我在Android应用中实施导航BottomNavigationView。我正在使用片段为每个标签设置内容。
我知道如何为每个标签设置一个片段，然后在单击标签时切换片段。但是，如何为每个选项卡分别设置一个后备堆栈？ 以下是设置一个片段的代码：
Fragment selectedFragment = ItemsFragment.newInstance();
FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
transaction.replace(R.id.content, selectedFragment);
transaction.commit();
例如，Fragment A和B将位于选项卡1下，Fragment C和D位于选项卡2下。当应用程序启动时，将显示片段A并选择选项卡1。然后Fragment A可能被片段B替换。当选择标签2时，应显示片段C.如果然后选择了标签1，则应再次显示Fragment B.此时，应该可以使用后退按钮显示片段A.
以下是在同一标签中设置下一个fragment的代码：
Fragment selectedFragment = ItemsFragment.newInstance();
FragmentTransaction ft = getFragmentManager().beginTransaction();
ft.replace(R.id.content, selectedFragment);
ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
ft.addToBackStack(null);
ft.commit();
答案 0 :(得分：23)
最后，我找到了解决方案，它的灵感来自之前对StackOverflow的回答：Separate Back Stack for each tab in Android using Fragments
我只用BottomNavigationView替换了TabHost，这里是代码：
主要活动
public class MainActivity extends AppCompatActivity {
private HashMap<String, Stack<Fragment>> mStacks;
public static final String TAB_HOME = "tab_home";
public static final String TAB_DASHBOARD = "tab_dashboard";
public static final String TAB_NOTIFICATIONS = "tab_notifications";
private String mCurrentTab;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation);
navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);
mStacks = new HashMap<String, Stack<Fragment>>();
mStacks.put(TAB_HOME, new Stack<Fragment>());
mStacks.put(TAB_DASHBOARD, new Stack<Fragment>());
mStacks.put(TAB_NOTIFICATIONS, new Stack<Fragment>());
navigation.setSelectedItemId(R.id.navigation_home);
}
private BottomNavigationView.OnNavigationItemSelectedListener mOnNavigationItemSelectedListener
= new BottomNavigationView.OnNavigationItemSelectedListener() {
@Override
public boolean onNavigationItemSelected(@NonNull MenuItem item) {
switch (item.getItemId()) {
case R.id.navigation_home:
selectedTab(TAB_HOME);
return true;
case R.id.navigation_dashboard:
selectedTab(TAB_DASHBOARD);
return true;
case R.id.navigation_notifications:
selectedTab(TAB_NOTIFICATIONS);
return true;
}
return false;
}
};
private void gotoFragment(Fragment selectedFragment)
{
FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
fragmentTransaction.replace(R.id.content, selectedFragment);
fragmentTransaction.commit();
}
private void selectedTab(String tabId)
{
mCurrentTab = tabId;
if(mStacks.get(tabId).size() == 0){
/*
* First time this tab is selected. So add first fragment of that tab.
* Dont need animation, so that argument is false.
* We are adding a new fragment which is not present in stack. So add to stack is true.
*/
if(tabId.equals(TAB_HOME)){
pushFragments(tabId, new HomeFragment(),true);
}else if(tabId.equals(TAB_DASHBOARD)){
pushFragments(tabId, new DashboardFragment(),true);
}else if(tabId.equals(TAB_NOTIFICATIONS)){
pushFragments(tabId, new NotificationsFragment(),true);
}
}else {
/*
* We are switching tabs, and target tab is already has atleast one fragment.
* No need of animation, no need of stack pushing. Just show the target fragment
*/
pushFragments(tabId, mStacks.get(tabId).lastElement(),false);
}
}
public void pushFragments(String tag, Fragment fragment, boolean shouldAdd){
if(shouldAdd)
mStacks.get(tag).push(fragment);
FragmentManager manager = getSupportFragmentManager();
FragmentTransaction ft = manager.beginTransaction();
ft.replace(R.id.content, fragment);
ft.commit();
}
public void popFragments(){
/*
* Select the second last fragment in current tab's stack..
* which will be shown after the fragment transaction given below
*/
Fragment fragment = mStacks.get(mCurrentTab).elementAt(mStacks.get(mCurrentTab).size() - 2);
/*pop current fragment from stack.. */
mStacks.get(mCurrentTab).pop();
/* We have the target fragment in hand.. Just show it.. Show a standard navigation animation*/
FragmentManager manager = getSupportFragmentManager();
FragmentTransaction ft = manager.beginTransaction();
ft.replace(R.id.content, fragment);
ft.commit();
}
@Override
public void onBackPressed() {
if(mStacks.get(mCurrentTab).size() == 1){
// We are already showing first fragment of current tab, so when back pressed, we will finish this activity..
finish();
return;
}
/* Goto previous fragment in navigation stack of this tab */
popFragments();
}
}
主页片段示例
public class HomeFragment extends Fragment {
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_home, container, false);
Button gotoNextFragment = (Button) view.findViewById(R.id.gotoHome2);
gotoNextFragment.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
((MainActivity)getActivity()).pushFragments(MainActivity.TAB_HOME, new Home2Fragment(),true);
}
});
return view;
}
}
答案 1 :(得分：2)
值得注意的是，您描述的行为违反了Google准则。 https://material.io/guidelines/components/bottom-navigation.html#bottom-navigation-behavior
通过底部导航栏导航应重置任务状态。
换句话说，将片段A和片段B放在“内部”选项卡1中就可以了，但是如果用户打开片段B，单击Tab 2，然后再次单击Tab 1，它们应该看到片段A.
答案 2 :(得分：2)
新的导航体系结构组件（https://developer.android.com/topic/libraries/architecture/navigation/）支持此行为。
本质上，人们可以使用NavHostFragment，这是一个控制自己的后堆栈的片段：
每个NavHostFragment都有一个NavController，用于定义导航主机中的有效导航。这包括导航图以及导航状态（例如当前位置和后退堆栈），它们将与NavHostFragment本身一起保存和恢复。 https://developer.android.com/reference/androidx/navigation/fragment/NavHostFragment
答案 3 :(得分：0)
假设您有5（A，B，C，D，E）BottomNavigationView菜单项，然后在Activity中以平行重叠的方式创建5 FrameLayout（frmlytA，frmlytB，frmlytC，frmlytD，frmlytE）作为这些容器的容器菜单项。当按下BottomNavigation菜单项A时，隐藏所有其他FrameLayouts（可见性= GONE），然后仅显示（Visibility = VISIBLE）将托管FragmentA的FrameLayout'frmlytA'，并在此容器上进行进一步的事务，例如（FragmentA-> FragmentX -> FragmentY）。然后，如果用户单击BottomNavigation菜单项B，则只需隐藏此（frmlytA）容器并显示'frmlytB'。然后，如果用户再次按下菜单项A，然后显示“ frmlytA”，则它应保持较早的状态。因此，您可以像这样在容器的FrameLayouts之间切换，并可以维护每个容器的后堆栈。
答案 4 :(得分：-4)
使用add fragment，
而不是使用替换方法而不是这种方法 ft.replace（R.id.content，selectedFragment）;
使用此 ft.add（R.id.content，selectedFragment）;
Fragment selectedFragment = ItemsFragment.newInstance();
FragmentTransaction ft = getFragmentManager().beginTransaction();
ft.(R.id.content, selectedFragment);
ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
ft.addToBackStack(null);
ft.commit();