使用Android导航组件获取Backstack状态

Question

我要实现backpress行为，以便在您将backstack设为空时按回车时会提示确认弹出窗口，否则它将弹出堆栈中的下一个片段。

我正在尝试获取backstack计数，但是我总是从两个片段管理器中获得0

 getSupportFragmentManager().getBackStackEntryCount();
 getFragmentManager().getBackStackEntryCount();

我猜它应该工作，因为我检查了片段导航器的google代码，并且通过规范的片段事务将其添加到了backstack中：

FragmentNavigator.java：

 if (initialNavigation || isClearTask) {
        backStackEffect = BACK_STACK_DESTINATION_ADDED;
    } else if (isSingleTopReplacement) {
        // Single Top means we only want one instance on the back stack
        if (mBackStack.size() > 1) {
            // If the Fragment to be replaced is on the FragmentManager's
            // back stack, a simple replace() isn't enough so we
            // remove it from the back stack and put our replacement
            // on the back stack in its place
            mFragmentManager.popBackStack();
            ft.addToBackStack(Integer.toString(destId));
            mIsPendingBackStackOperation = true;
        }
        backStackEffect = BACK_STACK_UNCHANGED;
    } else {
        ft.addToBackStack(Integer.toString(destId));
        mIsPendingBackStackOperation = true;
        backStackEffect = BACK_STACK_DESTINATION_ADDED;
    }
    ft.setReorderingAllowed(true);
    ft.commit();

我没有找到任何API来通过NavController或Navigator来检索此信息。

谢谢

Answer 1

也可能对某人有帮助：

<fragment
    android:id="@+id/YOUR_NAVIGATION_HOST_ID"
    android:name="androidx.navigation.fragment.NavHostFragment"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:defaultNavHost="true"
    app:navGraph="@navigation/YOUR_NAVIGATION_HOST" />

val navHostFragment = supportFragmentManager.findFragmentById(R.id.YOUR_NAVIGATION_HOST_ID)
val backStackEntryCount = navHostFragment?.childFragmentManager?.backStackEntryCount

Answer 2

要检查导航主机片段的返回堆栈计数，您应该使用 navHostFragment 的 childFragmentManager ：

navHostFragment.childFragmentManager.addOnBackStackChangedListener {
            if (navHostFragment.childFragmentManager.backStackEntryCount == 0) {
                // First fragment is open, backstack is empty
            } else {
                // there are fragments in backstack
            }
        }