Lets first know what exactly happens in the low-level code.
There are 4 threads in the React Native App:
1) UI Thread : Also known as Main Thread. This is used for native android or iOS UI rendering. For example, In android this thread is used for android measure/layout/draw happens.
One exception are the native views that happen completely in UI thread, for example, navigatorIOS or scrollview run completely in UI thread and hence are not blocked due to a slow js thread.
3) Native Modules Thread: Sometimes an app needs access to platform API, and this happens as part of native module thread.
4) Render Thread: Only in Android L (5.0), react native render thread is used to generate actual OpenGL commands used to draw your UI.
Generally, we can separate React Native into 3 parts:
This is often called “The 3 Parts of React Native”.