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; }
     }
 
 }