write swift class like stringbuilder in visual basic

we need a replacement in swift for the stringbuilder object in visual basic. example usage: =================================================================== var mystring As stringbuilder() or var mystring As stringbuilder("my initial text") [login to view URL] "jhkhkjhkjhkjh" - conactenates to the current value a string [login to view URL] - returns the value of the stringbuilder as string [login to view URL] - clears the content [login to view URL] "fromstring", "replacestring" - substitute in the cuurent string all "fromstring" with "replacestring" ===================================================================