表示表示选择器的表达式。所有选择器均以 @ 开头。例如 @size 在表达式中：select{t:material, @size}。
[Description("Convert arguments to a string allowing you to format the result.")]
[SearchExpressionEvaluator(SearchExpressionType.Selector | SearchExpressionType.Text, SearchExpressionType.Iterable | SearchExpressionType.Literal | SearchExpressionType.Variadic)]
[SearchExpressionEvaluatorSignatureOverload(SearchExpressionType.Iterable | SearchExpressionType.Literal | SearchExpressionType.Variadic)]
public static IEnumerable<SearchItem> FormatItems(SearchExpressionContext c)
{
var skipCount = 0;
if (SearchExpression.GetFormatString(c.args[0], out var formatStr))
skipCount++;
var items = c.args.Skip(skipCount).SelectMany(e => e.Execute(c));
var dataSet = SearchExpression.ProcessValues(items, null, item => SearchExpression.FormatItem(c.search, item, formatStr));
return dataSet;
}