我有一个管理4个片段的Activity。我在活动中使用导航抽屉和折叠工具栏。当我切换片段时，我只是希望ActionBar标题成为我以编程方式设置它的内容，但由于某种原因它没有发生。
我知道它最初有效，因为在我的onCreate（）方法中，我将标题设置为“Home”并显示正常。但是，当我开始导航时，代码不会执行。有谁知道为什么？
这是我的MainActivity XML：
<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">
<include
layout="@layout/app_bar"
android:layout_width="match_parent"
android:layout_height="match_parent"/>
<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"
app:menu="@menu/activity_main_drawer" />
</android.support.v4.widget.DrawerLayout>
这是app_bar布局：
<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="wrap_content">
<android.support.design.widget.AppBarLayout
android:layout_width="match_parent"
android:layout_height="200dp"
android:fitsSystemWindows="true"
android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar">
<android.support.design.widget.CollapsingToolbarLayout
android:id="@+id/collapsing_toolbar"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true"
app:layout_scrollFlags="scroll|exitUntilCollapsed">
<ImageView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@drawable/task_header"
android:scaleType="centerCrop"
android:fitsSystemWindows="true"
app:layout_collapseMode="parallax"/>
<android.support.v7.widget.Toolbar
android:id="@+id/toolbar"
android:layout_height="?attr/actionBarSize"
android:layout_width="match_parent" />
</android.support.design.widget.CollapsingToolbarLayout>
</android.support.design.widget.AppBarLayout>
<FrameLayout
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
app:layout_behavior="@string/appbar_scrolling_view_behavior"
tools:context="com.markfeldman.tasktrack.MainActivity">
</FrameLayout>
</android.support.design.widget.CoordinatorLayout>
这是我的主要活动：
public class MainActivity extends AppCompatActivity implements Home
.onFragmentClicked,NavigationView.OnNavigationItemSelectedListener {
private Fragment selectedFragment;
private DrawerLayout drawerLayout;
private ActionBarDrawerToggle drawerToggle;
private String title;
private final String INSTANCE_STATE_SAVE_TITLE = "save_title";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
selectedFragment = new Home();
if (savedInstanceState==null){
title = getString(R.string.fragment_home_title);
getSupportFragmentManager()
.beginTransaction()
.add(R.id.container,selectedFragment)
.addToBackStack(title)//WORKS HERE
.commit();
if (getSupportActionBar()!=null){
getSupportActionBar().setTitle(title);
}
}else{
title = savedInstanceState.getString(INSTANCE_STATE_SAVE_TITLE);
if (getSupportActionBar()!=null){
getSupportActionBar().setTitle(title);
}
}
title = getTitle().toString();
drawerLayout = (DrawerLayout)findViewById(R.id.drawer_layout);
drawerToggle = new ActionBarDrawerToggle(this,drawerLayout,toolbar,R.string.drawer_open,R.string.drawer_close);
drawerLayout.addDrawerListener(drawerToggle);
NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
navigationView.setNavigationItemSelectedListener(this);
if (getSupportActionBar()!=null){
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
getSupportActionBar().setHomeButtonEnabled(true);
}
drawerToggle.syncState();
}
@Override
public boolean onNavigationItemSelected(@NonNull MenuItem item) {
int id = item.getItemId();
switch (id){
case (R.id.home_title_nav_drawer) :{
selectedFragment = new Home();
title = getString(R.string.fragment_home_title);
break;
}
case (R.id.cal_title_nav_drawer) :{
selectedFragment = new Calendar();
title = getString(R.string.fragment_cal_title);
break;
}
case (R.id.contacts_title_nav_drawer) :{
selectedFragment = new Contacts();
title = getString(R.string.fragment_contacts_title);
break;
}
case (R.id.tasks_title_nav_drawer) :{
selectedFragment = new Tasks();
title = getString(R.string.fragment_task_title);
break;
}
}
FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
ft.replace(R.id.container,selectedFragment)
.addToBackStack(title)
if (getSupportActionBar()!=null){
getSupportActionBar().setTitle(title);//NO LONGER WORKS
}
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
drawer.closeDrawer(GravityCompat.START);
return true;
}
@Override
protected void onPostCreate(Bundle savedInstanceState) {
super.onPostCreate(savedInstanceState);
// Sync the toggle state after onRestoreInstanceState has occurred.
drawerToggle.syncState();
}
@Override
public void onConfigurationChanged(Configuration newConfig) {
super.onConfigurationChanged(newConfig);
drawerToggle.onConfigurationChanged(newConfig);
}
@Override
protected void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
outState.putString(INSTANCE_STATE_SAVE_TITLE, title);
}
@Override
public void onBackPressed() {
super.onBackPressed();
int count = this.getSupportFragmentManager().getBackStackEntryCount();
if(count==0){
this.finish();
}else if (count>0){
title = getSupportFragmentManager().getBackStackEntryAt(count-1).getName();
if (getSupportActionBar()!=null){
getSupportActionBar().setTitle(title);//DOESN'T WORK
}
}
}
@Override
public void buttonClicked(View v) {
int id = v.getId();
switch (id){
case (R.id.cal_button):{
selectedFragment = new Calendar();
title = getString(R.string.fragment_cal_title);
break;
}
case (R.id.tasks_button):{
selectedFragment = new Tasks();
title = getString(R.string.fragment_task_title);
break;
}
case (R.id.contacts_button):{
selectedFragment = new Contacts();
title = getString(R.string.fragment_contacts_title);
break;
}
}
FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
fragmentTransaction.replace(R.id.container,selectedFragment)
.addToBackStack(title)
.commit();
if (getSupportActionBar()!=null){
getSupportActionBar().setTitle(title);//DOESN'T WORK
}
}}