Improve input validation mechanism

Author: ctrueden

pom.xml

@@ -10,7 +10,7 @@
 	</parent>
 
 	<artifactId>scijava-common</artifactId>
-	<version>2.99.4-SNAPSHOT</version>
+	<version>2.100.0-SNAPSHOT</version>
 
 	<name>SciJava Common</name>
 	<description>SciJava Common is a shared library for SciJava software. It provides a plugin framework, with an extensible mechanism for service discovery, backed by its own annotation processor, so that plugins can be loaded dynamically. It is used by downstream projects in the SciJava ecosystem, such as ImageJ and SCIFIO.</description>

src/main/java/org/scijava/module/AbstractModule.java

@@ -160,8 +160,13 @@ public void resolveInput(final String name) {
 				item.validate(this);
 			}
 			catch (final MethodCallException exc) {
-				// NB: Hacky, but avoids changing the API signature.
-				throw new RuntimeException(exc);
+				// NB: resolveInput cannot declare checked exceptions, so we wrap.
+				// Prefer the cause's message (the user-facing validation error) when
+				// available; otherwise fall back to the MethodCallException's message.
+				final Throwable cause = exc.getCause();
+				final String message = (cause != null && cause.getMessage() != null &&
+					!cause.getMessage().isEmpty()) ? cause.getMessage() : exc.getMessage();
+				throw new RuntimeException(message, exc);
 			}
 		}
 		resolvedInputs.add(name);

src/main/java/org/scijava/module/AbstractModuleItem.java

@@ -210,7 +210,12 @@ public void validate(final Module module) throws MethodCallException {
 		if (validaterRef == null) {
 			validaterRef = new MethodRef(delegateObject.getClass(), getValidater());
 		}
-		validaterRef.execute(module.getDelegateObject());
+		final Object result = validaterRef.executeWithResult(module.getDelegateObject());
+		// If the validater returns a non-empty String, treat it as an error message.
+		if (result instanceof String) {
+			final String message = (String) result;
+			if (!message.isEmpty()) throw new MethodCallException(message);
+		}
 	}
 
 	@Override

src/main/java/org/scijava/module/MethodRef.java

@@ -62,9 +62,15 @@ public MethodRef(final Class<?> clazz, final String methodName,
 	public void execute(final Object obj, final Object... args)
 		throws MethodCallException
 	{
-		if (method == null) return;
+		executeWithResult(obj, args);
+	}
+
+	public Object executeWithResult(final Object obj, final Object... args)
+		throws MethodCallException
+	{
+		if (method == null) return null;
 		try {
-			method.invoke(obj, args);
+			return method.invoke(obj, args);
 		}
 		catch (final Exception exc) {
 			// NB: Several types of exceptions; simpler to handle them all the same.

src/main/java/org/scijava/module/ModuleItem.java

@@ -126,11 +126,52 @@ public interface ModuleItem<T> extends BasicDetails {
 
 	/**
 	 * Invokes this item's validation function, if any, on the given module.
-	 * 
+	 * <p>
+	 * The validation function may signal failure either by throwing an exception
+	 * or by returning a non-empty {@link String} error message.
+	 * </p>
+	 *
+	 * @throws MethodCallException if validation fails or the method cannot be
+	 *           invoked. When the validater returns a non-empty String, a
+	 *           {@link MethodCallException} is thrown with that string as its
+	 *           message.
 	 * @see #getValidater()
+	 * @see #validateMessage(Module)
 	 */
 	void validate(Module module) throws MethodCallException;
 
+	/**
+	 * Validates this item's value in the given module, returning any error
+	 * message rather than throwing.
+	 * <p>
+	 * The validation function may signal failure either by throwing an exception
+	 * or by returning a non-empty {@link String} error message. This method
+	 * catches both cases and returns the error message as a string, or
+	 * {@code null} if the value is valid.
+	 * </p>
+	 *
+	 * @return an error message if the value is invalid, or {@code null} if valid.
+	 * @see #getValidater()
+	 * @see #validate(Module)
+	 */
+	default String validateMessage(final Module module) {
+		try {
+			validate(module);
+			return null;
+		}
+		catch (final MethodCallException exc) {
+			// Unwrap to find the most informative message.
+			final Throwable cause = exc.getCause();
+			if (cause != null && cause.getMessage() != null &&
+				!cause.getMessage().isEmpty())
+			{
+				return cause.getMessage();
+			}
+			final String msg = exc.getMessage();
+			return msg != null && !msg.isEmpty() ? msg : exc.toString();
+		}
+	}
+
 	/**
 	 * Gets the function that is called whenever this item changes.
 	 * <p>

src/main/java/org/scijava/widget/DefaultWidgetModel.java

@@ -79,6 +79,8 @@ public class DefaultWidgetModel extends AbstractContextual implements WidgetMode
 
 	private boolean initialized;
 
+	private String validationMessage;
+
 	public DefaultWidgetModel(final Context context, final InputPanel<?, ?> inputPanel,
 		final Module module, final ModuleItem<?> item, final List<?> objectPool)
 	{
@@ -167,6 +169,11 @@ public void setValue(final Object value) {
 		if (initialized) {
 			threadService.queue(() -> {
 				callback();
+				// Revalidate all inputs: changing one value may affect others' validity.
+				for (final ModuleItem<?> anyItem : module.getInfo().inputs()) {
+					final InputWidget<?, ?> w = inputPanel.getWidget(anyItem.getName());
+					if (w != null) w.get().updateValidation();
+				}
 				inputPanel.refresh(); // must be on AWT thread?
 				module.preview();
 			});
@@ -284,6 +291,16 @@ public boolean isInitialized() {
 		return initialized;
 	}
 
+	@Override
+	public String getValidationMessage() {
+		return validationMessage;
+	}
+
+	@Override
+	public void updateValidation() {
+		validationMessage = item.validateMessage(module);
+	}
+
 	// -- Helper methods --
 
 	/**

src/main/java/org/scijava/widget/WidgetModel.java

@@ -187,6 +187,27 @@ public interface WidgetModel extends Contextual {
 	/** Gets whether the input is compatible with the given type. */
 	boolean isType(Class<?> type);
 
+	/**
+	 * Gets the current validation error message for this widget.
+	 *
+	 * @return the error message from the most recent validation run, or
+	 *         {@code null} if the last validation passed or no validation has
+	 *         been run yet.
+	 * @see org.scijava.module.ModuleItem#validateMessage(org.scijava.module.Module)
+	 */
+	String getValidationMessage();
+
+	/**
+	 * Re-runs this item's validation and updates the stored validation message.
+	 * <p>
+	 * This should be called on all widgets whenever any parameter value changes,
+	 * since a change to one parameter may affect the validity of others.
+	 * </p>
+	 *
+	 * @see #getValidationMessage()
+	 */
+	void updateValidation();
+
 	/**
 	 * Toggles the widget's initialization state. An initialized widget can be
 	 * assumed to be an active part of a container {@link InputPanel}.