Add method signatures and preprocessor to select multiple files

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

@@ -297,6 +297,8 @@ public <T> void save(final ModuleItem<T> item, final T value) {
 			return;
 		}
 
+		// FIXME: Convert to string, instead of just calling toString.
+		// Otherwise many things (e.g. File[]) are persisted improperly.
 		final String sValue = value == null ? "" : value.toString();
 
 		// do not persist if object cannot be converted back from a string

src/main/java/org/scijava/ui/DefaultUIService.java

@@ -33,6 +33,7 @@
 package org.scijava.ui;
 
 import java.io.File;
+import java.io.FileFilter;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
@@ -319,6 +320,18 @@ public File chooseFile(final File file, final String style) {
 		return ui == null ? null : ui.chooseFile(title, file, style);
 	}
 
+	@Override
+	public File[] chooseFiles(File[] files, FileFilter filter) {
+		final UserInterface ui = getDefaultUI();
+		return ui == null ? null : ui.chooseFiles(files, filter);
+	}
+
+	@Override
+	public List<File> chooseFiles(List<File> fileList, FileFilter filter) {
+		final UserInterface ui = getDefaultUI();
+		return ui == null ? null : ui.chooseFiles(fileList, filter);
+	}
+
 	@Override
 	public void showContextMenu(final String menuRoot, final Display<?> display,
 		final int x, final int y)

src/main/java/org/scijava/ui/FileListPreprocessor.java

@@ -0,0 +1,64 @@
+package org.scijava.ui;
+
+import java.io.File;
+
+import org.scijava.module.Module;
+import org.scijava.module.ModuleItem;
+import org.scijava.module.process.AbstractPreprocessorPlugin;
+import org.scijava.module.process.PreprocessorPlugin;
+import org.scijava.plugin.Parameter;
+import org.scijava.plugin.Plugin;
+import org.scijava.widget.InputHarvester;
+
+@Plugin(type = PreprocessorPlugin.class, priority = InputHarvester.PRIORITY + 1.0)
+public class FileListPreprocessor extends AbstractPreprocessorPlugin {
+
+	@Parameter(required = false)
+	private UIService uiService;
+
+	@Override
+	public void process(final Module module) {
+		if (uiService == null) return;
+		final ModuleItem<File[]> fileInput = getFilesInput(module);
+		if (fileInput == null) return;
+
+		final File[] files = fileInput.getValue(module);
+
+		// show file chooser dialog box
+		final File[] result = uiService.chooseFiles(files, null);
+		if (result == null) {
+			cancel("");
+			return;
+		}
+
+		fileInput.setValue(module, result);
+		module.resolveInput(fileInput.getName());
+	}
+
+	// -- Helper methods --
+
+	/**
+	 * Gets the single unresolved {@link File} input parameter. If there is not
+	 * exactly one unresolved {@link File} input parameter, or if there are other
+	 * types of unresolved parameters, this method returns null.
+	 */
+	private ModuleItem<File[]> getFilesInput(final Module module) {
+		ModuleItem<File[]> result = null;
+		for (final ModuleItem<?> input : module.getInfo().inputs()) {
+			if (module.isInputResolved(input.getName())) continue;
+			final Class<?> type = input.getType();
+			if (!File[].class.isAssignableFrom(type)) {
+				// not a File[] parameter; abort
+				return null;
+			}
+			if (result != null) {
+				// second File parameter; abort
+				return null;
+			}
+			@SuppressWarnings("unchecked")
+			final ModuleItem<File[]> fileInput = (ModuleItem<File[]>) input;
+			result = fileInput;
+		}
+		return result;
+	}
+}

src/main/java/org/scijava/ui/UIService.java

@@ -33,6 +33,7 @@
 package org.scijava.ui;
 
 import java.io.File;
+import java.io.FileFilter;
 import java.util.List;
 
 import org.scijava.app.StatusService;
@@ -294,6 +295,28 @@ DialogPrompt.Result showDialog(String message, String title,
 	 */
 	File chooseFile(String title, File file, String style);
 
+	/**
+	 * Prompts the user to select one or multiple files.
+	 * <p>
+	 * The prompt is displayed in the default user interface.
+	 * </p>
+	 * 
+	 * @param files The initial value displayed in the file chooser prompt.
+	 * @param filter A filter allowing to restrict the choice of files
+	 */
+	File[] chooseFiles(File[] files, FileFilter filter);
+
+	/**
+	 * Prompts the user to select one or multiple files.
+	 * <p>
+	 * The prompt is displayed in the default user interface.
+	 * </p>
+	 * 
+	 * @param fileList The initial value displayed in the file chooser prompt.
+	 * @param filter A filter allowing to restrict the choice of files
+	 */
+	List<File> chooseFiles(List<File> fileList, FileFilter filter);
+
 	/**
 	 * Displays a popup context menu for the given display at the specified
 	 * position.
@@ -309,5 +332,4 @@ DialogPrompt.Result showDialog(String message, String title,
 	 * @see StatusService#getStatusMessage(String, StatusEvent)
 	 */
 	String getStatusMessage(StatusEvent statusEvent);
-
 }

src/main/java/org/scijava/ui/UserInterface.java

@@ -33,6 +33,9 @@
 package org.scijava.ui;
 
 import java.io.File;
+import java.io.FileFilter;
+import java.util.Arrays;
+import java.util.List;
 
 import org.scijava.Disposable;
 import org.scijava.display.Display;
@@ -185,6 +188,32 @@ default File chooseFile(String title, File file, String style) {
 		throw new UnsupportedOperationException();
 	}
 
+	/**
+	 * Prompts the user to choose a list of files.
+	 *
+	 * @param files The initial value displayed in the file chooser prompt.
+	 * @param filter A filter allowing to restrict file choice.
+	 * @return The selected {@link File}s chosen by the user, or null if the
+	 *         user cancels the prompt.
+	 */
+	default File[] chooseFiles(File[] files, FileFilter filter) {
+		throw new UnsupportedOperationException();
+	}
+
+	/**
+	 * Prompts the user to choose a list of files.
+	 *
+	 * @param fileList The initial value displayed in the file chooser prompt.
+	 * @param filter A filter allowing to restrict file choice.
+	 * @return The selected {@link File}s chosen by the user, or null if the
+	 *         user cancels the prompt.
+	 */
+	default List<File> chooseFiles(List<File> fileList, FileFilter filter) {
+		final File[] initialFiles = fileList.toArray(new File[fileList.size()]);
+		final File[] chosenFiles = chooseFiles(initialFiles, filter);
+		return chosenFiles == null ? null : Arrays.asList(chosenFiles);
+	}
+
 	/**
 	 * Displays a popup context menu for the given display at the specified
 	 * position.
@@ -199,5 +228,4 @@ default File chooseFile(String title, File file, String style) {
 
 	/** Returns true if this UI requires the EDT. */
 	boolean requiresEDT();
-
 }

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

@@ -0,0 +1,7 @@
+package org.scijava.widget;
+
+import java.io.File;
+
+public interface FileListWidget<U> extends InputWidget<File[], U> {
+	// NB: No changes to interface.
+}