Added observer and observable interface, implemented observable on ChannelBase

src/main/java/cz/cvut/fel/sit/omo/foodchain/channel/ChannelBase.java

@@ -3,16 +3,20 @@ package cz.cvut.fel.sit.omo.foodchain.channel;
 import cz.cvut.fel.sit.omo.foodchain.operation.Transaction;
 import cz.cvut.fel.sit.omo.foodchain.product.ProductType;
 import cz.cvut.fel.sit.omo.foodchain.requirement.Requirement;
+import cz.cvut.fel.sit.omo.foodchain.util.ChannelObserver;
+import cz.cvut.fel.sit.omo.foodchain.util.Observable;
 
 import java.util.PriorityQueue;
 import java.util.Queue;
+import java.util.Set;
 
-public abstract class ChannelBase implements IChannel
+public abstract class ChannelBase implements IChannel, Observable {
-{
     protected ProductType productType;
     protected Queue<Requirement> requirements;
     protected Queue<Transaction> transactions;
 
+    private Set<ChannelObserver> subscribers;
+
     public ChannelBase(ProductType type)
     {
         this.requirements = new PriorityQueue<>();
@@ -47,6 +51,22 @@ public abstract class ChannelBase implements IChannel
     public void addRequirement(Requirement requirement)
     {
         this.requirements.add(requirement);
+        notifyAllObservers();
+    }
+
+    @Override
+    public void notifyAllObservers(){
+        subscribers.forEach(subscriber -> subscriber.update(requirements.peek()));
+    }
+
+    @Override
+    public void attach(ChannelObserver observer){
+        subscribers.add(observer);
+    }
+
+    @Override
+    public void detach(ChannelObserver observer){
+        subscribers.remove(observer);
     }
 
     /**

src/main/java/cz/cvut/fel/sit/omo/foodchain/util/ChannelObserver.java

@@ -0,0 +1,7 @@
+package cz.cvut.fel.sit.omo.foodchain.util;
+
+import cz.cvut.fel.sit.omo.foodchain.requirement.Requirement;
+
+public interface ChannelObserver {
+    void update(Requirement requirement);
+}

src/main/java/cz/cvut/fel/sit/omo/foodchain/util/Observable.java

@@ -0,0 +1,7 @@
+package cz.cvut.fel.sit.omo.foodchain.util;
+
+public interface Observable {
+    void attach(ChannelObserver channelObserver);
+    void detach(ChannelObserver channelObserver);
+    void notifyAllObservers();
+}