Model interface: Add method to get predicate of filteredPersonList

The Model interface does not contain any method to
get the predicate of its filteredPersonList.

This prevents other classes from getting the predicate if
necessary.

Let's add a method in Model to get the predicate of its
filteredPersonList

src/main/java/seedu/address/model/Model.java

@@ -39,6 +39,9 @@ void updatePerson(ReadOnlyPerson target, ReadOnlyPerson editedPerson)
     /** Returns an unmodifiable view of the filtered person list */
     ObservableList<ReadOnlyPerson> getFilteredPersonList();
 
+    /** Returns the predicate used in the filtered person list */
+    Predicate<ReadOnlyPerson> getFilteredPersonListPredicate();
+
     /**
      * Updates the filter of the filtered person list to filter by the given {@code predicate}.
      * @throws NullPointerException if {@code predicate} is null.

src/main/java/seedu/address/model/ModelManager.java

@@ -92,6 +92,11 @@ public ObservableList<ReadOnlyPerson> getFilteredPersonList() {
         return FXCollections.unmodifiableObservableList(filteredPersons);
     }
 
+    @Override
+    public Predicate<ReadOnlyPerson> getFilteredPersonListPredicate() {
+        return (Predicate<ReadOnlyPerson>) filteredPersons.getPredicate();
+    }
+
     @Override
     public void updateFilteredPersonList(Predicate<ReadOnlyPerson> predicate) {
         requireNonNull(predicate);

src/test/java/seedu/address/logic/commands/AddCommandTest.java

@@ -129,6 +129,12 @@ public ObservableList<ReadOnlyPerson> getFilteredPersonList() {
             return null;
         }
 
+        @Override
+        public Predicate<ReadOnlyPerson> getFilteredPersonListPredicate() {
+            fail("This method should not be called.");
+            return null;
+        }
+
         @Override
         public void updateFilteredPersonList(Predicate<ReadOnlyPerson> predicate) {
             fail("This method should not be called.");
@@ -148,6 +154,9 @@ public void addPerson(ReadOnlyPerson person) throws DuplicatePersonException {
         public ReadOnlyAddressBook getAddressBook() {
             return new AddressBook();
         }
+
+        @Override
+        public Predicate<ReadOnlyPerson> getFilteredPersonListPredicate() { return PREDICATE_SHOW_ALL_PERSONS; }
     }
 
     /**
@@ -165,6 +174,9 @@ public void addPerson(ReadOnlyPerson person) throws DuplicatePersonException {
         public ReadOnlyAddressBook getAddressBook() {
             return new AddressBook();
         }
+
+        @Override
+        public Predicate<ReadOnlyPerson> getFilteredPersonListPredicate() { return PREDICATE_SHOW_ALL_PERSONS; }
     }
 
 }