Allow expressions in @:priority tags.

Also, rewrite `AdvancedFunctionalityTest.testPriority()` to no longer rely on a map (`fixedPriorityUpdateListeners`) having a stable order.

README.md

@@ -444,6 +444,8 @@ class Main {
 }
 ```
 
+The argument to `@:priority` can be any integer expression. For instance, `@:priority(OtherSystem.DEFAULT_PRIORITY + 1)` will make the listener run before `OtherSystem`. The expression will be evaluated during the constructor, meaning it can access the constructor's `priority` argument.
+
 ### Update length
 
 As Glenn Fielder explains in his article ["Fix Your Timestep!"](https://www.gafferongames.com/post/fix_your_timestep/), games and physics simulations can be very sensitive to the length of each update. `@:update` listeners in Echoes are no exception.

src/echoes/macro/SystemBuilder.hx

@@ -70,12 +70,16 @@ class SystemBuilder {
 		return null;
 	}
 
-	private static function getPriority(meta:Metadata):Null<Int> {
+	private static function getPriority(meta:Metadata, knownPriorities:Map<String, Expr>):String {
 		final entry:MetadataEntry = getMeta(meta, PRIORITY_META);
 		switch(entry) {
 			case null:
-			case _.params => [_.expr => EConst(CInt(v))]:
-				return Std.parseInt(v);
+			case _.params => [expr]:
+				final key:String = new Printer().printExpr(expr);
+				if(!knownPriorities.exists(key)) {
+					knownPriorities[key] = expr;
+				}
+				return key;
 			default:
 		}
 		return null;
@@ -195,9 +199,11 @@ class SystemBuilder {
 		//Listener function priorities
 		//============================
 
-		final updateListeners:Array<ListenerFunction> = fields.map(ListenerFunction.fromField.bind(_, UPDATE_META)).filter(notNull);
-		final addListeners:Array<ListenerFunction> = fields.map(ListenerFunction.fromField.bind(_, ADD_META)).filter(notNull);
-		final removeListeners:Array<ListenerFunction> = fields.map(ListenerFunction.fromField.bind(_, REMOVE_META)).filter(notNull);
+		final knownPriorities:Map<String, Expr> = new Map();
+
+		final updateListeners:Array<ListenerFunction> = fields.map(ListenerFunction.fromField.bind(_, UPDATE_META, knownPriorities)).filter(notNull);
+		final addListeners:Array<ListenerFunction> = fields.map(ListenerFunction.fromField.bind(_, ADD_META, knownPriorities)).filter(notNull);
+		final removeListeners:Array<ListenerFunction> = fields.map(ListenerFunction.fromField.bind(_, REMOVE_META, knownPriorities)).filter(notNull);
 		for(listener in addListeners.concat(removeListeners)) {
 			if(listener.wrapperFunction == null) {
 				Context.error("An @:add or @:remove listener must take at least one component. (Optional arguments don't count.)", listener.pos);
@@ -208,7 +214,7 @@ class SystemBuilder {
 		 * Update listeners that have `@:priority` tags. Each group of these
 		 * will be used to create a `ChildSystem`.
 		 */
-		final fixedPriorityUpdateListeners:Map<Int, Array<ListenerFunction>> = new Map();
+		final fixedPriorityUpdateListeners:Map<String, Array<ListenerFunction>> = new Map();
 		for(listener in updateListeners) {
 			if(listener.priority != null) {
 				if(!fixedPriorityUpdateListeners.exists(listener.priority)) {
@@ -219,11 +225,11 @@ class SystemBuilder {
 			}
 		}
 
-		final defaultPriority:Null<Int> = getPriority(classType.meta.get());
+		final defaultPriority:Null<String> = getPriority(classType.meta.get(), knownPriorities);
 		if(defaultPriority != null) {
 			fields.pushFields(macro class DefaultPriority {
 				private override function __getDefaultPriority__():Int {
-					return $v{ defaultPriority };
+					return ${ knownPriorities.get(defaultPriority) };
 				}
 			});
 		}
@@ -235,7 +241,7 @@ class SystemBuilder {
 			final body:Array<Expr> = [for(listener in listeners) listener.callDuringUpdate()];
 			body.unshift(macro __dt__ = dt);
 
-			macro __addListenersWithPriority__($v{ priority }, function(dt:Float) $b{ body });
+			macro __addListenersWithPriority__(${ knownPriorities[priority] }, function(dt:Float) $b{ body });
 		}];
 		initializeChildren.push(macro if(parent != null) {
 			for(child in __children__) {
@@ -425,7 +431,7 @@ class SystemBuilder {
 	name:String,
 	args:Array<FunctionArg>,
 	pos:Position,
-	priority:Null<Int>,
+	priority:Null<String>,
 	?components:Array<ComplexType>,
 	?optionalComponents:Array<ComplexType>,
 	?viewName:String,
@@ -434,7 +440,7 @@ class SystemBuilder {
 
 @:forward
 abstract ListenerFunction(ListenerFunctionData) from ListenerFunctionData {
-	public static function fromField(field:Field, listenerType:String):ListenerFunction {
+	public static function fromField(field:Field, listenerType:String, knownPriorities:Map<String, Expr>):ListenerFunction {
 		switch(field.kind) {
 			case FFun(func):
 				if(SystemBuilder.getMeta(field.meta, listenerType) == null) {
@@ -445,7 +451,7 @@ abstract ListenerFunction(ListenerFunctionData) from ListenerFunctionData {
 					name: field.name,
 					args: func.args,
 					pos: field.pos,
-					priority: SystemBuilder.getPriority(field.meta)
+					priority: SystemBuilder.getPriority(field.meta, knownPriorities)
 				};
 			default:
 				return null;

test/AdvancedFunctionalityTest.hx

@@ -191,14 +191,16 @@ class AdvancedFunctionalityTest extends Test {
 		//Next, add a system with children.
 		final parent:UpdateOrderSystem = new UpdateOrderSystem();
 		Assert.equals(0, parent.priority);
-		Assert.equals(1, parent.__children__[0].priority);
-		Assert.equals(-1, parent.__children__[1].priority);
+		final positiveChild:System = Lambda.find(parent.__children__, child -> child.priority == 1);
+		Assert.notNull(positiveChild);
+		final negativeChild:System = Lambda.find(parent.__children__, child -> child.priority == -1);
+		Assert.notNull(negativeChild);
 
 		list.add(parent);
 		assertListContents([
-			high, parent.__children__[0], //1
+			high, positiveChild, //1
 			middle, parent, //0
-			low, parent.__children__[1] //-1
+			low, negativeChild //-1
 		]);
 
 		final updateOrder:Array<String> = [];
@@ -214,8 +216,8 @@ class AdvancedFunctionalityTest extends Test {
 		low.priority = -1;
 		assertListContents([
 			parent, //2
-			high, parent.__children__[0], //1
-			parent.__children__[1], middle, low //-1
+			high, positiveChild, //1
+			negativeChild, middle, low //-1
 		]);
 
 		updateOrder.resize(0);

test/Systems.hx

@@ -79,7 +79,9 @@ class TimeCountSystem extends System implements IMethodCounter {
 }
 
 class UpdateOrderSystem extends System {
-	@:update @:priority(-1) private function post_update(order:Array<String>):Void {
+	private static final CONSTANT:Int = 5;
+
+	@:update @:priority(CONSTANT - 6) private function post_update(order:Array<String>):Void {
 		order.push("post_update");
 	}