I have applied to iOS developer position via ThoughtWorks site. They called me for technical online test on both iOS & Android(Main focus on iOS). And later a week, they called me for coding interview on their promises. It was simple program. You can use your own laptop, wifi access will be provided. They are mostly checking how you are approaching the program & your design architecture rather than output. Once you have cleared this round, you will called for F2F interview on the same day. It was very helpful experience for me.