add coalesce expression, evaluate text anchor, allow simpler expressi…

…ons for layer selector

lib/src/expressions/coalesce_expression.dart

@@ -0,0 +1,17 @@
+import 'expression.dart';
+
+class CoalesceExpression<T> extends Expression<T> {
+  final Iterable<Expression<T>> _delegates;
+
+  CoalesceExpression(this._delegates);
+
+  @override
+  T? evaluate(Map<String, dynamic> args) {
+    for (final delegate in _delegates) {
+      final result = delegate.evaluate(args);
+      if (result != null) return result;
+    }
+
+    return null;
+  }
+}

lib/src/features/text_renderer.dart

@@ -115,7 +115,10 @@ class TextRenderer {
     if (_painter == null) {
       return null;
     }
-    final anchor = style.textLayout!.anchor;
+    final anchorString = style.textLayout!.anchor?.evaluate(
+      toArgsMap(context, feature),
+    );
+    final anchor = LayoutAnchor.fromName(anchorString);
     final size = _painter!.size;
     switch (anchor) {
       case LayoutAnchor.center:

lib/src/parsers/coalesce.dart

@@ -0,0 +1,16 @@
+import 'package:vector_tile_renderer/src/expressions/coalesce_expression.dart';
+import 'package:vector_tile_renderer/src/expressions/expression.dart';
+
+import 'parser.dart';
+import 'parsers.dart' as Parsers;
+
+class CoalesceParser<T> extends Parser<T> {
+  @override
+  Expression<T>? parseSpecial(data) {
+    if (data is! List) return null;
+
+    final delegates =
+        data.skip(1).map((i) => Parsers.parse<T>(i)).whereType<Expression<T>>();
+    return CoalesceExpression(delegates);
+  }
+}

lib/src/parsers/parser.dart

@@ -5,6 +5,7 @@ import 'package:vector_tile_renderer/src/logger.dart';
 import 'package:vector_tile_renderer/src/parsers/match.dart';
 
 import 'case.dart';
+import 'coalesce.dart';
 import 'step.dart';
 
 abstract class Parser<T> {
@@ -39,11 +40,12 @@ abstract class Parser<T> {
       return null;
     }
 
+    if (data[0] == 'case') return CaseParser<T>().parseSpecial(data);
+    if (data[0] == 'coalesce') return CoalesceParser<T>().parseSpecial(data);
     if (data[0] == 'get') return ArgumentExpression<T>(data[1]);
-    if (data[0] == 'zoom') return ArgumentExpression<T>('zoom');
     if (data[0] == 'match') return MatchParser<T>().parseSpecial(data);
     if (data[0] == 'step') return StepParser<T>().parseSpecial(data);
-    if (data[0] == 'case') return CaseParser<T>().parseSpecial(data);
+    if (data[0] == 'zoom') return ArgumentExpression<T>('zoom');
 
     return null;
   }

lib/src/themes/selector_factory.dart

@@ -1,6 +1,5 @@
-import 'selector.dart';
-
 import '../logger.dart';
+import 'selector.dart';
 
 class SelectorFactory {
   final Logger logger;
@@ -65,7 +64,9 @@ class SelectorFactory {
     if (filter.length != 3) {
       throw Exception('unexpected filter $filter');
     }
-    final propertyName = filter[1];
+    final propertyNameField = filter[1];
+    final propertyName =
+        propertyNameField is List ? propertyNameField.last : propertyNameField;
     final propertyValue = filter[2];
     return LayerSelector.withProperty(propertyName,
         values: [propertyValue], negated: negated);
@@ -85,7 +86,9 @@ class SelectorFactory {
 
   LayerSelector _numericComparisonSelector(List<dynamic> filter, String op) {
     ComparisonOperator comparison = _toComparisonOperator(op);
-    final propertyName = filter[1] as String;
+    final propertyNameField = filter[1];
+    final propertyName =
+        propertyNameField is List ? propertyNameField.last : propertyNameField;
     final propertyValue = filter[2] as num;
     return LayerSelector.comparingProperty(
         propertyName, comparison, propertyValue);

lib/src/themes/style.dart

@@ -53,7 +53,7 @@ class LayoutAnchor {
 
 class TextLayout {
   final LayoutPlacement placement;
-  final LayoutAnchor anchor;
+  final Expression<String>? anchor;
   final Expression<String> text;
   final Expression<bool> keepUpright;
   final Expression<double> textSize;

lib/src/themes/theme_reader.dart

@@ -124,7 +124,7 @@ class ThemeReader {
     final textLetterSpacing = parse<double>(layout?['text-letter-spacing']);
     final placement =
         LayoutPlacement.fromName(layout?['symbol-placement'] as String?);
-    final anchor = LayoutAnchor.fromName(layout?['text-anchor'] as String?);
+    final anchor = parse<String>(layout?['text-anchor']);
     final textFunction = parse<String>(layout?['text-field'])!;
     final keepUpright =
         parse<bool>(layout?['text-keep-upright']) ?? ValueExpression(true);