[main]: Add mdp to lecture notes

docs/lectures/mdp/05_services.md

@@ -0,0 +1,130 @@
+# Services
+
+### Uses of services
+
+- MP3 Playback
+    - Playing songs in the background
+- Network Access
+    - Long downloads, sending emails, polling an email server for new mail, etc.
+
+### Services
+
+An application **component** that
+
+- Has no UI
+- Represents a desire to perform a longer-running operation
+
+Activities are loaded and unloaded as users move around the app
+
+- Services remain for as long as they are needed
+- One service may be used by many other applications to avoid duplication of resources.
+
+### What are Services *not*?
+
+**Not** a separate process - runs in the same process as the application in which it is declared
+
+**Not** a thread - there's one thread per application
+
+Services are simple
+
+- It's a way of telling the system about a part of your app that is expected to run for a long time
+    - i.e. longer than a few seconds
+
+### Tasks, Services and Activities
+
+###### Email
+
+- Checks for new mail occasionally -> Service
+- Collects new mail and stores it -> Service
+- Notifies user that there is new mail -> Service / Activity
+    - New component - **notifications**
+- User switches to inbox and retrieves/views new mail -> Activity
+
+###### MP3 Playback task
+
+- Play music while user does something else -> Service
+- Create a playlist -> Activity
+- Change the song playing & display current song progress -> Service / Activity
+
+### Creating a Service
+
+- Services are designed to support communication with
+    - Local activities (in the same process)
+        - within VM
+    - Remote activities
+        - IPC (Inter-process communication)
+    - Multiple components
+        - providing a service for multiple clients
+
+Services are components, similar to an activity
+
+- Register the service in the manifest
+- Create a subclass of `android.app.Service`
+- Handle the relevant life cycle methods
+
+### Service Life Cycle
+
+- By nature, services are **singleton objects**
+    - A service might be used by many clients
+- A service is started by something
+    - via an `intent`
+    - The service subclass object is instantiated if necessary
+        - `onCreate()` is called
+        - either `onStartCommand` or `onBind` will be called depending on how the service has been invoked
+- `onCreate()`/`onStart()`/`onBind()` are called in the context of the main UI thread
+    - A single thread for all components in an application
+    - Must spawn a worker thread
+- Something calls `stopService()`
+    - Could be the service, one of the clients (the activity) or the OS
+- `onDestroy()`
+
+## Code Examples
+
+`MainActivity` class
+
+```java
+public class MainActivity extends AppCompatActivity {
+    @Override
+    protected void onCreate(Bundle ...) {
+        ...
+    }
+    
+    //if clicked twice - onStartCommand is called, onCreate is called ONCE
+    public void onClickStartService(View v){
+        this.startService(new Intent(MainActivity.this, SimpleService.class));
+    }
+    
+    public void onClickStopService(View v){
+        this.stopService(new Intent(MainActivity.this, SimpleService.class));
+    }
+    
+    public void onClickClobberService(View v) {
+        android.os.Process.killProcess(android.os.Process.myPid());
+    }
+}
+```
+
+`SimpleService` class
+
+```java
+public class SimpleService extends Service {
+    //called first
+    public void onCreate() { 
+        super.onCreate();
+    }
+    
+    //called second
+    public int onStartCommand(Intent i, int flags, int startId) {
+        return Service.START_REDELIVER_INTENT;
+    }
+}
+```
+
+Android Manifest
+
+```xml
+<service
+    android:name=".SimpleService"
+    android:exported="false">
+</service>
+```