Merge pull request #18127 from mkouba/qute-loop-else-block

Qute - loop section - introduce the {#else} block

docs/src/main/asciidoc/qute-reference.adoc

@@ -522,19 +522,19 @@ A section helper that defines the logic of a section can "execute" any of the bl
 [[loop_section]]
 ==== Loop Section
 
-The loop section makes it possible to iterate over an instance of `Iterable`, `Map` 's entry set, `Stream` and an `Integer`.
+The loop section makes it possible to iterate over an instance of `Iterable`, `Iterator`, array, `Map` 's entry set, `Stream` and an `Integer`.
 It has two flavors.
-The first one is using the `each` name alias.
+The first one is using the `each` name and `it` is an implicit alias for the iteration element.
 
 [source]
 ----
 {#each items}
   {it.name} <1>
 {/each}
 ----
-<1> `it` is an implicit alias. `name` is resolved against the current iteration element.
+<1> `name` is resolved against the current iteration element.
 
-The other form is using the `for` name alias and can specify the alias used to reference the iteration element:
+The other form is using the `for` name and can specify the alias used to reference the iteration element:
 
 [source]
 ----
@@ -551,7 +551,9 @@ It's also possible to access the iteration metadata inside the loop:
   {count}. {it.name} <1>
 {/each}
 ----
-<1> `count` represents one-based index. Metadata also include zero-based `index`, `hasNext`, `odd` and `even`.
+<1> `count` represents one-based index. 
+
+TIP: The iteration metadata also include zero-based `index`, `hasNext`, `odd` and `even` properties.
 
 The `for` statement also works with integers, starting from 1. In the example below, considering that `total = 3`:
 
@@ -562,13 +564,24 @@ The `for` statement also works with integers, starting from 1. In the example be
 {/for}
 ----
 
-The output will be:
+And the output will be:
 
 [source]
 ----
 1:2:3:
 ----
 
+A loop section may define the `{#else}` block that is executed when there are no items to iterate:
+
+[source]
+----
+{#for item in items}
+  {item.name}
+{#else}
+  No items.
+{/for}
+----
+
 [[if_section]]
 ==== If Section
 

independent-projects/qute/core/src/main/java/io/quarkus/qute/LoopSectionHelper.java

@@ -2,9 +2,11 @@
 
 import static io.quarkus.qute.Parameter.EMPTY;
 
+import io.quarkus.qute.SectionHelperFactory.SectionInitContext;
 import java.lang.reflect.Array;
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
@@ -20,13 +22,18 @@
 public class LoopSectionHelper implements SectionHelper {
 
     private static final String DEFAULT_ALIAS = "it";
+    private static final String ELSE = "else";
+    private static final String ALIAS = "alias";
+    private static final String ITERABLE = "iterable";
 
     private final String alias;
     private final Expression iterable;
+    private final SectionBlock elseBlock;
 
-    LoopSectionHelper(String alias, Expression iterable) {
-        this.alias = Parameter.EMPTY.equals(alias) ? DEFAULT_ALIAS : alias;
-        this.iterable = Objects.requireNonNull(iterable);
+    LoopSectionHelper(SectionInitContext context) {
+        this.alias = context.getParameterOrDefault(ALIAS, DEFAULT_ALIAS);
+        this.iterable = Objects.requireNonNull(context.getExpression(ITERABLE));
+        this.elseBlock = context.getBlock(ELSE);
     }
 
     @SuppressWarnings("unchecked")
@@ -47,7 +54,12 @@ public CompletionStage<ResultNode> resolve(SectionResolutionContext context) {
                 results.add(nextElement(iterator.next(), idx++, iterator.hasNext(), context));
             }
             if (results.isEmpty()) {
-                return ResultNode.NOOP;
+                // Execute the {#else} block if present
+                if (elseBlock != null) {
+                    return context.execute(elseBlock, context.resolutionContext());
+                } else {
+                    return ResultNode.NOOP;
+                }
             }
             if (results.size() == 1) {
                 return results.get(0);
@@ -129,9 +141,7 @@ public static class Factory implements SectionHelperFactory<LoopSectionHelper> {
 
         public static final String HINT_ELEMENT = "<loop-element>";
         public static final String HINT_PREFIX = "<loop#";
-        private static final String ALIAS = "alias";
         private static final String IN = "in";
-        private static final String ITERABLE = "iterable";
 
         @Override
         public List<String> getDefaultAliases() {
@@ -147,9 +157,13 @@ public ParametersInfo getParameters() {
                     .build();
         }
 
+        public List<String> getBlockLabels() {
+            return Collections.singletonList(ELSE);
+        }
+
         @Override
         public LoopSectionHelper initialize(SectionInitContext context) {
-            return new LoopSectionHelper(context.getParameter(ALIAS), context.getExpression(ITERABLE));
+            return new LoopSectionHelper(context);
         }
 
         @Override

independent-projects/qute/core/src/main/java/io/quarkus/qute/SectionHelperFactory.java

@@ -116,14 +116,34 @@ default public Map<String, String> getParameters() {
             return getBlocks().get(0).parameters;
         }
 
+        /**
+         * 
+         * @return {@code true} if the main block declares a parameter of the given name
+         */
         default public boolean hasParameter(String name) {
             return getParameters().containsKey(name);
         }
 
+        /**
+         * 
+         * @return the parameter, or null/{@link Parameter.EMPTY} if the main block does not declare a parameter of the given
+         *         name
+         */
         default public String getParameter(String name) {
             return getParameters().get(name);
         }
 
+        /**
+         * 
+         * @param name
+         * @param defaultValue
+         * @return the param or the default value if not specified
+         */
+        default public String getParameterOrDefault(String name, String defaultValue) {
+            String param = getParameter(name);
+            return param == null || Parameter.EMPTY.equals(param) ? defaultValue : param;
+        }
+
         /**
          * Note that the expression must be registered in the {@link SectionHelperFactory#initializeBlock(Scope, BlockInfo)}
          * first.
@@ -144,6 +164,20 @@ default public String getParameter(String name) {
 
         public List<SectionBlock> getBlocks();
 
+        /**
+         * 
+         * @param label
+         * @return the first block with the given label, or {code null} if no such exists
+         */
+        default SectionBlock getBlock(String label) {
+            for (SectionBlock block : getBlocks()) {
+                if (label.equals(block.label)) {
+                    return block;
+                }
+            }
+            return null;
+        }
+
         public Engine getEngine();
 
     }

independent-projects/qute/core/src/test/java/io/quarkus/qute/LoopSectionTest.java

@@ -7,6 +7,7 @@
 
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -184,4 +185,11 @@ void testScope() {
         assertTrue(result.contains("hellllllo"), result);
     }
 
+    @Test
+    public void testElseBlock() {
+        Engine engine = Engine.builder().addDefaults().build();
+        assertEquals("No items.",
+                engine.parse("{#for i in items}{item}{#else}No items.{/for}").data("items", Collections.emptyList()).render());
+    }
+
 }