[英]DrawerLayout or TabLayout is null after resuming activity from background?
每當我最初創建我的活動時,一切都很順利,因為我能夠正確地擴充我的布局。 然而,問題是每當我切換到使用另一個應用程序,然后回到我自己的應用程序,我得到一個錯誤說TabLayout
為空,導致我的應用程序崩潰。 如果我切換到另一個應用程序幾分鍾,我的DrawerLayout
為空,也導致我的應用程序崩潰。
如果我正確理解Android生命周期,我意識到當活動從背景到前景時重新創建活動,並且再次調用onCreate
方法,如下所示: Android活動生命周期 - 所有這些方法是什么? 。 但是,我不明白為什么當應用程序啟動時,一切正常,但是當我進入后台時, TabLayout
或DrawerLayout
都設置為null。 這是我的嘗試:
news_feed.java:
private static DrawerLayout drawer;
private static TabLayout tabLayout;
public class news_feed extends BatchAppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.news_feed);
drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
//Listen for navigation events
ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close){
public void onDrawerOpened(View view) {
super.onDrawerOpened(view);
}
/** Called when a drawer has settled in a completely closed state. */
public void onDrawerClosed(View view) {
super.onDrawerClosed(view);
}
};
System.out.println("news_feed: toolbar: " + toolbar);
System.out.println("news_feed: Drawer: " + drawer);
System.out.println("news_feed: toggle: " + toggle);
drawer.setDrawerListener(toggle);
toggle.syncState();
TabLayout tabLayout = (TabLayout) findViewById(R.id.tab_layout);
System.out.println("news_feed: Tab Layout: " + tabLayout);
tabLayout.addTab(tabLayout.newTab().setText("First"));
tabLayout.addTab(tabLayout.newTab().setText("Second"));
tabLayout.addTab(tabLayout.newTab().setText("Third"));
tabLayout.setTabGravity(TabLayout.GRAVITY_FILL);
}
}
news_feed.xml:
<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout 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:id="@+id/drawer_layout"
android:layout_width="match_parent" android:layout_height="match_parent"
android:fitsSystemWindows="true" tools:openDrawer="start">
<include layout="@layout/app_bar_news_feed" android:layout_width="match_parent"
android:layout_height="match_parent" android:id="@+id/app_bar_main" />
<android.support.design.widget.NavigationView
android:id="@+id/nav_view"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_gravity="start"
android:fitsSystemWindows="true">
<LinearLayout
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">
<include
layout="@layout/nav_header_friends_list"
android:id="@+id/navigation_header"/>
<ListView
android:id="@+id/friends_list"
android:layout_width="match_parent"
android:layout_height="wrap_content"></ListView>
</LinearLayout>
</android.support.design.widget.NavigationView>
</android.support.v4.widget.DrawerLayout>
app_bar_news_feed.xml:
<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout
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" android:fitsSystemWindows="true"
tools:context=".news_feed">
<RelativeLayout
android:id="@+id/main_layout"
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">
<android.support.design.widget.AppBarLayout android:layout_height="wrap_content"
android:layout_width="match_parent" android:theme="@style/AppTheme.AppBarOverlay" android:id="@+id/app_bar">
<android.support.v7.widget.Toolbar android:id="@+id/toolbar"
android:layout_width="match_parent" android:layout_height="?attr/actionBarSize"
android:background="?attr/colorPrimary" app:popupTheme="@style/AppTheme.PopupOverlay" />
</android.support.design.widget.AppBarLayout>
<!-- <include layout="@layout/content_news_feed" /> -->
<android.support.design.widget.TabLayout
android:id="@+id/tab_layout"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_below="@+id/app_bar"
android:background="?attr/colorPrimary"
android:elevation="6dp"
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"
android:layout_below="@id/tab_layout"/>
</RelativeLayout>
<android.support.design.widget.FloatingActionButton android:id="@+id/fab"
android:layout_width="wrap_content" android:layout_height="wrap_content"
android:layout_gravity="bottom|end" android:layout_margin="@dimen/fab_margin"
android:src="@android:drawable/ic_dialog_email" />
</android.support.design.widget.CoordinatorLayout>
我確保findViewById
都引用了正確的id,但我似乎無法弄清楚出了什么問題。 這很奇怪:如果我快速從app -> another app -> app
切換,那么TabLayout
為空。 如果我從app -> another app -> wait a few minutes -> app
切換app -> another app -> wait a few minutes -> app
,那么DrawerLayout
為空。 我不知道為什么會這樣! 任何幫助,將不勝感激。
編輯:應用程序在我的活動的onCreate
方法崩潰。 而且,我提供了一個stacktrace:如下所示:
03-16 09:31:21.116 25852-25852 / com.test.tabs.tabs I / System.out:news_feed:toolbar:android.support.v7.widget.Toolbar {380bcc25 VE .... ..... .ID 0,0-0,0#7f0d009c app:id / toolbar} 03-16 09:31:21.116 25852-25852 / com.test.tabs.tabs I / System.out:news_feed:Drawer:android.support。 v4.widget.DrawerLayout {3ed761fa VFE .......我。 0,0-0,0#7f0d0095應用程序:id / drawer_layout} 03-16 09:31:21.116 25852-25852 / com.test.tabs.tabs I / System.out:news_feed:toggle:com.test.tabs。 tabs.com.tabs.activity.news_feed$1@22eb13ab 03-16 09:31:21.116 25852-25852 / com.test.tabs.tabs I / System.out:news_feed:Tab Layout:null 03-16 09:31: 21.116 25852-25852 / com.test.tabs.tabs E / AndroidRuntime:FATAL EXCEPTION:main進程:com.test.tabs.tabs,PID:25852 java.lang.RuntimeException:無法啟動活動ComponentInfo {com.test.tabs .tabs / com.test.tabs.tabs.com.tabs.activity.news_feed}:java.lang.NullPointerException:嘗試調用虛方法'android.support.design.widget.TabLayout $ Tab android.support.design.widget .TabLayout.newTab()'在android.app.A活動中的android.app.ActivityThread.perleLaunchActivity(ActivityThread.java:2661)上的空對象引用,位於android.app.ActivityThread.access的android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) $ 900(ActivityThread.java:172)在android.app.ActivityThread $ H.handleMessage(活動 Thread.java:1421)android.app.Handler.dispatchMessage(Handler.java:102)android.app.Looper.loop(Looper.java:145)android.app.ActivityThread.main(ActivityThread.java:5835) )at java.lang.reflect.Method.invoke(Native Method)at java.lang.reflect.Method.invoke(Method.java:372)at com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run(ZygoteInit.java) :1399)at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)引起:java.lang.NullPointerException:嘗試調用虛擬方法'android.support.design.widget.TabLayout $ Tab android。 support.design.widget.TabLayout.newTab()'在android.app.Activity.performCreate的com.test.tabs.tabs.com.tabs.activity.news_feed.onCreate(news_feed.java:174)上的空對象引用上(Activity.java:6221)在Android.app.ActivityThread.perleLaunchActivity(ActivityThread.java:2614)的android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)android.app.ActivityThread.handleLaunchActivity(ActivityThread.java: 2726)a t android.app.ActivityThread.access $ 900(ActivityThread.java:172)在android.app.ActivityThread $ H.handleMessage(ActivityThread.java:1421)在android.os.Handler.dispatchMessage(Handler.java:102)在android位於android.app.ActivityThread.main(ActivityThread.java:5835)的.os.Looper.loop(Looper.java:145)位於java.lang.reflect.Method的java.lang.reflect.Method.invoke(Native Method) .invoke(Method.java:372)at com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run(ZygoteInit.java:1399)at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)
編輯:包含的BatchAppCompatActivity類:
public class BatchAppCompatActivity extends AppCompatActivity {
@Override
protected void onStart()
{
super.onStart();
Batch.onStart(this);
}
@Override
protected void onStop()
{
Batch.onStop(this);
super.onStop();
}
@Override
protected void onDestroy()
{
Batch.onDestroy(this);
super.onDestroy();
}
@Override
protected void onNewIntent(Intent intent)
{
Batch.onNewIntent(this, intent);
super.onNewIntent(intent);
}
}
您對抽屜和tabLayout變量的聲明似乎有點奇怪。 除非它是一個錯字,我甚至感到驚訝它甚至有效。 就像是
public class news_feed extends BatchAppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {
private DrawerLayout drawer;
private TabLayout tabLayout;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.news_feed);
drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
//Listen for navigation events
ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close){
public void onDrawerOpened(View view) {
super.onDrawerOpened(view);
}
/** Called when a drawer has settled in a completely closed state. */
public void onDrawerClosed(View view) {
super.onDrawerClosed(view);
}
};
System.out.println("news_feed: toolbar: " + toolbar);
System.out.println("news_feed: Drawer: " + drawer);
System.out.println("news_feed: toggle: " + toggle);
drawer.setDrawerListener(toggle);
toggle.syncState();
tabLayout = (TabLayout) findViewById(R.id.tab_layout);
System.out.println("news_feed: Tab Layout: " + tabLayout);
tabLayout.addTab(tabLayout.newTab().setText("First"));
tabLayout.addTab(tabLayout.newTab().setText("Second"));
tabLayout.addTab(tabLayout.newTab().setText("Third"));
tabLayout.setTabGravity(TabLayout.GRAVITY_FILL);
}
}
應該工作假設工具欄是在你班級的其他地方定義的。
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