highly WIP

core/src/main/java/org/mapstruct/ConditionStrategy.java

@@ -20,4 +20,8 @@ public enum ConditionStrategy {
      * The condition method should be applied to check if a source parameters should be mapped.
      */
     SOURCE_PARAMETERS,
+    /**
+     * The condition method should be applied whether an element should be added to the iterable target.
+     */
+    ITERABLE_ELEMENTS,
 }

core/src/main/java/org/mapstruct/IterableElementCondition.java

@@ -0,0 +1,25 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * FIXME
+ *
+ * @author Oliver Erhart
+ * @since 1.7
+ * @see Condition @Condition
+ */
+@Target({ ElementType.METHOD })
+@Retention(RetentionPolicy.CLASS)
+@Condition(appliesTo = ConditionStrategy.ITERABLE_ELEMENTS)
+public @interface IterableElementCondition {
+
+}

processor/src/main/java/org/mapstruct/ap/internal/gem/ConditionStrategyGem.java

@@ -11,5 +11,6 @@ package org.mapstruct.ap.internal.gem;
 public enum ConditionStrategyGem {
 
     PROPERTIES,
-    SOURCE_PARAMETERS
+    SOURCE_PARAMETERS,
+    ITERABLE_ELEMENTS
 }

processor/src/main/java/org/mapstruct/ap/internal/model/source/ConditionOptions.java

@@ -93,6 +93,9 @@ public class ConditionOptions {
         else if ( strategy == ConditionStrategyGem.PROPERTIES ) {
             return hasValidStrategyForProperties( condition, method, parameters, messager );
         }
+        else if ( strategy == ConditionStrategyGem.ITERABLE_ELEMENTS ) {
+            return true;
+        }
         else {
             throw new IllegalStateException( "Invalid condition strategy: " + strategy );
         }

processor/src/main/resources/org/mapstruct/ap/internal/model/IterableMappingMethod.ftl

@@ -24,7 +24,7 @@
             <#if resultType.arrayType>
                 <#if existingInstanceMapping>
                     <#-- we can't clear an existing array, so we've got to clear by setting values to default -->
-                    for (int ${index2Name} = 0; ${index2Name} < ${resultName}.length; ${index2Name}++ ) {
+                    // test
                         ${resultName}[${index2Name}] = ${resultElementType.null};
                     }
                     return<#if returnType.name != "void"> ${resultName}</#if>;
@@ -74,7 +74,9 @@
         }
     <#else>
         for ( <@includeModel object=sourceElementType/> ${loopVariableName} : ${sourceParameter.name} ) {
-            <@includeModel object=elementAssignment targetBeanName=resultName targetWriteAccessorName="add" targetType=resultElementType/>
+            if ( countryIsNotNull( employeeDto ) ) {
+                <@includeModel object=elementAssignment targetBeanName=resultName targetWriteAccessorName="add" targetType=resultElementType/>
+            }
         }
     </#if>
     <#list afterMappingReferences as callback>

processor/src/test/java/org/mapstruct/ap/test/conditional/iterable/IterableElementConditionMapper.java

@@ -0,0 +1,37 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.iterable;
+
+import java.util.List;
+
+import org.mapstruct.IterableElementCondition;
+import org.mapstruct.Mapper;
+import org.mapstruct.Mapping;
+import org.mapstruct.ap.test.conditional.Employee;
+import org.mapstruct.ap.test.conditional.EmployeeDto;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Oliver Erhart
+ */
+@Mapper
+public interface IterableElementConditionMapper {
+
+    IterableElementConditionMapper INSTANCE =
+        Mappers.getMapper( IterableElementConditionMapper.class );
+
+    @Mapping(target = "nin", source = "name")
+    @Mapping(target = "ssid", source = "uniqueIdNumber")
+    Employee map(EmployeeDto employee);
+
+    List<Employee> map(List<EmployeeDto> employees);
+
+    @IterableElementCondition
+    default boolean countryIsNotNull(EmployeeDto value) {
+        return value.getCountry() != null;
+    }
+
+}

processor/src/test/java/org/mapstruct/ap/test/conditional/iterable/IterableElementConditionTest.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.conditional.iterable;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import org.mapstruct.ap.test.conditional.Employee;
+import org.mapstruct.ap.test.conditional.EmployeeDto;
+import org.mapstruct.ap.testutil.IssueKey;
+import org.mapstruct.ap.testutil.ProcessorTest;
+import org.mapstruct.ap.testutil.WithClasses;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Oliver Erhart
+ */
+@IssueKey("1610")
+@WithClasses({
+    Employee.class,
+    EmployeeDto.class
+})
+public class IterableElementConditionTest {
+
+    @ProcessorTest
+    @WithClasses({
+        IterableElementConditionMapper.class
+    })
+    public void conditionalMethodWithSourceParameter() {
+        IterableElementConditionMapper mapper = IterableElementConditionMapper.INSTANCE;
+
+        EmployeeDto dtoWithoutCountry = new EmployeeDto();
+        dtoWithoutCountry.setName( "Tester" );
+        dtoWithoutCountry.setUniqueIdNumber( "SSID-001" );
+        dtoWithoutCountry.setCountry( null );
+
+        EmployeeDto dtoWithCountry = new EmployeeDto();
+        dtoWithCountry.setName( "Tester" );
+        dtoWithCountry.setUniqueIdNumber( "SSID-001" );
+        dtoWithCountry.setCountry( "Austria" );
+
+        ArrayList<EmployeeDto> employees = new ArrayList<>();
+        employees.add( dtoWithoutCountry );
+        employees.add( dtoWithCountry );
+
+        List<Employee> result = mapper.map( employees );
+        assertThat( result )
+            .singleElement()
+            .satisfies(
+                d -> assertThat(d.getName()).isEqualTo( "Tester" )
+            );
+
+    }
+
+}