How to jump forward/backward from Xcode split window?

Question

I used to split my Xcode window to 4 sub windows (not popup). I wonder if there's shortcut key to move cursor from one sub window to another ?

+++++++++++++++++++++++++++++
+     +          +          +
+ G / +   F1     +    F2    +
+ r F +          +          +
+ o i +++++++++++++++++++++++
+ u l +          +          +
+ p e +   F3     +    F4    +
+     +          +          +
+++++++++++++++++++++++++++++