Android：从活动更新选项卡布局（片段）文本视图

Question

我正在使用 tablayout 和 viewpager 创建选项卡视图。我为每个选项卡使用一个片段。在我的 MainActivity 中，我通过蓝牙从设备中获取了一些值。这些值作为单个字符串接收，我将其拆分并将其设置为相应的选项卡。但是，当我尝试从我的活动中为片段中的 textview 调用 setText 方法时，我得到了 NullPointerException。 谁能告诉我如何从我的 MainActivity 更新每个片段的文本视图？就像，如果我选择温度选项卡，温度片段中的文本视图应该使用 MainActivity 中的值进行更新。请帮忙

活动

    public class MainActivity extends AppCompatActivity implements TabLayout.OnTabSelectedListener {

   Handler bluetoothIn;
   private static TextView tmpF, humF, CoF;
   String tempGL, HumGL, coGL, devname;
   double dblTemp, dblCo;
   final int handlerState = 0; // used to identify handler message
   private BluetoothAdapter btAdapter = null;
   private TabLayout tabLayout;
   private ViewPager viewPager;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_frag_home);

 tmpF = (TextView)findViewById(R.id.txt_temp_val);
 humF = (TextView)findViewById(R.id.txt_hum_val);
 co = (TextView)findViewById(R.id.txt_co_val);

//Adding toolbar to the activity
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
//Initializing the tablayout
tabLayout = (TabLayout) findViewById(R.id.tabLayout);
//Initializing viewPager
viewPager = (ViewPager) findViewById(R.id.pager);
//Adding the tabs using addTab() method
tabLayout.addTab(tabLayout.newTab().setText("Temperature"));
tabLayout.addTab(tabLayout.newTab().setText("Humidity"));
tabLayout.addTab(tabLayout.newTab().setText("CO"));
tabLayout.setTabGravity(TabLayout.GRAVITY_FILL);

//Creating our pager adapter
Pager adapter = new Pager(getSupportFragmentManager(), tabLayout.getTabCount());

//Adding adapter to pager
viewPager.setAdapter(adapter);
viewPager.setOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout));
//Adding onTabSelectedListener to swipe views
tabLayout.setOnTabSelectedListener(this);

bluetoothIn=new Handler() {

    String readMessage;
    String[] values = new String[]{""};

    public void handleMessage(android.os.Message msg) {

        if (msg.what == handlerState) {

            readMessage = (String) msg.obj;
            values = readMessage.split("#");
            for (int j = 0; j < values.length; j++) {

                int rem = j % 3;

                if (rem == 0) {

                    tmpF.setText(values[j] + " C");
                    tempGL = String.valueOf(values[j]);

                    try {
                        dblTemp = Double.parseDouble(tempGL);
                    } catch (NumberFormatException e) {
                        e.printStackTrace();
                    }

                } else if (rem == 1) {

                    CoF.setText(values[j] + " ppm");
                    coGL = values[j];
                    try {
                        dblCo = Double.parseDouble(coGL);
                    } catch (NumberFormatException e) {
                        e.printStackTrace();
                    }

                } else if (rem == 2) {
                    humF.setText(values[j] + " %");
                    HumGL = values[j];
                }
            }
        }
    }
};

btAdapter=BluetoothAdapter.getDefaultAdapter(); // get Bluetooth
}

 @Override
 public void onTabSelected(TabLayout.Tab tab) {
// mViewPager.setCurrentItem(tab.getPosition());

}

@Override
public void onTabUnselected(TabLayout.Tab tab) {

}

 @Override
 public void onTabReselected(TabLayout.Tab tab) {

 }
 }

Tab1

  public class OneFragment extends Fragment {

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

 }
 }

fragment_one.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"
 tools:context="shinil.tablayout.OneFragment"
 android:id="@+id/rltnvnv">

<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Temperature"
android:textSize="40dp"
android:textStyle="bold"
android:id="@+id/txt_temp_val"
android:layout_centerInParent="true"/>

</RelativeLayout>

Logcat

  Caused by: java.lang.NullPointerException: Attempt to invoke virtual       method 'void       android.support.v4.view.ViewPager.setAdapter(android.support.v4.view.PagerAdapter)' on a null object reference at   shinil.airopure.MainActivity.onCreate(MainActivity.java:194)

frag_home.xml

    <LinearLayout
    android:id="@+id/main_layout"
    android:orientation="vertical"
    xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:app="http://schemas.android.com/apk/res-auto"
   xmlns:tools="http://schemas.android.com/tools"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   tools:context=".MainActivity">


  <android.support.v7.widget.Toolbar
 android:id="@+id/toolbar"
 android:layout_width="match_parent"
 android:layout_height="wrap_content"
 android:background="?attr/colorPrimary"
 android:minHeight="?attr/actionBarSize"
 android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
 app:popupTheme="@style/ThemeOverlay.AppCompat.Light"/>


  <android.support.design.widget.TabLayout
  android:id="@+id/tabLayout"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:background="?attr/colorPrimary"

  android:minHeight="?attr/actionBarSize"
  android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"/>


 <android.support.v4.view.ViewPager
  android:id="@+id/pager"
  android:layout_width="match_parent"
 android:layout_height="fill_parent"/>

</LinearLayout>

Answer 1

您好，我不确定我是否正确理解您的问题，但让我尝试提供帮助。

我假设您的 ViewPager 适配器扩展自 FragmentStatePagerAdapter

首先在您的寻呼机适配器中为您的片段定义一个SparseArray，如下所示：

SparseArray<Fragment> myPagerFragments= new SparseArray<>();

在您的适配器的instantiateItem 方法中，将您的片段添加到您的稀疏数组中。

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

并在您的 destroyItem 方法中从您的稀疏数组中删除您的片段。

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

向您的 adpater 添加一个方法，该方法返回您的寻呼机片段，如下所示：

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

在你的片段中定义一个公共方法来改变你的 TextView 的文本，如下所示：

public void setText(String text){
    myTextView.setText(text);
}

在您的活动中定义您的适配器并将onPageChangeListener 添加到您的 ViewPager，如下所示：

MyPagerAdapter mAdapter = new MyPagerAdapter(context,items);
myViewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
    @Override
    public void onPageScrolled(int position, float positionOffset, int  positionOffsetPixels) {

    }

    @Override
    public void onPageSelected(int position) {
       Fragment myPagerFragment = mAdapter.getRegisteredFragment(position);
       if(fragment != null){
          ((YourFragment)fragment).setText("Hello World!");
       }
    }

    @Override
    public void onPageScrollStateChanged(int state) {

    }
});

我希望这会对你有所帮助。祝你好运。