Got it, let's walk through exactly how to build this feature—this is a common use case for ensuring users have connectivity before using your app, and it's pretty straightforward once you lay out the steps. I'll use Android as the example since that's where this scenario pops up most often.

1. Add Network Permissions to Your Manifest #

First, you need to let the app access network state info. Open your AndroidManifest.xml and add these permissions:

<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.INTERNET" /> <!-- Optional but useful if your app needs internet later -->

The ACCESS_NETWORK_STATE permission is mandatory for checking connectivity status.

2. Create a Network Utility Class #

Make a reusable helper method to check if the device has an active network connection. This keeps your code clean and easy to reuse elsewhere:

import android.content.Context
import android.net.ConnectivityManager
import android.net.NetworkCapabilities

object NetworkUtils {
    fun isNetworkAvailable(context: Context): Boolean {
        val connectivityManager = context.getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
        val network = connectivityManager.activeNetwork ?: return false
        val networkCapabilities = connectivityManager.getNetworkCapabilities(network) ?: return false
        
        return networkCapabilities.hasCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET) &&
                networkCapabilities.hasCapability(NetworkCapabilities.NET_CAPABILITY_VALIDATED)
    }
}

This method checks for both internet access and validated connectivity (so it's not just a cached network signal).

3. Check Network Status on App Launch #

In your launch Activity (usually MainActivity or a splash screen), call the utility method right in onCreate():

import android.app.AlertDialog
import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity

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

        // Check network status as soon as the app starts
        if (!NetworkUtils.isNetworkAvailable(this)) {
            showNoNetworkDialog()
        }
    }

    // Method to show the alert dialog
    private fun showNoNetworkDialog() {
        AlertDialog.Builder(this)
            .setTitle("No Network Connection")
            .setMessage("Please check your internet connection and try again.")
            .setCancelable(false) // Prevent user from dismissing the dialog by tapping outside
            .setPositiveButton("Ok") { _, _ ->
                // Close the app when Ok is clicked
                finishAffinity() // Closes all activities in the app's task
                // Alternatively, use System.exit(0) for a hard exit (use cautiously)
            }
            .show()
    }
}

Key Notes Here: #

• setCancelable(false) ensures the user can't bypass the dialog by tapping outside—they have to press "Ok" to close the app.
• finishAffinity() is the recommended way to close the app on Android, as it properly cleans up all open activities in your app's task. System.exit(0) works too but is a more forceful exit and should be used only if finishAffinity() doesn't work for your setup.

4. Handle Edge Cases (Optional) #

• If your app targets Android 12 (API 31+) or higher, you don't need any extra permissions beyond what we added earlier—ACCESS_NETWORK_STATE is still sufficient for foreground checks.
• If you're using Java instead of Kotlin, the logic is identical; you just need to adjust the syntax to Java's class structure.

That's all! This setup will detect no network on app launch, show the dialog, and close the app when the user taps "Ok".

内容的提问来源于stack exchange，提问作者Gustavo Gbn