Question

我正在尝试向片段视图添加一个按钮，通过单击该按钮，它应该将我带到另一个视图，但是现在我的应用程序崩溃了，因为我为了获得按钮的viewId而创建的变量一直在返回null ...我不知道为什么!!请帮忙！

package com.example.app;

import android.support.v4.app.FragmentActivity;
import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;

public class MainActivity extends FragmentActivity {

    Button nextPage;

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



        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment())
                    .commit();
        }

        nextPage = (Button) findViewById(R.id.nextpage);
     //   nextPage.setOnClickListener(buttonNextPage);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

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


   /* private View.OnClickListener buttonNextPage = new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            getFragmentManager().beginTransaction()
                    .replace(R.id.container, new FragmentViewOne())
                    .commit();
        }
    };*/

}

这就是我上面的MainActivity：

package com.example.app;

import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

/**
 * Created by Jerry on 2014/1/4.
 */
public class FragmentViewOne extends Fragment {
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
    }

        @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        View myFragmentView = inflater.inflate(R.layout.fragmentview_one, container, false);

        return myFragmentView;
    }

}

我的第二个片段：

<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.example.app.MainActivity"
    tools:ignore="MergeRootFrame" />

容器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:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context="com.example.app.MainActivity$PlaceholderFragment">

    <TextView
        android:text="@string/hello_world"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Next Page"
        android:id="@+id/nextpage"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true" />

</RelativeLayout>

第一个片段xml。

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Fragment View"
        android:id="@+id/textView" />
</LinearLayout>

第二个片段。

01-07 21:57:20.470  25549-25549/? E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.app, PID: 25549
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.app/com.example.app.MainActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
            at android.app.ActivityThread.access$800(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5017)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at com.example.app.MainActivity.setupButton(MainActivity.java:37)
            at com.example.app.MainActivity.onCreate(MainActivity.java:21)
            at android.app.Activity.performCreate(Activity.java:5231)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
            at android.app.ActivityThread.access$800(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5017)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
            at dalvik.system.NativeStart.main(Native Method)

logcat的

Answer 1

R.id.nextpage是片段的xml布局，而不是活动，你直接在活动中找不到它。

最好的办法就是在里面找到这个按钮吧。并在片段内附加onClickListener。

现在创建一个这样的界面：

interface FragmentCallback{
   void nextButtonClicked();
}

让您的活动实现它并编写代码以更改活动内nextButtonClicked函数内的视图。

现在在Fragment和FragmentCallback类型的实例变量中：

FragmentCallback callback;

并为它写一个setter：

public void setFragmentCallback(FragmentCallback callback){
    this.callback=callback;
}

现在在您的活动中创建片段调用setFragmentCallback。

<FragmentObject>.setFragmentCallback(this);

点击Fragment中的nextButton只需调用：

callback.nextButtonClicked();

片段：

public class FragmentViewOne extends Fragment implements OnClickListener{
    FragmentCallback callback;
    public void setFragmentCallback(FragmentCallback callback){
        this.callback=callback;
    }
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
    }

        @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        View myFragmentView = inflater.inflate(R.layout.fragmentview_one, container, false);
        myFragmentView.findViewById(R.id.nextpage).setOnClickListener(this);
        return myFragmentView;
    }
    @Override
public void onClick(View v) {
    callback.nextButtonClicked();
    }
}

的活动：

public class MainActivity extends FragmentActivity implements FragmentCallback{

Button nextPage;

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



    if (savedInstanceState == null) {
        PlaceholderFragment fragment= new PlaceholderFragment();
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, fragment)
                .commit();
        fragment.setFragmentCallback(this);
    }

     void nextButtonClicked(){
        //code to change view
     }
 //rest of the code

Android按钮setonclick返回null

1 个答案: