function parseUpdate$1(spec) {
    var ctx = this,
      srcid = isObject(srcid = spec.source) ? srcid.$ref : srcid,
      source = ctx.get(srcid),
      target = null,
      update = spec.update,
      params = undefined;
    if (!source) error('Source not defined: ' + spec.source);
    target = spec.target && spec.target.$expr ? ctx.eventExpression(spec.target.$expr) : ctx.get(spec.target);
    if (update && update.$expr) {
      if (update.$params) {
        params = ctx.parseParameters(update.$params);
      }
      update = ctx.handlerExpression(update.$expr);
    }
    ctx.update(spec, source, target, update, params);
  }
Parse an event-driven operator update.