单击 ActionBar 菜单时显示片段 return null

Question

我想在单击操作栏菜单项时 show/hide 片段。但是当我点击时我的应用程序给我错误。片段首先出现运行。当我想隐藏片段 App 时给出 null 错误。

我该如何解决这个问题？

谢谢。

Logcat

01-26 03:48:40.942    2295-2295/test.sy.myapplication E/AndroidRuntime﹕ FATAL EXCEPTION: main
java.lang.NullPointerException
        at android.app.BackStackRecord.run(BackStackRecord.java:661)
        at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1435)
        at android.app.FragmentManagerImpl.run(FragmentManager.java:441)
        at android.os.Handler.handleCallback(Handler.java:730)
        at android.os.Handler.dispatchMessage(Handler.java:92)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:5103)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:525)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
        at dalvik.system.NativeStart.main(Native Method)

LayerList.Java

package test.sy.myapplication;

import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;


public class LayerList extends Fragment {

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

}
}

MainActivity.Java

package test.sy.myapplication;

import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;


public class MainActivity extends ActionBarActivity {

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


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.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.
    switch (item.getItemId()) {
        case R.id.action_settings:
            return true;
        case R.id.showsth:

            FragmentManager fm = getFragmentManager();
            FragmentTransaction ft = getFragmentManager().beginTransaction();
            ft.setCustomAnimations(android.R.animator.fade_in,
                    android.R.animator.fade_out);
            ft.show(fm.findFragmentById(R.id.fragment));
            ft.commit();
            return true;
        default:
            return super.onOptionsItemSelected(item);
    }
}
}

Answer 1

ft.show(fm.findFragmentById(R.id.fragment));

这一行导致了问题

fm.findFragmentById(R.id.fragment) returns null.

通过这一行，您试图在 layout/activity_main 中找到之前添加到 R.id.fragment 的片段。如果之前没有添加或者替换fragment，很明显return null AMK

Answer 2

我解决了这个问题。

这种情况是由于添加片段而没有任何框架布局造成的。

将框架布局添加到主activity。并给它 id;

<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=".MainActivity">

<FrameLayout
    android:layout_width="200dp"
    android:layout_height="450dp"

    android:layout_centerVertical="true"
    android:layout_alignParentRight="true"
    android:layout_alignParentEnd="false"
    android:id="@+id/frameLayout"></FrameLayout>

第二；

创建片段及其活动。在这个例子中，我们的片段名称可能是 FragmentTest.java 而我们的片段 activity 可能是 fragment_activity.xml。

并将片段添加到主要活动；在 Oncreate 方法中，您应该添加这一行；

FragmentTest Mfragmenttest = new FragmentTest();
FragmentManager fm = getSupportFragmentManager();
                    FragmentTransaction ft = fm.beginTransaction();
//R.id.frameLayout is our frame layouts id. 
 ft.add(R.id.frameLayout, Mfragmenttest , "Hello Fragment");
ft.commit

单击 ActionBar 菜单时显示片段 return null

Show fragment when click ActionBar menu return null

java

android

nullpointerexception

android-fragments