react-native android fontFamily does not take effect

Question 1:

I add a fontFamily to index.android.js's welcome style, but it takes no effect. Does fontFamily actually work on android?

welcome:{ fontSize:20, fontFamily:'roboto-thin', textAlign:'center', margin:10}

Question 2:

if fontFamily works on android, is there a way to load custom font from assets? Or is this some feature to be implemented by react-native?