为什么getActionBar（）在Android的TabHost中获得空值？
我正在尝试在Android中编写代码，以在向下滚动事件期间隐藏Actionbar。我有一个10x50的网格。因此，当我向下滚动时，操作栏应该隐藏。我正确地在Log Cat中获得滚动状态结果，所以每当我申请getActionBar（）时，我都会收到NullPointerException错误。
我的代码：
package com.example.complexdatepicker;
import android.app.TabActivity;
import android.content.Intent;
import android.os.Bundle;
import android.widget.TabHost;
import android.widget.TabHost.TabSpec;
public class MainActivity extends TabActivity
{
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
TabHost tabHost = getTabHost();
TabSpec tab1 = tabHost.newTabSpec("Home");
tab1.setIndicator("Home");
Intent photosIntent = new Intent(this, Home.class);
tab1.setContent(photosIntent);
tabHost.addTab(tab1);
}
}
Home.java
package com.example.complexdatepicker;
import java.util.ArrayList;
import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.widget.GridView;
public class Home extends ActionBarActivity
{
ArrayList<String> abc;
TestGrid tg;
GridView gv;
int mLastFirstVisibleItem;
int mLastVisibleItemCount;
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.home);
gv = (GridView) findViewById(R.id.gridView1);
abc = new ArrayList<String>();
for(int i=0;i<500;i++)
{
abc.add(String.valueOf(i));
}
tg = new TestGrid(Home.this,Home.this,abc);
gv.setAdapter(tg);
getSupportActionBar().hide(); /// Getting Error at this point..
/*
gv.setOnScrollListener(new AbsListView.OnScrollListener() {
public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {
if (mLastFirstVisibleItem > firstVisibleItem) {
Log.e(getClass().toString(), "scrolling up");
getActionBar().show();
} else if (mLastFirstVisibleItem < firstVisibleItem) {
Log.e(getClass().toString(), "scrolling down");
getActionBar().hide();
} else if (mLastVisibleItemCount < visibleItemCount) {
Log.e(getClass().toString(), "scrolling down");
getActionBar().hide();
} else if (mLastVisibleItemCount > visibleItemCount) {
Log.e(getClass().toString(), "scrolling up");
getActionBar().show();
}
mLastFirstVisibleItem = firstVisibleItem;
mLastVisibleItemCount = visibleItemCount;
}
public void onScrollStateChanged(AbsListView listView, int scrollState) {
}
});
*/
}
}
TestGrid.java
package com.example.complexdatepicker;
import java.util.ArrayList;
import java.util.Calendar;
import android.annotation.SuppressLint;
import android.app.DatePickerDialog;
import android.app.Dialog;
import android.content.Context;
import android.content.SharedPreferences;
import android.content.SharedPreferences.Editor;
import android.graphics.Color;
import android.os.Bundle;
import android.support.v4.app.DialogFragment;
import android.support.v4.app.FragmentActivity;
import android.util.Log;
import android.view.Gravity;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.View.OnClickListener;
import android.widget.BaseAdapter;
import android.widget.DatePicker;
import android.widget.TextView;
import android.widget.Toast;
public class TestGrid extends BaseAdapter
{
private ArrayList<String> abc;
private FragmentActivity activity;
private Context context;
String text;
int c = 0;
int x = 0;
SelectDateFragment newFragment;
public TestGrid(FragmentActivity activity , Context cont,ArrayList<String> abc)
{
super();
this.activity=activity;
this.context = cont;
this.abc = abc;
}
@Override
public int getCount()
{
return abc.size();
}
@Override
public Object getItem(int in)
{
return abc.get(in);
}
@Override
public long getItemId(int arg0)
{
return 0;
}
public class ViewHolder
{
public TextView txt;
}
@Override
public View getView(final int arg0, View arg1, ViewGroup arg2)
{
final SharedPreferences pref = context.getApplicationContext().getSharedPreferences("TestDate", 0);
ViewHolder view;
LayoutInflater inflator = activity.getLayoutInflater();
if(arg1==null)
{
view = new ViewHolder();
arg1 = inflator.inflate(R.layout.test_grid, null);
view.txt = (TextView) arg1.findViewById(R.id.txt);
arg1.setTag(view);
}
else
{
view = (ViewHolder) arg1.getTag();
}
View.OnClickListener alert = new OnClickListener()
{
public void onClick(View v)
{
Toast.makeText(activity.getApplicationContext(), abc.get(arg0)+" - Clicked ", Toast.LENGTH_SHORT).show();
newFragment = new SelectDateFragment();
Bundle args = new Bundle();
newFragment.setArguments(args);
newFragment.setOnDateSetListener(new DatePickerDialog.OnDateSetListener()
{
@Override
public void onDateSet(DatePicker view, int yy, int mm, int dd)
{
String saved_date = pref.getString("SavedDate", null);
Toast.makeText(activity.getApplicationContext(), "SavedDate : "+saved_date,Toast.LENGTH_LONG).show();
}
});
newFragment.show(activity.getSupportFragmentManager(), "DatePicker");
}
};
view.txt.setOnClickListener(alert);
text = String.valueOf(abc.get(arg0));
view.txt.setBackgroundResource(R.drawable.txtbk);
view.txt.setTextSize(18);
view.txt.setTextColor(Color.BLACK);
view.txt.setGravity(Gravity.CENTER);
view.txt.setText(text);
return arg1;
}
@SuppressLint({ "ValidFragment", "NewApi" })
public class SelectDateFragment extends DialogFragment implements DatePickerDialog.OnDateSetListener
{
private DatePickerDialog.OnDateSetListener externalListener;
public void setOnDateSetListener(DatePickerDialog.OnDateSetListener listener)
{
this.externalListener = listener;
}
@Override
public Dialog onCreateDialog(Bundle savedInstanceState)
{
final Calendar calendar = Calendar.getInstance();
int yy = calendar.get(Calendar.YEAR);
int mm = calendar.get(Calendar.MONTH);
int dd = calendar.get(Calendar.DAY_OF_MONTH);
return new DatePickerDialog(getActivity(), this, yy, mm, dd);
}
public void onDateSet(DatePicker view, int yy, int mm, int dd)
{
SharedPreferences pref = context.getApplicationContext().getSharedPreferences("TestDate", 0);
Editor et = pref.edit();
et.putString("SavedDate", String.valueOf(dd+"/"+mm+"/"+yy));
et.commit();
Log.d("SavedDate : ", String.valueOf(dd+"/"+mm+"/"+yy));
if(externalListener != null)
externalListener.onDateSet(view, yy, mm, dd);
}
public void onFinishEditDialog(String inputText)
{
}
}
}
的AndroidManifest.xml
<manifest
xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.complexdatepicker"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="14"
android:targetSdkVersion="17" />
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/Theme.AppCompat" >
<activity
android:name="com.example.complexdatepicker.MainActivity"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:name="com.example.complexdatepicker.Home"
android:label="@string/app_name" ></activity>
</application>
Style.xml
<resources>
<!--
Base application theme, dependent on API level. This theme is replaced
by AppBaseTheme from res/values-vXX/styles.xml on newer devices.
-->
<style name="AppBaseTheme" parent="android:Theme.Light">
<!--
Theme customizations available in newer API levels can go in
res/values-vXX/styles.xml, while customizations related to
backward-compatibility can go here.
-->
</style>
<!-- Application theme. -->
<style name="AppTheme" parent="AppBaseTheme">
<!-- All customizations that are NOT specific to a particular API-level can go here. -->
</style>
</resources>
错误图片：
错误：
05-25 11:10:37.374: E/AndroidRuntime(872): FATAL EXCEPTION: main
05-25 11:10:37.374: E/AndroidRuntime(872): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.complexdatepicker/com.example.complexdatepicker.MainActivity}: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.complexdatepicker/com.example.complexdatepicker.Home}: java.lang.NullPointerException
05-25 11:10:37.374: E/AndroidRuntime(872): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
05-25 11:10:37.374: E/AndroidRuntime(872): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
05-25 11:10:37.374: E/AndroidRuntime(872): at android.app.ActivityThread.access$600(ActivityThread.java:141)
05-25 11:10:37.374: E/AndroidRuntime(872): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
05-25 11:10:37.374: E/AndroidRuntime(872): at android.os.Handler.dispatchMessage(Handler.java:99)
05-25 11:10:37.374: E/AndroidRuntime(872): at android.os.Looper.loop(Looper.java:137)
05-25 11:10:37.374: E/AndroidRuntime(872): at android.app.ActivityThread.main(ActivityThread.java:5103)
05-25 11:10:37.374: E/AndroidRuntime(872): at java.lang.reflect.Method.invokeNative(Native Method)
05-25 11:10:37.374: E/AndroidRuntime(872): at java.lang.reflect.Method.invoke(Method.java:525)
05-25 11:10:37.374: E/AndroidRuntime(872): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
05-25 11:10:37.374: E/AndroidRuntime(872): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
05-25 11:10:37.374: E/AndroidRuntime(872): at dalvik.system.NativeStart.main(Native Method)
05-25 11:10:37.374: E/AndroidRuntime(872): Caused by: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.complexdatepicker/com.example.complexdatepicker.Home}: java.lang.NullPointerException
05-25 11:10:37.374: E/AndroidRuntime(872): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
05-25 11:10:37.374: E/AndroidRuntime(872): at android.app.ActivityThread.startActivityNow(ActivityThread.java:2054)
05-25 11:10:37.374: E/AndroidRuntime(872): at android.app.LocalActivityManager.moveToState(LocalActivityManager.java:135)
05-25 11:10:37.374: E/AndroidRuntime(872): at android.app.LocalActivityManager.startActivity(LocalActivityManager.java:347)
05-25 11:10:37.374: E/AndroidRuntime(872): at android.widget.TabHost$IntentContentStrategy.getContentView(TabHost.java:749)
05-25 11:10:37.374: E/AndroidRuntime(872): at android.widget.TabHost.setCurrentTab(TabHost.java:413)
05-25 11:10:37.374: E/AndroidRuntime(872): at android.widget.TabHost.addTab(TabHost.java:240)
05-25 11:10:37.374: E/AndroidRuntime(872): at com.example.complexdatepicker.MainActivity.onCreate(MainActivity.java:42)
05-25 11:10:37.374: E/AndroidRuntime(872): at android.app.Activity.performCreate(Activity.java:5133)
05-25 11:10:37.374: E/AndroidRuntime(872): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
05-25 11:10:37.374: E/AndroidRuntime(872): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
05-25 11:10:37.374: E/AndroidRuntime(872): ... 11 more
05-25 11:10:37.374: E/AndroidRuntime(872): Caused by: java.lang.NullPointerException
05-25 11:10:37.374: E/AndroidRuntime(872): at android.support.v7.app.ActionBarImplICS.hide(ActionBarImplICS.java:289)
05-25 11:10:37.374: E/AndroidRuntime(872): at com.example.complexdatepicker.Home.onCreate(Home.java:50)
05-25 11:10:37.374: E/AndroidRuntime(872): at android.app.Activity.performCreate(Activity.java:5133)
05-25 11:10:37.374: E/AndroidRuntime(872): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
05-25 11:10:37.374: E/AndroidRuntime(872): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
05-25 11:10:37.374: E/AndroidRuntime(872): ... 21 more
这些都是上面的代码和错误详情，请告诉我如何在 TabWidget 中使用 getActionBar（）。
请建议我一些好的解决方案。
1 个答案:
答案 0 :(得分：7)
您的主页Activity必须延长ActionBarActivity而不是FragmentActivity，以便您可以调用getActionBar()方法。
ActionBarActivity延伸FragmentActivity。此外，您应该使用getSupportActionBar()而不是getActionBar()，因为最新版本需要API级别11。
另请注意，您需要导入v4支持库才能调用getSupportActionBar()并支持v7库以使用ActionBarActivity。
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