string properties return empty string if not set

To avoid null checks everywhere in the client
code, the type of string properties is now `String` instead of `String?`.
An additional '_raw_...' property is generated,
like it is for bool/int, that is of type `String?`

Author: slisson

metamodel-generator/src/main/kotlin/org/modelix/metamodel/generator/MetaModelGenerator.kt

@@ -246,13 +246,11 @@ class MetaModelGenerator(val outputDir: Path) {
                             .mutable(true)
                             .delegate("""${accessorClass.qualifiedName}(unwrap(), "${feature.originalName}")""")
                             .build())
-                        if (data.type != PropertyType.STRING) {
-                            addProperty(PropertySpec.builder("_raw_" + feature.validName, PropertyType.STRING.asKotlinType())
-                                .addModifiers(KModifier.OVERRIDE)
-                                .mutable(true)
-                                .delegate("""${StringPropertyAccessor::class.qualifiedName}(unwrap(), "${feature.originalName}")""")
-                                .build())
-                        }
+                        addProperty(PropertySpec.builder("_raw_" + feature.validName, String::class.asTypeName().copy(nullable = true))
+                            .addModifiers(KModifier.OVERRIDE)
+                            .mutable(true)
+                            .delegate("""${RawPropertyAccessor::class.qualifiedName}(unwrap(), "${feature.originalName}")""")
+                            .build())
                     }
                     is ChildLinkData -> {
                         // TODO resolve link.type and ensure it exists
@@ -290,11 +288,9 @@ class MetaModelGenerator(val outputDir: Path) {
                         addProperty(PropertySpec.builder(feature.validName, data.type.asKotlinType())
                             .mutable(true)
                             .build())
-                        if (data.type != PropertyType.STRING) {
-                            addProperty(PropertySpec.builder("_raw_" + feature.validName, PropertyType.STRING.asKotlinType())
-                                .mutable(true)
-                                .build())
-                        }
+                        addProperty(PropertySpec.builder("_raw_" + feature.validName, String::class.asTypeName().copy(nullable = true))
+                            .mutable(true)
+                            .build())
                     }
                     is ChildLinkData -> {
                         // TODO resolve link.type and ensure it exists
@@ -318,7 +314,7 @@ class MetaModelGenerator(val outputDir: Path) {
 
 fun PropertyType.asKotlinType(): TypeName {
     return when (this) {
-        PropertyType.STRING -> String::class.asTypeName().copy(nullable = true)
+        PropertyType.STRING -> String::class.asTypeName()
         PropertyType.BOOLEAN -> Boolean::class.asTypeName()
         PropertyType.INT -> Int::class.asTypeName()
     }

metamodel-runtime/src/commonMain/kotlin/org/modelix/metamodel/PropertyAccessor.kt

@@ -16,11 +16,16 @@ abstract class PropertyAccessor<ValueT>(val node: INode, val role: String) {
     abstract fun convertWrite(value: ValueT): String?
 }
 
-class StringPropertyAccessor(node: INode, role: String) : PropertyAccessor<String?>(node, role) {
+class RawPropertyAccessor(node: INode, role: String) : PropertyAccessor<String?>(node, role) {
     override fun convertRead(value: String?): String? = value
     override fun convertWrite(value: String?): String? = value
 }
 
+class StringPropertyAccessor(node: INode, role: String) : PropertyAccessor<String>(node, role) {
+    override fun convertRead(value: String?): String = value ?: ""
+    override fun convertWrite(value: String): String? = value
+}
+
 class BooleanPropertyAccessor(node: INode, role: String) : PropertyAccessor<Boolean>(node, role) {
     override fun convertRead(value: String?): Boolean = value == "true"
     override fun convertWrite(value: Boolean): String? = if (value) "true" else "false"