应用程序关闭时如何显示通知？

Question

I have a video calling application (it's called FtoF and it's currently on the play store). I would like to show a notification when one user calls another, and this I did (lines 110-132 and 255), the problem is that this only works when the app is open. How could I make notifications work even when the app is closed? I leave you the code:

CallingActivity.java

public class CallingActivity extends AppCompatActivity {
    private TextView nameContact;
    private ImageView profileImage;
    private ImageView cancelCallBtn, acceptCallBtn; //private ImageView cancelCallBtn, acceptCallBtn;
    private String callingId="", ringingId="";
    private String reciverUserId="", receiverUserImage="", receiverUserName="";
    private String senderUserId="", senderUserImage="", senderUserName="", checker="";
    private DatabaseReference userRef;
    private MediaPlayer mediaPlayer;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_calling);
        senderUserId = FirebaseAuth.getInstance().getCurrentUser().getUid();
        reciverUserId = getIntent().getExtras().get("visit_user_id").toString();
        userRef = FirebaseDatabase.getInstance().getReference().child("Users");
        nameContact = findViewById(R.id.name_calling);
        profileImage = findViewById(R.id.profile_image_calling);
        cancelCallBtn = findViewById(R.id.cancel_call);
        acceptCallBtn = findViewById(R.id.make_call);
        cancelCallBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                checker = "clicked";
                cancelCallingUser();
            }
        });
        acceptCallBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                final HashMap<String, Object> callingPickUpMap = new HashMap<>();
                callingPickUpMap.put("picked", "picked");
                userRef.child(senderUserId).child("Ringing")
                        .updateChildren(callingPickUpMap)
                        .addOnCompleteListener(new OnCompleteListener<Void>() {
                            @Override
                            public void onComplete(@NonNull Task<Void> task) {
                                Intent intent = new Intent(CallingActivity.this, VideoCallActivity.class);
                                startActivity(intent);
                            }
                        });
            }
        });
        getAndSetUserProfileInfo();
    }

    private void addNotification() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O){
            NotificationChannel channel =
                    new NotificationChannel("MyNotifications", "MyNotifications", NotificationManager.IMPORTANCE_DEFAULT);
            NotificationManager manager = getSystemService(NotificationManager.class);
            manager.createNotificationChannel(channel);
        }
        NotificationCompat.Builder builder = new NotificationCompat.Builder(this, "MyNotifications")
                .setContentTitle("Chiamata in arrivo da "+ receiverUserName)
                .setSmallIcon(R.mipmap.ic_launcher)
                .setAutoCancel(true)
                .setContentText("Apri FtoF per partecipare alla videochiamata");
        NotificationManagerCompat manager = NotificationManagerCompat.from(this);
        manager.notify(999, builder.build());
    }

    private void getAndSetUserProfileInfo() {
        userRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                if(dataSnapshot.child(reciverUserId).exists()){
                    receiverUserImage = dataSnapshot.child(reciverUserId).child("image").getValue().toString();
                    receiverUserName = dataSnapshot.child(reciverUserId).child("name").getValue().toString();
                    nameContact.setText(receiverUserName);
                    Picasso.get().load(receiverUserImage).placeholder(R.drawable.profile_image).into(profileImage);
                }
                if(dataSnapshot.child(senderUserId).exists()){
                    senderUserImage = dataSnapshot.child(senderUserId).child("image").getValue().toString();
                    senderUserName = dataSnapshot.child(senderUserId).child("name").getValue().toString();
                }
                TextView info_txt = (TextView) findViewById(R.id.txt);
                info_txt.setText("in attesa che " + receiverUserName + " accetti l'invito..."); //concatenazione per testo sotto al nome
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) { }
        });
    }

    @Override
    protected void onStart() {
        super.onStart();
        userRef.child(reciverUserId)
                .addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                        if (!checker.equals("clicked") && !dataSnapshot.hasChild("Calling") && !dataSnapshot.hasChild("Ringing")){
                            final HashMap<String, Object> callingInfo = new HashMap<>();
                            callingInfo.put("calling", reciverUserId);
                            userRef.child(senderUserId)
                                    .child("Calling")
                                    .updateChildren(callingInfo)
                                    .addOnCompleteListener(new OnCompleteListener<Void>() {
                                        @Override
                                        public void onComplete(@NonNull Task<Void> task) {
                                            if (task.isSuccessful()){
                                                final HashMap<String, Object> ringingInfo = new HashMap<>();
                                                ringingInfo.put("ringing", senderUserId);
                                                userRef.child(reciverUserId)
                                                        .child("Ringing")
                                                        .updateChildren(ringingInfo);
                                            }
                                        }
                                    });
                        }
                    }

                    @Override
                    public void onCancelled(@NonNull DatabaseError databaseError) { }
                });

        userRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                if(dataSnapshot.child(senderUserId).hasChild("Ringing") && !dataSnapshot.child(senderUserId).hasChild("Calling")){
                    addNotification();
                    acceptCallBtn.setVisibility(View.VISIBLE);
                }
                if (dataSnapshot.child(reciverUserId).child("Ringing").hasChild("picked"))
                {
                    Intent intent = new Intent(CallingActivity.this, VideoCallActivity.class);
                    startActivity(intent);
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) { }
        });
    }

    private void cancelCallingUser() {
        //sender
        userRef.child(senderUserId)
                .child("Calling")
                .addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                        if (dataSnapshot.exists() && dataSnapshot.hasChild("calling"))
                        {
                            callingId = dataSnapshot.child("calling").getValue().toString();
                            userRef.child(callingId)
                                    .child("Ringing")
                                    .removeValue()
                                    .addOnCompleteListener(new OnCompleteListener<Void>() {
                                        @Override
                                        public void onComplete(@NonNull Task<Void> task) {
                                            if (task.isSuccessful()){
                                                userRef.child(senderUserId)
                                                        .child("Calling")
                                                        .removeValue()
                                                        .addOnCompleteListener(new OnCompleteListener<Void>() {
                                                            @Override
                                                            public void onComplete(@NonNull Task<Void> task) {
                                                                startActivity(new Intent(CallingActivity.this, RegistrationActivity.class));
                                                                finish();
                                                            }
                                                        });
                                            }
                                        }
                                    });
                        } else {
                            startActivity(new Intent(CallingActivity.this, RegistrationActivity.class));
                            finish();
                        }
                    }

                    @Override
                    public void onCancelled(@NonNull DatabaseError databaseError) {
                    }
                });
        //receiver
        userRef.child(senderUserId)
                .child("Ringing")
                .addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                        if (dataSnapshot.exists() && dataSnapshot.hasChild("ringing"))

                        {
                            ringingId = dataSnapshot.child("ringing").getValue().toString();
                            userRef.child(ringingId)
                                    .child("Calling")
                                    .removeValue()
                                    .addOnCompleteListener(new OnCompleteListener<Void>() {
                                        @Override
                                        public void onComplete(@NonNull Task<Void> task) {
                                            if (task.isSuccessful()){
                                                userRef.child(senderUserId)
                                                        .child("Ringing")
                                                        .removeValue()
                                                        .addOnCompleteListener(new OnCompleteListener<Void>() {
                                                            @Override
                                                            public void onComplete(@NonNull Task<Void> task) {
                                                                startActivity(new Intent(CallingActivity.this, RegistrationActivity.class));
                                                                finish();
                                                            }
                                                        });
                                            }
                                        }
                                    });
                        }
                        else {
                            startActivity(new Intent(CallingActivity.this, RegistrationActivity.class));
                            finish();
                        }
                    }

                    @Override
                    public void onCancelled(@NonNull DatabaseError databaseError) {
                    }
                });
    }
}

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    package="com.conta.ftof">

    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
    <uses-permission android:name="android.permission.ACCESS_WIFI_STATE"/>
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.USE_FULL_SCREEN_INTENT" />

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".donate" />
        <activity android:name=".VideoCallActivity" />
        <activity android:name=".CallingActivity" />
        <activity android:name=".ProfileActivity" />
        <activity android:name=".FindPeopleActivity" />
        <activity android:name=".NotificationsActivity" />
        <activity android:name=".SettingsActivity" />
        <activity
            android:name=".ContactsActivity"
            android:label="@string/title_activity_main" />
        <activity android:name=".RegistrationActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <meta-data
            android:name="com.google.android.gms.ads.APPLICATION_ID"
            android:value="ca-app-pub-3400243939195187~6852949310"/>

        <receiver android:name=".ReminderBroadcast"/>
    </application>
</manifest>

Answer 1

According to the NotificationCompat documentation:

public Builder (Context context, String channelId)

Constructs a new Builder with the defaults:

• context A Context that will be used by the Builder to construct the RemoteViews .
• channelId String: The constructed Notification will be posted on this NotificationChannel.

The Context is an interface to global information about an application environment. This is an abstract class whose implementation is provided by the Android system. It allows access to application-specific resources and classes, as well as up-calls for application-level operations such as launching activities, broadcasting and receiving intents, etc.

In this way, the Android operating system controls through Context what the application may or may not present to the user, thus preventing any application from displaying what it wants at the time it wants.

So the way you can present a notification is passing a Context do builder that have permission to construct Remote Views, and your app receives this context when the user open it or the app receives a broadcast message like a push notification .

public class PushNotificationReceiver extends BroadcastReceiver {

    @Override
    public void onReceive(Context context, Intent intent) {
        //at this point the context has the permission to create remote views
        addNotification(context);
        setResultCode(Activity.RESULT_OK);
    }

    private void addNotification(Context context) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O){
            NotificationChannel channel =
                    new NotificationChannel("MyNotifications", "MyNotifications", NotificationManager.IMPORTANCE_DEFAULT);
            NotificationManager manager = getSystemService(NotificationManager.class);
            manager.createNotificationChannel(channel);
        }
        NotificationCompat.Builder builder = new NotificationCompat.Builder(context, "MyNotifications")
                .setContentTitle("Chiamata in arrivo")
                .setSmallIcon(R.mipmap.ic_launcher)
                .setAutoCancel(true)
                .setContentText("Apri FtoF per partecipare alla videochiamata");
        NotificationManagerCompat manager = NotificationManagerCompat.from(context);
        manager.notify(999, builder.build());
    }

}