add function interface to assertItem

documentation/src/test/java/guides/TestSubscribersTest.java

@@ -51,4 +51,37 @@ void failing() {
                 .assertFailedWith(IOException.class, "Boom");
         // </failing>
     }
+
+    @Test
+    void uniFunction() {
+        // <uni-function>
+        Uni<Integer> uni = Uni.createFrom().item(63);
+
+        UniAssertSubscriber<Integer> subscriber = uni
+            .subscribe().withSubscriber(UniAssertSubscriber.create());
+
+        subscriber.assertSubscribed().assertItem(String::valueOf, "63");
+        // </uni-function>
+    }
+
+    @Test
+    void uniFunctionWithObjects() {
+        // <uni-function-with-objects>
+        class Person {
+            String name;
+            int age;
+        }
+
+        Person person = new Person();
+        person.name = "John";
+        person.age = 42;
+        Uni<Person> uni = Uni.createFrom().item(person);
+
+        UniAssertSubscriber<Person> subscriber = uni
+            .subscribe().withSubscriber(UniAssertSubscriber.create());
+
+        subscriber.assertSubscribed().assertItem(p -> p.name, person.name);
+        subscriber.assertSubscribed().assertItem(p -> p.age, person.age);
+        // </uni-function-with-objects>
+    }
 }

implementation/src/main/java/io/smallrye/mutiny/helpers/test/UniAssertSubscriber.java

@@ -10,6 +10,7 @@
 import java.util.List;
 import java.util.concurrent.*;
 import java.util.function.Consumer;
+import java.util.function.Function;
 
 import io.smallrye.mutiny.Context;
 import io.smallrye.mutiny.subscription.UniSubscriber;
@@ -341,6 +342,17 @@ public UniAssertSubscriber<T> assertItem(T expected) {
         return this;
     }
 
+    /**
+     * Assert that the {@link io.smallrye.mutiny.Uni} has received an item.
+     * @param function the function to apply the item to assert on
+     * @param expected the expected item
+     * @param <R> the return type of the item
+     */
+    public <R> void assertItem(Function<T, R> function, R expected) {
+        shouldHaveCompleted(hasCompletedSuccessfully, failure, null);
+        shouldHaveReceived(function.apply(getItem()), expected);
+    }
+
     /**
      * Assert that the {@link io.smallrye.mutiny.Uni} has failed.
      *