feature: made TaskList observable

src/main/java/ch/zhaw/gartenverwaltung/io/JsonTaskList.java

@@ -13,6 +13,7 @@ import java.net.URISyntaxException;
 import java.net.URL;
 import java.time.LocalDate;
 import java.time.format.DateTimeFormatter;
+import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
@@ -29,6 +30,7 @@ public class JsonTaskList implements TaskList {
     private final static String INVALID_DATASOURCE_MSG = "Invalid datasource specified!";
 
     private Map<Long, Task> taskMap = Collections.emptyMap();
+    private final List<TaskListObserver> subscribers = new ArrayList<>();
 
     /**
      * Creating constant objects required to deserialize the {@link LocalDate} classes
@@ -87,6 +89,7 @@ public class JsonTaskList implements TaskList {
             loadTaskListFromFile();
         }
         taskMap.values().removeIf(task -> task.getCropId() == cropId);
+        notifySubscribers();
     }
 
     /**
@@ -107,6 +110,7 @@ public class JsonTaskList implements TaskList {
 
         taskMap.put(id, task.withId(id));
         writeTaskListToFile();
+        notifySubscribers();
     }
 
     /**
@@ -126,6 +130,25 @@ public class JsonTaskList implements TaskList {
             taskMap.remove(taskId);
             writeTaskListToFile();
         }
+        notifySubscribers();
+    }
+
+    /**
+     * {@inheritDoc}
+     * @param observer The change handler
+     */
+    @Override
+    public void subscribe(TaskListObserver observer) {
+        subscribers.add(observer);
+    }
+
+    /**
+     * Calls the change handler method on all registered observers.
+     */
+    private void notifySubscribers() {
+        for (TaskListObserver subscriber : subscribers) {
+            subscriber.onChange(taskMap.values().stream().toList());
+        }
     }
 
     /**

src/main/java/ch/zhaw/gartenverwaltung/io/TaskList.java

@@ -51,4 +51,22 @@ public interface TaskList {
      * @throws IOException If the database cannot be accessed
      */
     void removeTask(Task task) throws IOException;
+
+    /**
+     * Registers an observer to be notified of Changes in the TaskList
+     * @param observer The change handler
+     */
+    void subscribe(TaskListObserver observer);
+
+    /**
+     * Specifies an observer for a TaskList
+     */
+    @FunctionalInterface
+    interface TaskListObserver {
+        /**
+         * Method which will be called when changes occur.
+         * @param newTaskList The new values
+         */
+        void onChange(List<Task> newTaskList);
+    }
 }