[main]: Add mdp to lecture notes

docs/lectures/mdp/04_threads.md

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+# Threads (and Services)
+
+Activities are regularly transitioned to the background
+
+* i.e. when they are stopped or destroyed
+* As the user continues with the *task*
+
+### Thread of Execution
+
+Android applications use a **single thread model**
+
+* A single thread of execution called *main*
+    * Started when a *process* is created
+* Handles and dispatches user interface events
+    * Drawing the interface, responding to `onClick()`
+* Handles activity life cycle events
+    * `onCreate()`, `onDestroy()`
+    * For all components in an application
+
+### Looper and Handler
+
+`HandlerThread` - extension of thread with support for looper
+
+**Looper**
+
+* Each `HandlerThread` can have one looper
+* A Java thread dies when the run method returns
+* Maintains a message queue
+* `looper.loop()` - loops through the message queue and processes waiting messages
+
+**Message**
+
+* A task to be completed
+    * Might contain data or a reference to a `Runnable` object
+
+**Handler**
+
+* Attached to a looper
+* Enqueues messages in the looper message queue
+* Handles messages from the message queue
+* **Thread-safe**
+* One looper can have many handlers associated with it
+
+![img](img/m.png)
+
+If we run code that takes >5 s, the application will appear to hang.
+
+* Put longer-running / non-instantaneous code in a separate thread of execution
+    * Network access, file access
+* 2 rules
+    1. Do not block the UI thread
+    2. Do not access the UI thread from outside the UI thread
+        - Android view objects are not thread-safe
+        - *Post* messages via the Handler to the UI thread's looper
+
+We can programmatically interact with UI components
+
+- e.g. `textField.setText(string)`
+- But cannot call this method from outside the UI thread (rule 2)
+
+Instead, split code into two parts
+
+1. Long(ish)-running code that *does not* involve the UI
+2. Instantaneous code that *does* involve the UI
+    - Which needs to be posted to the UI thread responsible for the view
+
+We need to remember the activity life-cycle; otherwise, we risk having **orphaned threads**.
+
+#### AsyncTask and others
+
+###### Accessing the UI thread from another thread
+
+```java
+Activity.runOnUiThread(Runnable);
+View.post(Runnable);
+View.postDelayed(Runnable, long);
+MutableLiveData.postValue;
+```