Very basic example showing how to share state via the context chain

basic_context_example.js

Macro.add('scene', {
	tags    : null,
	handler : function () {
		/* Argument sanity checking. */

		// Extend our execution context with the `_scene` object so our
		// descendants may share state, by looking up our context.
		Object.defineProperty(this, '_scene', {
			value : /* Whatever you need to share, probably an object. */
		});

		/* Whatever else the macro needs to do. */
	}
});

/*
	`<<branch>>` variant that requires a `<<scene>>` be its immediate parent.
*/
Macro.add('branch', {
	tags    : null,
	handler : function () {
		// Get our `<<scene>>` parent's shared scene state.
		var scene = (function () {
			var parent = this.parent;

			if (parent === null || parent.name !== 'scene' || !parent.hasOwnProperty('_scene')) {
				return null;
			}

			return parent._scene;
		})();

		if (scene === null) {
			return this.error('must only be used in conjunction with its parent macro <<scene>>');
		}

		/* Argument sanity checking. */

		/* Whatever else the macro needs to do. */
	}
});

/*
	`<<branch>>` variant that requires a `<<scene>>` be an ancestor.
*/
Macro.add('branch', {
	tags    : null,
	handler : function () {
		// Get our first `<<scene>>` ancestor's shared scene state.
		var scene = (function () {
			var parent = this.contextSelect(function (ctx) {
				return ctx.name === 'scene';
			});

			if (parent === null || !parent.hasOwnProperty('_scene')) {
				return null;
			}

			return parent._scene;
		})();

		if (scene === null) {
			return this.error('must only be used in conjunction with its parent macro <<scene>>');
		}

		/* Argument sanity checking. */

		/* Whatever else the macro needs to do. */
	}
});