Watchman
Watchman is an open source tool created by Facebook (https://facebook.github.io/watchman/). React Native's packager uses Watchman to recursively watch for changes to our source code files across one or more directory trees. Once it detects a change, it automatically rebuilds the JavaScript bundle. This allows us to sidestep one of the slowest and most painful parts of native development.
Much like several of our other tools, once Watchman is installed, you won't have to worry about it. The React Native Package Manager handles running Watchman for us.
Flow
Unlike the other tools mentioned, Flow is entirely optional. Flow is yet another open source tool created by the Facebook team (http://flowtype.org/). It's used to add type annotations ...
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