题，

  

所以唯一剩下的就是如何访问制表符片段以从我的FragmentActivity中调用方法

您不应该使用片段来调用片段活动中的方法。

同样，我将通过此视频 "Programming Android with fragments"向您指出正确的方向。它有点长，但它将解释如何使用碎片。

使用视频的片段示例，并将其与操作栏标签页中的示例代码相结合。

祝你好运

**快速编辑以澄清：您应该使用操作栏标签，因为这是使用标签的最新和最简单的方法。这是新的“最佳实践”。 TabWidget和TabActivity更老，更复杂，甚至可能已被弃用。

Answer 4

试试GrilledUI库。它将为您完成所有脏工作，甚至可以从XML加载选项卡列表。