Xcode's refactoring support has always been a bit of a joke, but if you've switched to Swift in the last 18 months then the joke is over because it simply doesn't work at all. I'm sure it's being worked on, and I'm sure it'll be great when it comes, but until then you might want to check out this plugin from John Holdsworth. It only supports renaming, but its better than nothing.
Playgrounds are great, but what if the code is already inside your app? This plugin from Shengjia Wang allows you to select some code in Xcode and quickly run it. The biggest limitation is (fairly obviously) that you can't make reference to anything outside the scope of what's selected, but the same limitation exists with copying some code into a playground, so you're not really any worse off.