android fragment中使用viewpager报空指针，vpBanner 这里就是空了

Question

1.MainActivity

public class MainActivity extends FragmentActivity {

    private ViewPager vpBanner;
    private ViewpagerAdapter vpAdapter;
    private List<View> views;
    private ImageView[] dots;
    private int[] ivids = { R.drawable.banner1, R.drawable.banner2,
            R.drawable.banner3, R.drawable.banner4 };
    private int[] ids = { R.id.bannerDot1, R.id.bannerDot2, R.id.bannerDot3,
            R.id.bannerDot4 };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if (savedInstanceState == null) {
            getFragmentManager().beginTransaction()
                    .add(R.id.container, new MainFragment()).commit();
        }

        initViews();
        initDots();
    }

    private void initViews() {
        
        views = new ArrayList<View>();
        for (int i = 0; i < ivids.length; i++) {
            ImageView iv = new ImageView(this);
            iv.setLayoutParams(new LayoutParams(LayoutParams.MATCH_PARENT,
                    LayoutParams.MATCH_PARENT));
            iv.setBackgroundResource(ivids[i]);
            views.add(iv);
        }

        // views.add(inflater.inflate(R.layout.banner1, null));
        // views.add(inflater.inflate(R.layout.banner2, null));
        // views.add(inflater.inflate(R.layout.banner3, null));
        // views.add(inflater.inflate(R.layout.banner4, null));

        vpAdapter = new ViewpagerAdapter(views, this);

        vpBanner = (ViewPager) findViewById(R.id.vpBanner);

        vpBanner.setAdapter(vpAdapter);

        vpBanner.addOnPageChangeListener(new OnPageChangeListener() {

            @Override
            public void onPageSelected(int arg0) {
                for (int i = 0; i < ids.length; i++) {
                    if (arg0 == i) {
                        dots[i].setImageResource(R.drawable.dot_select);
                    } else {
                        dots[i].setImageResource(R.drawable.dot);
                    }

                }
            }

            @Override
            public void onPageScrolled(int arg0, float arg1, int arg2) {

            }

            @Override
            public void onPageScrollStateChanged(int arg0) {

            }
        });
    }

    private void initDots() {
        dots = new ImageView[views.size()];
        for (int i = 0; i < views.size(); i++) {
            dots[i] = (ImageView) findViewById(ids[i]);
        }
    }
}

2.MainFragment.java

public class MainFragment extends Fragment {

    public MainFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container,
                false);
        return rootView;
    }
    
}

3.ViewpageAdaper.java

public class ViewpagerAdapter extends PagerAdapter{
    
    private List<View> views;
    private Context context;
    
    public ViewpagerAdapter(List<View> views,Context context){
        this.views = views;
        this.context = context;
    }
    
    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        container.removeView(views.get(position));
    }

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        container.addView(views.get(position));
        return views.get(position);
    }
    
    @Override
    public int getCount() {
        return views.size();
    }

    @Override
    public boolean isViewFromObject(View arg0, Object arg1) {
        return (arg0 == arg1);
    }

}

4.activity_main.xml

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.demo_viewpagerfm.MainActivity"
    tools:ignore="MergeRootFrame" />

5.fragment_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.demo_viewpagerfm.MainActivity$PlaceholderFragment" >

    <RelativeLayout
        android:layout_width="fill_parent"
        android:layout_height="200dp" >
    
        <android.support.v4.view.ViewPager
            android:id="@+id/vpBanner"
            android:layout_width="fill_parent"
            android:layout_height="fill_parent"
            android:background="#888" >
        </android.support.v4.view.ViewPager>

        <LinearLayout
            android:id="@+id/llBanner"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:layout_alignParentBottom="true"
            android:gravity="center_horizontal"
            android:orientation="horizontal"
            android:paddingBottom="10dp" 
            android:background="#888">

            <ImageView
                android:id="@+id/bannerDot1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:paddingRight="3dp"
                android:src="@drawable/dot_select" />

            <ImageView
                android:id="@+id/bannerDot2"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:paddingLeft="3dp"
                android:src="@drawable/dot" />
            
            <ImageView
                android:id="@+id/bannerDot3"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:paddingLeft="3dp"
                android:src="@drawable/dot" />
            
            <ImageView
                android:id="@+id/bannerDot4"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:paddingLeft="3dp"
                android:src="@drawable/dot" />
        </LinearLayout>
        
        </RelativeLayout>

</RelativeLayout>

我不用Fragment 直接在MainActivity中写是可以运行的，用了Fragment后vpBanner赋值这块就找不到了

Answer 1

vpBanner 在Fragment里面进行初始化，用rootView去findView

Answer 2

你可以把这个initView和initDot放到fragment中去。

Answer 3

vpBanner = (ViewPager) findViewById(R.id.vpBanner);这个赋值的意思是在当前的activity的布局中寻找id为vpBanner的控件，你的activity_main.xml里没有这个控件。你应该先加载fragment_main.xml这个布局，
fragmentView = getLayoutInflater().inflate(R.layout.more_view,null);然后
vpBanner =(ViewPager) fragmentView.findViewById(R.id.vpBanner);