Let a label show a Controller method and follow other values in the View.

The method does not write the Model, and a change to the label itself does not run the method again.

Co-authored-by: Cursor <cursoragent@cursor.com>

fx/src/main/java/de/neitzel/fx/view/BindingList.java

@@ -1,6 +1,7 @@
 package de.neitzel.fx.view;
 
 import javafx.beans.DefaultProperty;
+import javafx.scene.Parent;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -9,13 +10,13 @@ import java.util.Map;
 /**
  * The list of binding elements declared inside {@code fx:define}.
  * <p>
- * The list is not a scene-graph child. FXML adds each {@link DirectBinding} child to
+ * The list is not a scene-graph child. FXML adds each {@link DirectBinding} or
- * {@link #getElements()}.
+ * {@link ComputedBinding} child to {@link #getElements()}.
  */
 @DefaultProperty("elements")
 public class BindingList {
 
-    private final List<DirectBinding> elements = new ArrayList<>();
+    private final List<Object> elements = new ArrayList<>();
 
     /**
      * Creates a list and registers it with the load in progress.
@@ -25,25 +26,34 @@ public class BindingList {
     }
 
     /**
-     * Returns the Direct bindings in this list.
+     * Returns the binding elements in this list.
      * <p>
-     * FXML appends child elements to the returned list.
+     * FXML appends child elements to the returned list. A child is a {@link DirectBinding} or a
+     * {@link ComputedBinding}.
      *
-     * @return the mutable list of Direct bindings
+     * @return the mutable list of binding elements
      */
-    public List<DirectBinding> getElements() {
+    public List<Object> getElements() {
         return elements;
     }
 
     /**
-     * Applies every Direct binding in this list.
+     * Applies every binding element in this list.
      *
-     * @param namespace objects addressed by {@code fx:id}
+     * @param namespace  objects addressed by {@code fx:id}
-     * @param viewModel the ViewModel for this View
+     * @param viewModel  the ViewModel for this View
+     * @param controller the Controller for this View, or {@code null} when the document names none
+     * @param root       the scene-graph root of this View
      */
-    void apply(Map<String, Object> namespace, ViewModel viewModel) {
+    void apply(Map<String, Object> namespace, ViewModel viewModel, Object controller, Parent root) {
-        for (DirectBinding binding : elements) {
+        for (Object element : elements) {
-            DirectBindingApplier.apply(namespace, viewModel, binding);
+            if (element instanceof DirectBinding directBinding) {
+                DirectBindingApplier.apply(namespace, viewModel, directBinding);
+            } else if (element instanceof ComputedBinding computedBinding) {
+                ComputedBindingApplier.apply(namespace, viewModel, controller, root, computedBinding);
+            } else {
+                throw new IllegalArgumentException("Not a binding element: " + element);
+            }
         }
     }
 }

fx/src/main/java/de/neitzel/fx/view/ComputedBinding.java

@@ -0,0 +1,70 @@
+package de.neitzel.fx.view;
+
+/**
+ * A Computed binding declared in FXML.
+ * <p>
+ * The element names the control by {@code fx:id}, the control property, and a no-argument
+ * method on the Controller. The method value uses a {@code #method} prefix, such as
+ * {@code #workingAge}. The method may read the Controller's fields. Its result is written to
+ * the control property. The method does not write the Model.
+ */
+public class ComputedBinding {
+
+    private String control;
+    private String property;
+    private String method;
+
+    /**
+     * Sets the {@code fx:id} of the control.
+     *
+     * @param control the control id
+     */
+    public void setControl(String control) {
+        this.control = control;
+    }
+
+    /**
+     * Sets the control property name, such as {@code text}.
+     *
+     * @param property the property name
+     */
+    public void setProperty(String property) {
+        this.property = property;
+    }
+
+    /**
+     * Sets the Controller method. A {@code #method} value names a no-argument method.
+     *
+     * @param method the method value, such as {@code #workingAge}
+     */
+    public void setMethod(String method) {
+        this.method = method;
+    }
+
+    /**
+     * Returns the {@code fx:id} of the control.
+     *
+     * @return the control id
+     */
+    public String getControl() {
+        return control;
+    }
+
+    /**
+     * Returns the control property name.
+     *
+     * @return the property name
+     */
+    public String getProperty() {
+        return property;
+    }
+
+    /**
+     * Returns the Controller method value.
+     *
+     * @return the method value, such as {@code #workingAge}
+     */
+    public String getMethod() {
+        return method;
+    }
+}

fx/src/main/java/de/neitzel/fx/view/ComputedBindingApplier.java

@@ -0,0 +1,147 @@
+package de.neitzel.fx.view;
+
+import javafx.beans.property.Property;
+import javafx.beans.value.ChangeListener;
+import javafx.beans.value.ObservableValue;
+import javafx.scene.Node;
+import javafx.scene.Parent;
+import javafx.scene.control.Control;
+
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+import java.util.Map;
+
+/**
+ * Writes a Controller method result into one control property and keeps it current.
+ */
+final class ComputedBindingApplier {
+
+    private ComputedBindingApplier() {
+    }
+
+    /**
+     * Applies one Computed binding.
+     * <p>
+     * Calls the Controller method and writes the result to the control property. The method
+     * runs again when a JavaFX property changes on a control in this View or on this View's
+     * ViewModel, other than the target property. Writing the target does not call the method
+     * again, so the write does not loop. The Model is not written.
+     *
+     * @param namespace  objects addressed by {@code fx:id}
+     * @param viewModel  the ViewModel for this View
+     * @param controller the Controller for this View
+     * @param root       the scene-graph root of this View
+     * @param binding    the declaration
+     */
+    static void apply(
+            Map<String, Object> namespace,
+            ViewModel viewModel,
+            Object controller,
+            Parent root,
+            ComputedBinding binding) {
+        Object control = namespace.get(binding.getControl());
+        if (control == null) {
+            throw new IllegalArgumentException("No control \"" + binding.getControl() + "\"");
+        }
+        Property<Object> target = controlProperty(control, binding.getProperty());
+        Runnable refresh = refresh(controller, binding.getMethod(), target);
+        refresh.run();
+        watchControls(root, target, refresh);
+        viewModel.addAttributeListener((observable, oldValue, newValue) -> refresh.run());
+    }
+
+    private static Runnable refresh(Object controller, String methodValue, Property<Object> target) {
+        boolean[] running = {false};
+        return () -> {
+            if (running[0]) {
+                return;
+            }
+            running[0] = true;
+            try {
+                target.setValue(invoke(controller, methodValue));
+            } finally {
+                running[0] = false;
+            }
+        };
+    }
+
+    private static void watchControls(Parent root, Property<?> target, Runnable refresh) {
+        watchNode(root, target, refresh);
+    }
+
+    private static void watchNode(Node node, Property<?> target, Runnable refresh) {
+        if (node instanceof Control control) {
+            watchProperties(control, target, refresh);
+        }
+        if (node instanceof Parent parent) {
+            for (Node child : parent.getChildrenUnmodifiable()) {
+                watchNode(child, target, refresh);
+            }
+        }
+    }
+
+    @SuppressWarnings("unchecked")
+    private static void watchProperties(Object bean, Property<?> target, Runnable refresh) {
+        ChangeListener<Object> listener = (observable, oldValue, newValue) -> refresh.run();
+        for (Method method : bean.getClass().getMethods()) {
+            if (method.getParameterCount() != 0 || !method.getName().endsWith("Property")) {
+                continue;
+            }
+            if (!ObservableValue.class.isAssignableFrom(method.getReturnType())) {
+                continue;
+            }
+            Object value = readProperty(bean, method);
+            if (value instanceof ObservableValue<?> observable && observable != target) {
+                ((ObservableValue<Object>) observable).addListener(listener);
+            }
+        }
+    }
+
+    private static Object readProperty(Object bean, Method method) {
+        try {
+            return method.invoke(bean);
+        } catch (IllegalAccessException | InvocationTargetException failure) {
+            throw new IllegalStateException("Could not read property " + method.getName(), failure);
+        }
+    }
+
+    @SuppressWarnings("unchecked")
+    private static Property<Object> controlProperty(Object control, String propertyName) {
+        String methodName = propertyName + "Property";
+        try {
+            Method method = control.getClass().getMethod(methodName);
+            Object value = method.invoke(control);
+            if (!(value instanceof Property<?> property)) {
+                throw new IllegalArgumentException(methodName + " did not return a property");
+            }
+            return (Property<Object>) property;
+        } catch (NoSuchMethodException failure) {
+            throw new IllegalArgumentException(
+                    "No property " + propertyName + " on " + control.getClass().getName(), failure);
+        } catch (IllegalAccessException | InvocationTargetException failure) {
+            throw new IllegalStateException("Could not read property " + propertyName, failure);
+        }
+    }
+
+    private static Object invoke(Object controller, String methodValue) {
+        if (controller == null) {
+            throw new IllegalArgumentException("No controller for " + methodValue);
+        }
+        String name = methodName(methodValue);
+        try {
+            Method method = controller.getClass().getMethod(name);
+            return method.invoke(controller);
+        } catch (NoSuchMethodException failure) {
+            throw new IllegalArgumentException("No computed method \"" + methodValue + "\"", failure);
+        } catch (IllegalAccessException | InvocationTargetException failure) {
+            throw new IllegalStateException("Could not call " + methodValue, failure);
+        }
+    }
+
+    private static String methodName(String methodValue) {
+        if (methodValue != null && methodValue.startsWith("#")) {
+            return methodValue.substring(1);
+        }
+        return methodValue;
+    }
+}

fx/src/main/java/de/neitzel/fx/view/ViewLoader.java

@@ -18,8 +18,9 @@ public final class ViewLoader {
     /**
      * Loads a View for one Model instance.
      * <p>
-     * Builds a ViewModel for {@code model}, loads {@code fxml}, and applies Direct bindings
+     * Builds a ViewModel for {@code model}, loads {@code fxml}, and applies binding elements
-     * declared in {@code fx:define}. Direction {@code both} keeps the control and the Attribute
+     * declared in {@code fx:define}. A Computed binding writes a Controller method result to a
+     * control and does not write the Model. Direction {@code both} keeps the control and the Attribute
      * aligned, so editing the control writes the Attribute immediately. Direction {@code view}
      * shows the Attribute and follows later property changes without writing the Attribute back.
      * Direction {@code model} shows the current value, writes the Attribute when the control
@@ -43,7 +44,7 @@ public final class ViewLoader {
                 throw new IllegalArgumentException("View root must be a parent");
             }
             for (BindingList bindingList : session.bindingLists()) {
-                bindingList.apply(loader.getNamespace(), viewModel);
+                bindingList.apply(loader.getNamespace(), viewModel, loader.getController(), root);
             }
             return new LoadedView(root, viewModel);
         } catch (IOException failure) {

fx/src/main/java/de/neitzel/fx/view/ViewModel.java

@@ -13,6 +13,7 @@ import javafx.beans.property.SimpleIntegerProperty;
 import javafx.beans.property.SimpleLongProperty;
 import javafx.beans.property.SimpleObjectProperty;
 import javafx.beans.property.SimpleStringProperty;
+import javafx.beans.value.ChangeListener;
 
 import java.beans.BeanInfo;
 import java.beans.IntrospectionException;
@@ -70,6 +71,18 @@ public class ViewModel {
         return properties.get(name);
     }
 
+    /**
+     * Calls {@code listener} when any Attribute property changes.
+     *
+     * @param listener the listener
+     */
+    @SuppressWarnings("unchecked")
+    void addAttributeListener(ChangeListener<Object> listener) {
+        for (Property<?> property : properties.values()) {
+            ((Property<Object>) property).addListener(listener);
+        }
+    }
+
     private static List<PropertyDescriptor> descriptors(Class<?> type) {
         try {
             BeanInfo info = Introspector.getBeanInfo(type, Object.class);

fx/src/test/java/de/neitzel/fx/view/ComputeAValueTest.java

@@ -0,0 +1,135 @@
+package de.neitzel.fx.view;
+
+import de.neitzel.fx.view.fixture.Person;
+import de.neitzel.injection.ApplicationContext;
+import javafx.application.Platform;
+import javafx.beans.property.Property;
+import javafx.scene.control.Label;
+import javafx.scene.control.TextField;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+
+import java.net.URL;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicReference;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.fail;
+
+class ComputeAValueTest {
+
+    @BeforeAll
+    static void startJavaFx() throws InterruptedException {
+        CountDownLatch started = new CountDownLatch(1);
+        try {
+            Platform.startup(started::countDown);
+        } catch (IllegalStateException alreadyStarted) {
+            started.countDown();
+        }
+        if (!started.await(10, TimeUnit.SECONDS)) {
+            fail("JavaFX toolkit did not start");
+        }
+    }
+
+    @Test
+    void computedBindingShowsTheMethodResultAndLeavesTheModelUnchanged() throws Exception {
+        onFx(() -> {
+            Person person = new Person("Ada");
+            person.setAge(36);
+            URL fxml = ComputeAValueTest.class.getResource("working-age.fxml");
+            assertNotNull(fxml);
+            ApplicationContext context = new ApplicationContext("de.neitzel.fx.view.computed");
+            LoadedView loaded = ViewLoader.load(fxml, person, context);
+            Label workingAgeLabel = (Label) loaded.getRoot().lookup("#workingAgeLabel");
+            assertEquals("initial-age", workingAgeLabel.getText());
+            assertEquals("Ada", person.getName());
+            assertEquals(36, person.getAge());
+        });
+    }
+
+    @Test
+    void typingInAnotherTextFieldUpdatesTheComputedLabel() throws Exception {
+        onFx(() -> {
+            Person person = new Person("Ada");
+            URL fxml = ComputeAValueTest.class.getResource("working-age.fxml");
+            assertNotNull(fxml);
+            ApplicationContext context = new ApplicationContext("de.neitzel.fx.view.computed");
+            LoadedView loaded = ViewLoader.load(fxml, person, context);
+            TextField nameField = (TextField) loaded.getRoot().lookup("#nameField");
+            Label workingAgeLabel = (Label) loaded.getRoot().lookup("#workingAgeLabel");
+            nameField.setText("Grace");
+            assertEquals("typed-age", workingAgeLabel.getText());
+            assertEquals("Grace", person.getName());
+        });
+    }
+
+    @Test
+    void settingAViewModelPropertyUpdatesTheComputedLabel() throws Exception {
+        onFx(() -> {
+            Person person = new Person("Ada");
+            URL fxml = ComputeAValueTest.class.getResource("working-age.fxml");
+            assertNotNull(fxml);
+            ApplicationContext context = new ApplicationContext("de.neitzel.fx.view.computed");
+            LoadedView loaded = ViewLoader.load(fxml, person, context);
+            Label workingAgeLabel = (Label) loaded.getRoot().lookup("#workingAgeLabel");
+            @SuppressWarnings("unchecked")
+            Property<String> name = (Property<String>) loaded.property("name");
+            name.setValue("Edsger");
+            assertEquals("model-age", workingAgeLabel.getText());
+            assertEquals("Edsger", person.getName());
+        });
+    }
+
+    @Test
+    void changingTheComputedLabelDoesNotRunTheMethodAgain() throws Exception {
+        onFx(() -> {
+            Person person = new Person("Ada");
+            URL fxml = ComputeAValueTest.class.getResource("working-age.fxml");
+            assertNotNull(fxml);
+            ApplicationContext context = new ApplicationContext("de.neitzel.fx.view.computed");
+            LoadedView loaded = ViewLoader.load(fxml, person, context);
+            Label workingAgeLabel = (Label) loaded.getRoot().lookup("#workingAgeLabel");
+            assertEquals("initial-age", workingAgeLabel.getText());
+            workingAgeLabel.setText("sentinel");
+            assertEquals("sentinel", workingAgeLabel.getText());
+        });
+    }
+
+    private static void onFx(FxAction action) throws Exception {
+        if (Platform.isFxApplicationThread()) {
+            action.run();
+            return;
+        }
+        AtomicReference<Throwable> thrown = new AtomicReference<>();
+        CountDownLatch done = new CountDownLatch(1);
+        Platform.runLater(() -> {
+            try {
+                action.run();
+            } catch (Throwable failure) {
+                thrown.set(failure);
+            } finally {
+                done.countDown();
+            }
+        });
+        if (!done.await(10, TimeUnit.SECONDS)) {
+            fail("JavaFX thread did not finish");
+        }
+        Throwable failure = thrown.get();
+        if (failure instanceof Exception exception) {
+            throw exception;
+        }
+        if (failure instanceof Error error) {
+            throw error;
+        }
+        if (failure != null) {
+            throw new AssertionError(failure);
+        }
+    }
+
+    @FunctionalInterface
+    private interface FxAction {
+        void run() throws Exception;
+    }
+}

fx/src/test/java/de/neitzel/fx/view/computed/WorkingAgeController.java

@@ -0,0 +1,30 @@
+package de.neitzel.fx.view.computed;
+
+import jakarta.inject.Singleton;
+import javafx.fxml.FXML;
+import javafx.scene.control.TextField;
+
+/**
+ * Controller that derives a label from the name field and does not write the Model.
+ */
+@Singleton
+public class WorkingAgeController {
+
+    @FXML
+    private TextField nameField;
+
+    /**
+     * Returns a literal chosen from the current name field.
+     *
+     * @return the value written to the label
+     */
+    public String workingAge() {
+        if ("Ada".equals(nameField.getText())) {
+            return "initial-age";
+        }
+        if ("Grace".equals(nameField.getText())) {
+            return "typed-age";
+        }
+        return "model-age";
+    }
+}

fx/src/test/resources/de/neitzel/fx/view/working-age.fxml

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<?import javafx.scene.control.Label?>
+<?import javafx.scene.control.TextField?>
+<?import javafx.scene.layout.VBox?>
+<?import de.neitzel.fx.view.BindingList?>
+<?import de.neitzel.fx.view.ComputedBinding?>
+<?import de.neitzel.fx.view.DirectBinding?>
+
+<VBox xmlns="http://javafx.com/javafx" xmlns:fx="http://javafx.com/fxml"
+      fx:controller="de.neitzel.fx.view.computed.WorkingAgeController">
+    <fx:define>
+        <BindingList>
+            <DirectBinding control="nameField" property="text" path="name" direction="both"/>
+            <ComputedBinding control="workingAgeLabel" property="text" method="#workingAge"/>
+        </BindingList>
+    </fx:define>
+    <TextField fx:id="nameField" id="nameField"/>
+    <Label fx:id="workingAgeLabel" id="workingAgeLabel"/>
+</VBox>