This menu gives you access to several functions which are useful for debugging. It is also known as the Debug Menu. Invoking it depends on the device where you are running your application.
On an iOS Device
Shake the device a little bit.
On iOS Simulator
Hit Ctrl-Cmd-Z on a Mac in the emulator to simulate the shake gesture, or press Cmd+D.
On Android Virtual Device
Either hit Cmd+M, or run adb shell input keyevent 82 in your terminal window.
To ensure the best debugging experience, first change your host type in XDE to LAN or localhost. If you use Tunnel with debugging enabled, you are likely to experience so much latency that your app is unusable. While here, also ensure that Development Mode is checked.
If you are using LAN, make sure your device is on the same wifi network as your development machine. This may not work on some public networks. localhost will not work for iOS unless you are in the simulator, and it only work on Android if your device is connected to your machine via usb.
Line numbers for console.log statements don't work by default when using Chrome debugging. To get correct line numbers open up the Chrome Dev Tools settings, go to the "Blackboxing" tab, make sure that "Blackbox content scripts" is checked, and add expo/src/Logs.js as a pattern with "Blackbox" selected.
Troubleshooting localhost debugging
When you open a project in XDE and when you press Open on Android, XDE will automatically tell your device to forward localhost:19000 and 19001 to your development machine, as long as your device is plugged in or emulator is running. If you are using localhost for debugging and it isn't working, close the app and open it up again using Open on Android. Alternatively, you can manually forward the ports using the following command if you have the Android developer tools installed: adb reverse tcp:19000 tcp:19000 - adb reverse tcp:19001 tcp:19001
Source maps and async functions
Source maps and async functions aren't 100% reliable. React Native doesn't play well with Chrome's source mapping in every case, so if you want to make sure you're breakpointing in the correct place, you should use the debugger call directly from your code.
To debug your app's HTTP requests you should use a proxy. The following options will all work:
On Android, the Proxy Settings app is helpful for switch between debug and non-debug mode. Unfortunately it doesn't work with Android M yet.
There is future work to get network requests showing up in Chrome DevTools.
Hot Reloading and Live Reloading
Hot Module Reloading is a quick way to reload changes without losing your state in the screen or navigation stack. To enable, invoke the developer menu and tap the "Enable Hot Reloading" item. Whereas Live Reload will reload the entire JS context, Hot Module Reloading will make your debug cycles even faster. However, make sure you don't have both options turned on, as that is unsupported behavior.