I tried to create a UILabel
in playground but failed. Does playground only support OS X development for now?
YES, it does!
File: New > File... > iOS > Source > Playground
import UIKit
let lbl = UILabel(frame: CGRect(x: 0, y: 0, width: 300, height: 100))
lbl.text = "Hello StackOverflow!"
Then, save the file. (Or manually run it.) This will trigger the Playground to interpret UI related things. At this point, the word "UILabel" should appear on the right-hand side.
Now, to actually view what you've done, you've got to click on the "Quick View" eye on the right, or the white circle to open it in Assistant Editor:
Here's a screenshot of some basic things with UIImage working, etc.
(EDIT: minor text update to current CGRect syntax -- But, screenshots still show old syntax.)