It’s the hooks rule number one: “Only call hooks at the top level”.
But what if I told you that this rule does not apply to every hook? One of them can actually safely be used conditionally. 
To understand how useContext is different from other hooks, and why it is exempted from this major rule, we need to know why this rule exists in the first place. We will build our own mental model of react’s rendering behavior, focused on how react keeps track of data at runtime: props, states, refs… and context values.

How do you start your app on your simulator in the morning? Do you always run react-native run-ios? If so, you might be wasting a lot of time. Let’s check out together what a React Native app looks like under the hood. It will help us to be more efficient on a daily basis. We’ll identify the shortest steps to execute daily actions, like starting our apps as fast as possible (no more unnecessary compilations of native modules!), or dealing with pesky red errors that metro throws at us.