Expose default option in SingleItemSelector

- Tweak flow framework so that single item selector
  can be configured with a default item which is then
  "exposed" automatically so that user can just hit enter.
- Fixes #414

spring-shell-core/src/main/java/org/springframework/shell/component/flow/BaseSingleItemSelector.java

@@ -41,6 +41,7 @@ public abstract class BaseSingleItemSelector extends BaseInput<SingleItemSelecto
 	private String resultValue;
 	private ResultMode resultMode;
 	private Map<String, String> selectItems = new HashMap<>();
+	private String defaultSelect;
 	private Comparator<SelectorItem<String>> comparator;
 	private Function<SingleItemSelectorContext<String, SelectorItem<String>>, List<AttributedString>> renderer;
 	private Integer maxItems;
@@ -84,6 +85,12 @@ public abstract class BaseSingleItemSelector extends BaseInput<SingleItemSelecto
 		return this;
 	}
 
+	@Override
+	public SingleItemSelectorSpec defaultSelect(String name) {
+		this.defaultSelect = name;
+		return this;
+	}
+
 	@Override
 	public SingleItemSelectorSpec sort(Comparator<SelectorItem<String>> comparator) {
 		this.comparator = comparator;
@@ -160,6 +167,10 @@ public abstract class BaseSingleItemSelector extends BaseInput<SingleItemSelecto
 		return selectItems;
 	}
 
+	public String getDefaultSelect() {
+		return defaultSelect;
+	}
+
 	public Comparator<SelectorItem<String>> getComparator() {
 		return comparator;
 	}

spring-shell-core/src/main/java/org/springframework/shell/component/flow/ComponentFlow.java

@@ -555,8 +555,20 @@ public interface ComponentFlow {
 				List<SelectorItem<String>> selectorItems = input.getSelectItems().entrySet().stream()
 					.map(e -> SelectorItem.of(e.getKey(), e.getValue()))
 					.collect(Collectors.toList());
+
+				// setup possible item for initial expose
+				String defaultSelect = input.getDefaultSelect();
+				Stream<SelectorItem<String>> defaultCheckStream = StringUtils.hasText(defaultSelect)
+						? selectorItems.stream()
+						: Stream.empty();
+				SelectorItem<String> defaultExpose = defaultCheckStream
+					.filter(si -> ObjectUtils.nullSafeEquals(defaultSelect, si.getName()))
+					.findFirst()
+					.orElse(null);
+
 				SingleItemSelector<String, SelectorItem<String>> selector = new SingleItemSelector<>(terminal,
 						selectorItems, input.getName(), input.getComparator());
+				selector.setDefaultExpose(defaultExpose);
 				Function<ComponentContext<?>, ComponentContext<?>> operation = (context) -> {
 					if (input.getResultMode() == ResultMode.ACCEPT && input.isStoreResult()
 							&& StringUtils.hasText(input.getResultValue())) {

spring-shell-core/src/main/java/org/springframework/shell/component/flow/SingleItemSelectorSpec.java

@@ -76,6 +76,14 @@ public interface SingleItemSelectorSpec extends BaseInputSpec<SingleItemSelector
 	 */
 	SingleItemSelectorSpec selectItems(Map<String, String> selectItems);
 
+	/**
+	 * Automatically selects and exposes a given item.
+	 *
+	 * @param name the name
+	 * @return a builder
+	 */
+	SingleItemSelectorSpec defaultSelect(String name);
+
 	/**
 	 * Sets a {@link Comparator} for sorting items.
 	 *

spring-shell-core/src/main/java/org/springframework/shell/component/support/AbstractSelectorComponent.java

@@ -31,6 +31,7 @@ import org.springframework.shell.component.context.BaseComponentContext;
 import org.springframework.shell.component.context.ComponentContext;
 import org.springframework.shell.component.support.AbstractSelectorComponent.SelectorComponentContext;
 import org.springframework.util.Assert;
+import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
 
 import static org.jline.keymap.KeyMap.ctrl;
@@ -54,6 +55,8 @@ public abstract class AbstractSelectorComponent<T, C extends SelectorComponentCo
 	private boolean stale = false;
 	private AtomicInteger start = new AtomicInteger(0);
 	private AtomicInteger pos = new AtomicInteger(0);
+	private I defaultExpose;
+	private boolean expose = false;
 
 	public AbstractSelectorComponent(Terminal terminal, String name, List<I> items, boolean exitSelects,
 			Comparator<I> comparator) {
@@ -95,6 +98,18 @@ public abstract class AbstractSelectorComponent<T, C extends SelectorComponentCo
 		return itemMapper;
 	}
 
+	/**
+	 * Sets default expose item when component start.
+	 *
+	 * @param defaultExpose the default item
+	 */
+	public void setDefaultExpose(I defaultExpose) {
+		this.defaultExpose = defaultExpose;
+		if (defaultExpose != null) {
+			expose = true;
+		}
+	}
+
 	/**
 	 * Gets items.
 	 *
@@ -120,6 +135,7 @@ public abstract class AbstractSelectorComponent<T, C extends SelectorComponentCo
 	@Override
 	protected C runInternal(C context) {
 		C thisContext = getThisContext(context);
+		initialExpose(thisContext);
 		ItemStateViewProjection buildItemStateView = buildItemStateView(start.get(), thisContext);
 		List<ItemState<I>> itemStateView = buildItemStateView.items;
 		thisContext.setItemStateView(itemStateView);
@@ -224,6 +240,33 @@ public abstract class AbstractSelectorComponent<T, C extends SelectorComponentCo
 		return false;
 	}
 
+	private void initialExpose(C context) {
+		if (!expose) {
+			return;
+		}
+		expose = false;
+		List<ItemState<I>> itemStates = context.getItemStates();
+		if (itemStates == null) {
+			AtomicInteger index = new AtomicInteger(0);
+			itemStates = context.getItems().stream()
+					.sorted(comparator)
+					.map(item -> ItemState.of(item, item.getName(), index.getAndIncrement(), item.isEnabled()))
+					.collect(Collectors.toList());
+		}
+		for (int i = 0; i < itemStates.size(); i++) {
+			if (ObjectUtils.nullSafeEquals(itemStates.get(i).getName(), defaultExpose.getName())) {
+				if (i < maxItems) {
+					this.pos.set(i);
+				}
+				else {
+					this.pos.set(maxItems - 1);
+					this.start.set(i - maxItems + 1);
+				}
+				break;
+			}
+		}
+	}
+
 	private ItemStateViewProjection buildItemStateView(int skip, SelectorComponentContext<T, I, ?> context) {
 		List<ItemState<I>> itemStates = context.getItemStates();
 		if (itemStates == null) {