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Debugging

Using a Simulator / Emulator

There is no substitute to testing the performance and feel of your app on an actual device, but when it comes to debugging you might have an easier time using an emulator/simulator.

Apple refers to their emulator as a “Simulator” and Google refers to theirs as an “Emulator”.

iOS

Make sure you have the latest Xcode (e.g. from the Mac App Store). This includes the iOS Simulator, among several other tools.

Android

On Android we recommend the Genymotion emulator over the standard emulator — we have found it to be more feature complete, faster and easier to use.

Download Genymotion (free version) and follow the Genymotion installation guide. Once you’ve installed Genymotion, create a virtual device - we recommend a Nexus 5, the Android version is up to you. Start up the virtual device when it’s ready. If you run into any issues follow our Genymotion guide.

Developer Menu

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 Genymotion

Either press “Menu” button in Genymotion’s toolbar, or just hit Cmd-m.

Debugging Javascript

You can debug Expo apps using the Chrome debugger tools. Rather than running your app’s JavaScript on your phone, it will instead run it inside of a webworker in Chrome. You can then set breakpoints, inspect variables, execute code, etc, as you would when debugging a web app.

  • 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.

debugging host

  • 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.

  • Open the app on your device, reveal the developer menu then tap on Debug JS Remotely. This should open up a Chrome tab with the URL http://localhost:19001/debugger-ui. From there, you can set breakpoints and interact through the JavaScript console. Shake the device and stop Chrome debugging when you’re done.

  • 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.

Debugging HTTP

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.

Other Debugging Tips

Dotan Nahum outlined in his “Debugging React Native Applications” Medium post other useful tools such as spying on bridge messages and JSEventLoopWatchdog.