如果沒有 Internet，如何在 MainActivity 中顯示消息？

Question

如果沒有網絡，應用啟動時全屏顯示某個xml，表示沒有網絡，如果有，執行MainActivity。

public class MainActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener, MaterialSearchBar.OnSearchActionListener, PopupMenu.OnMenuItemClickListener {
    private static final String TAG = "MainActivity";

    @SuppressLint("StaticFieldLeak")
    public static MaterialSearchBar searchBar;
    private DrawerLayout drawer;
    private NavigationView navigationView;
    private AdView mAdView;
    private static List<String> listPermissionsNeeded;

    public boolean isInternetAvailable() {
        try {
            InetAddress address = InetAddress.getByName("www.google.com");
            return !address.equals("");
        } catch (UnknownHostException e) {
            // Log error
        }
        return false;
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        Log.d(TAG, "onCreate: setting things up");

        StaticUtils.requestQueue = (RequestQueue) Volley.newRequestQueue(getApplicationContext());
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //Admob banner
        MobileAds.initialize(this, new OnInitializationCompleteListener()  {
            @Override
            public void onInitializationComplete(InitializationStatus initializationStatus) {
            }
        });

        mAdView = findViewById(R.id.adView);
        AdRequest adRequest = new AdRequest.Builder().build();
        mAdView.loadAd(adRequest);
        mAdView.setAdListener(new AdListener() {
            @Override
            public void onAdLoaded() {
                super.onAdLoaded();
                mAdView.setVisibility(View.VISIBLE);
            }

            @Override
            public void onAdFailedToLoad(int i) {
                super.onAdFailedToLoad(i);
                mAdView.setVisibility(View.GONE);
            }


        });
        //restoring recent searches list
        StaticUtils.recentSearchesList = getArrayList(StaticUtils.KEY_LIST_PREFERENCCES);
        if (StaticUtils.recentSearchesList==null){
            StaticUtils.recentSearchesList = new ArrayList<>();
        }
        StaticUtils.savedImagesList = new ArrayList<>();
        searchBar = findViewById(R.id.searchToolBar);
        searchBar.setHint("Search Wallpapers");
        searchBar.setOnSearchActionListener(this);
        searchBar.hideSuggestionsList();

        drawer = findViewById(R.id.drawer_layout);
        navigationView = findViewById(R.id.nav_view);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
                this, drawer, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.addDrawerListener(toggle);
        toggle.syncState();
        navigationView.setNavigationItemSelectedListener(this);

        //sets home fragment open by default
        getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
                new HomeFragment()).commit();

        playLogoAudio();  //to play the logo audio
        searchEvents();  //advanced search events
    }

    @Override
    public void onBackPressed() {
        Log.d(TAG, "onBackPressed: back button invoked.");

        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
        } else {
            super.onBackPressed();
        }
    }

    public void grantPermissionBottomSheet(){
        View dialogView = getLayoutInflater().inflate(R.layout.layout_bottomsheet, null);
        final BottomSheetDialog dialog = new BottomSheetDialog(this);
        Button ok = dialogView.findViewById(R.id.bt_bottomsheet);
        ok.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Log.d(TAG, "onClick: bottomSheet button clicked.");
                requestPermissions(MainActivity.this);
                dialog.dismiss();
            }
        });
        dialog.setContentView(dialogView);
        dialog.show();
    }

    @Override
    public boolean onNavigationItemSelected(@NonNull MenuItem item) {
        Log.d(TAG, "onNavigationItemSelected: navigation item pressed.");

        // Handle navigation view item clicks here.
        int id = item.getItemId();

        if (id == R.id.nav_home) {
            getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
                    new HomeFragment()).commit();

        } else if (id == R.id.nav_saved) {
            NetworkUtils network = new NetworkUtils(this);
            if(network.checkConnection(drawer)){   //if network connected
                Intent i = new Intent(MainActivity.this, SecondActivity.class);
                i.putExtra(StaticUtils.KEY_FRAG_ID,2);
                i.putExtra(StaticUtils.KEY_SEARCH_DATA,"Saved");
                startActivity(i);
            }

        }else if (id == R.id.nav_downloads) {
            //downloaded images
            if (permissionsGranted(this)) {
                Intent i = new Intent(MainActivity.this, SecondActivity.class);
                i.putExtra(StaticUtils.KEY_FRAG_ID, 4);  //4 for downloads section
                i.putExtra(StaticUtils.KEY_SEARCH_DATA, "Downloads");
                startActivity(i);
            }else{
                grantPermissionBottomSheet();
            }

        } else if (id == R.id.nav_share) {
            //share the app intent
            Intent sendIntent = new Intent();
            sendIntent.setAction(Intent.ACTION_SEND);
            sendIntent.putExtra(Intent.EXTRA_TEXT, "Hey! look what I found from play store.\nDownload cool & amazing wallpapers for your device from this app, "+getResources().getString(R.string.app_name)+", among various categories.\n\nCheck this out:\n"+StaticUtils.playStoreUrlDefault+getPackageName()+"\nDownload now:)");
            sendIntent.setType("text/plain");
            startActivity(Intent.createChooser(sendIntent, "Share app through..."));

        } else if (id == R.id.nav_rate) {
            //rate the app intent
            Uri uri = Uri.parse("market://details?id=" + getApplicationContext().getPackageName());
            Intent goToMarket = new Intent(Intent.ACTION_VIEW, uri);
            // To count with Play market backstack, After pressing back button,
            // to taken back to our application, we need to add following flags to intent.
            goToMarket.addFlags(Intent.FLAG_ACTIVITY_NO_HISTORY |
                    Intent.FLAG_ACTIVITY_NEW_DOCUMENT |
                    Intent.FLAG_ACTIVITY_MULTIPLE_TASK);
            try {
                startActivity(goToMarket);
            } catch (Exception e) {
                startActivity(new Intent(Intent.ACTION_VIEW,
                        Uri.parse("http://play.google.com/store/apps/details?id=" + getApplicationContext().getPackageName())));
            }

        } else if (id == R.id.nav_about) {
            //about page
            Intent i = new Intent(MainActivity.this, SecondActivity.class);
            i.putExtra(StaticUtils.KEY_FRAG_ID,3);  //3 for about fragment
            i.putExtra(StaticUtils.KEY_SEARCH_DATA,"About");
            startActivity(i);
        }
        drawer.closeDrawer(GravityCompat.START);
        return true;
    }
    public void playLogoAudio(){
        Log.d(TAG, "playLogoAudio: playing logo audio");

        View headerView = navigationView.getHeaderView(0);
        ImageView drawerLogo = headerView.findViewById(R.id.imageLogo);
        drawerLogo.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                MediaPlayer mediaPlayer = MediaPlayer.create(MainActivity.this,R.raw.hello);
                mediaPlayer.start();
            }
        });
    }

    @Override
    public boolean onMenuItemClick(MenuItem item) {
        return false;
    }

    @Override
    public void onSearchStateChanged(boolean enabled) {
        if (enabled){
            getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
                    new SearchFragment()).commit();
        }else{
            getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
                    new HomeFragment()).commit();
        }
    }

    @Override
    public void onSearchConfirmed(CharSequence text) {
        Log.d(TAG, "onSearchConfirmed: confirmed search: "+text);

        Intent i = new Intent(this,SecondActivity.class);
        i.putExtra(StaticUtils.KEY_FRAG_ID,1);
        i.putExtra(StaticUtils.KEY_SEARCH_DATA,String.valueOf(text));

        if(new NetworkUtils(getApplicationContext()).checkConnection(drawer)) {  //start intent if network connected
            StaticUtils.recentSearchesList.add(String.valueOf(text));  //adds the query to the recents list

            if (StaticUtils.recentSearchesList.size()>20){
                StaticUtils.recentSearchesList.remove(0);
            }
            SearchFragment.updateAdapter(this,StaticUtils.recentSearchesList);
            searchBar.setText("");  //removes the search query
            startActivity(i);
        }
    }

    @Override
    public void onButtonClicked(int buttonCode) {
        Log.d(TAG, "onButtonClicked: search interface button clicked: "+buttonCode);
        switch (buttonCode){
            case MaterialSearchBar.BUTTON_NAVIGATION:
                drawer.openDrawer(GravityCompat.START);
                break;
            case MaterialSearchBar.BUTTON_BACK:
                searchBar.disableSearch();
        }
    }

    @Override
    public void onPointerCaptureChanged(boolean hasCapture) {}

    public void saveArrayList(ArrayList<String> list, String key){
        Log.d(TAG, "saveArrayList: saving recent searchList data");
        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
        SharedPreferences.Editor editor = prefs.edit();
        Gson gson = new Gson();
        String json = gson.toJson(list);
        editor.putString(key, json);
        editor.apply();
    }

    public ArrayList<String> getArrayList(String key){
        Log.d(TAG, "getArrayList: getting saved recent searchList data");
        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
        Gson gson = new Gson();
        String json = prefs.getString(key, null);
        Type type = new TypeToken<ArrayList<String>>() {}.getType();

        return gson.fromJson(json, type);
    }

    @Override
    protected void onPause() {
        super.onPause();
        //Saving arraylist when activity gets paused
        saveArrayList(StaticUtils.recentSearchesList,StaticUtils.KEY_LIST_PREFERENCCES);
    }

    public void searchEvents(){
        Log.d(TAG, "searchEvents: managing the search events");
        searchBar.addTextChangeListener(new TextWatcher() {
            ArrayList<String> tempList = new ArrayList<>();
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {
                Log.d(TAG, "beforeTextChanged: clearing the list");
                if (!tempList.isEmpty()){
                    tempList.clear();
                }
            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {
                Log.d(TAG, "onTextChanged: changing search query");
                for (String string : StaticUtils.recentSearchesList) {
                    if (s.length() > 0) {
                        if (string.matches("(?i)(" + s + ").*")) {
                            tempList.add(string);
                            SearchFragment.updateAdapter(getApplicationContext(), tempList);
                        }
                    }else{
                        SearchFragment.updateAdapter(getApplicationContext(), StaticUtils.recentSearchesList);
                    }
                }
            }

            @Override
            public void afterTextChanged(Editable s) {}
        });
    }

    @Override
    protected void onResume() {
        super.onResume();
        navigationView.setCheckedItem(R.id.nav_home);
        if (!permissionsGranted(this)){   //checking and requesting permissions
            grantPermissionBottomSheet();
        }
    }

    public static boolean permissionsGranted(Context context) {
        Log.d(TAG, "checkPermissions: checking if permissions granted.");
        int result;
        listPermissionsNeeded = new ArrayList<>();
        for (String p:permissions) {
            result = ContextCompat.checkSelfPermission(context,p);
            if (result != PackageManager.PERMISSION_GRANTED) {
                listPermissionsNeeded.add(p);
            }
        }
        //if all/some permissions not granted
        return listPermissionsNeeded.isEmpty();
    }

    public static void requestPermissions(Activity activity){
        Log.d(TAG, "requestPermissions: requesting permissions.");
        ActivityCompat.requestPermissions(activity, listPermissionsNeeded.toArray(new
                String[listPermissionsNeeded.size()]), StaticUtils.MULTIPLE_PERMISSIONS);
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        Log.d(TAG, "onRequestPermissionsResult: action when permission granted or denied");
        if (requestCode == StaticUtils.MULTIPLE_PERMISSIONS) {
            if (grantResults.length <= 0) {
                // no permissions granted.
                showPermissionDialog();
            }
        }
    }

    public void showPermissionDialog(){
        Log.d(TAG, "showPermissionDialog: requesting permissions");

        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setTitle("Are you sure?");
        builder.setMessage("You'll not be able to use this app properly without these permissions.");
        builder.setPositiveButton("Try Again", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                //re-requesting permissions
                requestPermissions(MainActivity.this);
            }
        });
        builder.setNegativeButton("Cancel", null);
        builder.show();
    }





}

Answer 1

我會建議某種不同的流程。 大多數應用程序都有一個稱為“啟動畫面”的東西（通常帶有徽標，有時是進度條）。

你可以做的是創建一個啟動活動， - 如果有互聯網調用你的 MainActiviy，否則使用你想要的 XML 調用另一個活動 NetworkErrorActivity。