Activity中多视图架构实现咨询：展示Fragment与断连全屏视图

Nice problem to solve! Let's walk through a clean, maintainable architecture for your Activity that handles all three view states you described. The key here is to separate concerns: your Activity will act as a controller that switches between states, while each state's UI is handled by dedicated Fragments or a standalone offline view.

1. Core Layout Structure #

First, set up your Activity's layout to accommodate all three states. We'll use a ConstraintLayout as the root to easily control visibility and positioning:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <!-- Stable AppBarLayout - hidden only in offline state -->
    <com.google.android.material.appbar.AppBarLayout
        android:id="@+id/app_bar_layout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:layout_constraintTop_toTopOf="parent"
        android:visibility="visible">

        <com.google.android.material.appbar.MaterialToolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            app:title="Your App Title"/>

    </com.google.android.material.appbar.AppBarLayout>

    <!-- Fragment Container - holds either the scrollable or tabbed Fragment -->
    <FrameLayout
        android:id="@+id/fragment_container"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        app:layout_constraintTop_toBottomOf="@id/app_bar_layout"
        app:layout_constraintBottom_toBottomOf="parent"
        android:visibility="visible"/>

    <!-- Full-screen offline view - hidden by default -->
    <LinearLayout
        android:id="@+id/offline_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        android:gravity="center"
        android:visibility="gone"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toBottomOf="parent">

        <ImageView
            android:layout_width="120dp"
            android:layout_height="120dp"
            android:src="@drawable/ic_no_internet"/>

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="No internet connection"
            android:textSize="18sp"
            android:layout_marginTop="16dp"/>

        <Button
            android:id="@+id/retry_button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Retry"
            android:layout_marginTop="24dp"/>

    </LinearLayout>

</androidx.constraintlayout.widget.ConstraintLayout>

2. Define State Enums #

Create an enum to represent your three states—this makes state switching explicit and avoids magic values:

enum class ScreenState {
    SCROLLABLE_CONTENT, // State 1: NestedScrollView in Fragment
    TABBED_CONTENT,     // State 2: Tabbed Fragment
    OFFLINE             // State 3: Full-screen offline view
}

3. State Switching Logic #

In your Activity, implement a method to switch between states. This method will handle showing/hiding the AppBar, swapping Fragments, and toggling the offline view:

class MainActivity : AppCompatActivity() {

    private lateinit var appBarLayout: AppBarLayout
    private lateinit var fragmentContainer: FrameLayout
    private lateinit var offlineView: LinearLayout
    private lateinit var retryButton: Button

    // Initialize Fragments once to preserve state
    private val scrollableFragment = ScrollableContentFragment()
    private val tabbedFragment = TabbedContentFragment()
    private var currentState: ScreenState = ScreenState.SCROLLABLE_CONTENT

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        // Initialize views
        appBarLayout = findViewById(R.id.app_bar_layout)
        fragmentContainer = findViewById(R.id.fragment_container)
        offlineView = findViewById(R.id.offline_view)
        retryButton = findViewById(R.id.retry_button)

        // Set up initial fragment
        supportFragmentManager.beginTransaction()
            .add(R.id.fragment_container, scrollableFragment, "scrollable")
            .add(R.id.fragment_container, tabbedFragment, "tabbed")
            .hide(tabbedFragment)
            .commit()

        // Retry button click listener
        retryButton.setOnClickListener {
            // Check internet connection and switch back to previous state
            if (isInternetAvailable()) {
                switchToState(currentState)
            }
        }
    }

    fun switchToState(newState: ScreenState) {
        currentState = newState
        when (newState) {
            ScreenState.SCROLLABLE_CONTENT -> {
                // Show AppBar and fragment container, hide offline view
                appBarLayout.visibility = View.VISIBLE
                fragmentContainer.visibility = View.VISIBLE
                offlineView.visibility = View.GONE

                // Switch to scrollable fragment
                supportFragmentManager.beginTransaction()
                    .hide(tabbedFragment)
                    .show(scrollableFragment)
                    .commit()
            }
            ScreenState.TABBED_CONTENT -> {
                // Show AppBar and fragment container, hide offline view
                appBarLayout.visibility = View.VISIBLE
                fragmentContainer.visibility = View.VISIBLE
                offlineView.visibility = View.GONE

                // Switch to tabbed fragment
                supportFragmentManager.beginTransaction()
                    .hide(scrollableFragment)
                    .show(tabbedFragment)
                    .commit()
            }
            ScreenState.OFFLINE -> {
                // Hide AppBar and fragment container, show offline view
                appBarLayout.visibility = View.GONE
                fragmentContainer.visibility = View.GONE
                offlineView.visibility = View.VISIBLE
            }
        }
    }

    // Helper method to check internet connectivity (simplified example)
    private fun isInternetAvailable(): Boolean {
        val connectivityManager = getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
        val network = connectivityManager.activeNetwork ?: return false
        val capabilities = connectivityManager.getNetworkCapabilities(network) ?: return false
        return capabilities.hasTransport(NetworkCapabilities.TRANSPORT_WIFI) ||
                capabilities.hasTransport(NetworkCapabilities.TRANSPORT_CELLULAR)
    }
}

4. Implement the Fragments #

Now create your two content Fragments:

ScrollableContentFragment (State 1) #

This fragment holds your NestedScrollView:

<!-- fragment_scrollable_content.xml -->
<androidx.core.widget.NestedScrollView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <!-- Your long scrollable content here -->
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        android:padding="16dp">

        <TextView
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Scrollable Content"
            android:textSize="24sp"
            android:layout_marginBottom="16dp"/>

        <!-- Add more views as needed -->

    </LinearLayout>

</androidx.core.widget.NestedScrollView>

TabbedContentFragment (State 2) #

This fragment uses a TabLayout and ViewPager2 for tab switching:

<!-- fragment_tabbed_content.xml -->
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <com.google.android.material.tabs.TabLayout
        android:id="@+id/tab_layout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:tabMode="fixed">

        <com.google.android.material.tabs.TabItem
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Tab 1"/>

        <com.google.android.material.tabs.TabItem
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Tab 2"/>

    </com.google.android.material.tabs.TabLayout>

    <androidx.viewpager2.widget.ViewPager2
        android:id="@+id/view_pager"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"/>

</LinearLayout>

In the Fragment class, set up the ViewPager2 adapter:

class TabbedContentFragment : Fragment() {

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        val view = inflater.inflate(R.layout.fragment_tabbed_content, container, false)
        val tabLayout = view.findViewById<TabLayout>(R.id.tab_layout)
        val viewPager = view.findViewById<ViewPager2>(R.id.view_pager)

        viewPager.adapter = TabAdapter(this)
        TabLayoutMediator(tabLayout, viewPager) { tab, position ->
            tab.text = when (position) {
                0 -> "Tab 1"
                1 -> "Tab 2"
                else -> ""
            }
        }.attach()

        return view
    }

    private class TabAdapter(fragment: Fragment) : FragmentStateAdapter(fragment) {
        override fun getItemCount() = 2
        override fun createFragment(position: Int) = when (position) {
            0 -> Tab1Fragment()
            1 -> Tab2Fragment()
            else -> Tab1Fragment()
        }
    }
}

5. Bonus Tips for Stability #

• Preserve Fragment State: By using hide() and show() instead of replace(), we keep the Fragments in memory, preserving scroll positions, tab selections, and user input.
• Internet Monitoring: Use a ConnectivityManager.NetworkCallback to automatically switch to the offline state when connectivity is lost, and switch back when it's restored.
• ViewModel Integration: For more complex state management, use a ViewModel to hold the current screen state, so it survives configuration changes (like screen rotations).
• Testability: Each Fragment handles its own business logic, making it easier to test in isolation.

内容的提问来源于stack exchange，提问作者yozhik