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Debugging

Seeing what the app is doing is an essential part of development. A variety of tools are available to help us do that.

Index:

Upstream advice from React Native

React Native's documentation has a "Debugging" page with a wide variety of tips.

Definitely read about:

  • the In-App Developer Menu
  • Reloading JavaScript
  • In-App Errors and Warnings

The Chrome Developer Tools are also essential; see the next section here for details.

Other advice on the page may be useful, but feel free to skim or skip.

Tools: React + Redux

These tools connect to the JavaScript environment that React Native sets up and most of our app code runs in. They provide JS-level debugging there, plus useful hooks specific to React and Redux.

Chrome Developer Tools / Remote JS Debugging

React Native supports debugging the app using Chrome's developer tools, in much the same way you would a web app. This provides you with prettily formatted debug messages and helpful additional information.

To use it, start the app. (Either in the emulator, or see here for additional instructions to do this on a physical device.) Then, open the Developer Menu. Here, select "Debug" (formerly "Debug JS Remotely"). This will open a new tab in your browser, at http://localhost:8081/debugger-ui . Go to this tab and open the developer console.

This console will show all console debug output from your app, which means that you can debug the app with statements like

console.debug(foobar)

Additionally, all Redux events are automatically logged to the console. See discussion of redux-logger below.

See also in the "Troubleshooting" section below.

React DevTools

The standalone version of the React Developer Tools, an Electron app, can be used to debug the React component hierarchy in our app. See RN's docs on how to use it with RN.

As of 2020-01, the latest version (v4) of React DevTools does not support any released version of React Native; see its release notes. Instead, install v3 with the command yarn global add react-devtools@3.

redux-logger: deeper info on Redux events

One extremely useful kind of information for debugging many kinds of issues -- as well as for getting to understand how the app works! -- is to see the Redux state, and a log of the Redux actions.

We have exactly that information logged to the console (in the Chrome Developer Tools; see above), thanks to the middleware redux-logger.

By default, it logs the previous state and next state of every action that is dispatched. You can control its behavior in more detail by editing the call to createLogger in src/boot/middleware.js.

  • diff: true will compute the diff (using deep-diff) between the old and new states. For example, the log output for the action SWITCH_NARROW can look like this:

    image

    This can be especially helpful for seeing what each action really does.

  • predicate can be used to filter which actions cause a log message. By default, all actions are logged; when looking at a long log, this option can help you cut noise and focus on actions relevant to what you're studying.

  • Many other options exist! See the doc.

immutable-devtools: inspect Immutable.js objects in Chrome

Some of the data in the Redux state is stored in Immutable.js data structures. It's normally awkward to inspect these data structures; an Immutable.Map object, for example, is full of properties like size, __altered, __hash, and __ownerID, which you have to dig around to get at a clear representation of the items contained.

immutable-devtools fixes this problem in Google Chrome v47+ by installing a "custom formatter".

(Alternatively, we might have recommended a Chrome extension, which immutable-devtools mentions in its setup doc. But using the NPM package provides the formatter for zulip-mobile just as effectively without making widespread changes in behavior when you browse the Web.)

To enable it, open Chrome Dev Tools and press F1 to open the settings. In the "Console" section, tick "Enable custom formatters". If it isn't working, please consult the project's issue tracker before opening an issue in zulip-mobile.

Tools: Debugging the message list (in WebView)

We render the message list using a native WebView component. Anything related to the rendering of messages, the behavior of the message list, or bugs inside it, you can debug with familiar tools.

Debugging is available both for Android and iOS, and on an emulator or a physical device via a browser's developer tools.

We've defined a function sendMessage that can send a message from within the WebView to React Native-land outside it. This can be used (with type: 'debug') if you want to see WebView logs alongisde React Native logs.

To debug code that runs during the initial load of the WebView, add alert("pause"); debugger; where you want a breakpoint. Then open the WebView and connect the debugger before clearing the alert.

Debug WebView on iOS

  1. Enable debugging on the device

    For iOS Simulator you can skip this step, as it is already enabled.

    To debug on your physical iOS device, go to Settings > Safari > Advanced and make sure Web Inspector is on.

  2. Connect to the device

    • Run Safari (even if your browser of choice is something else).
    • If you have not done so already, enable the developer tools by going to Safari’s menu, Preferences > Advanced, and checking the Show Develop menu in the menu bar checkbox.
    • In the app you are debugging, make sure you have navigated to a screen that is showing a message list.
    • In the Develop menu, find your device and select it.

  3. Debug

    You now have access to the rich developer tools you might be familiar with. You can inspect HTML elements, CSS styles and examine console.log output.

Debug WebView on Android

  1. Enable debugging on the device

    For the Android emulator you can skip this step, as it is already enabled.

    To debug on your physical Android device, go to Settings > About phone. Next, tap the Build number panel seven times. You will get a notice that now you are a developer. Go back to the main Settings screen. Go to the new Developer options menu and enable the USB debugging checkbox.

  2. Connect to the device

    • Run Chrome.
    • Navigate to about:inspect.
    • Check the Discover USB devices and the app will appear.

  3. Debug

    You now have access to the rich developer tools you might be familiar with. You can inspect HTML elements, CSS styles and examine console.log output.

Tools: Native

These tools operate outside the JavaScript environments set up by either React Native or our WebView for the message list. They're essential for debugging our platform-native code which runs outside those JS environments.

adb logcat

When running on Android, either in the emulator or on a physical device, you can use ADB (the Android debugger) to fetch or watch the device's logs. This will include any messages that you print with a statement like

console.debug(foobar)

To see the logs, run adb logcat. This accepts many command-line flags to filter and control the output, some of them extremely useful -- see upstream documentation. Start with the section on filtering log output; feel free to skim the whole rest of the document, but definitely read that section.

Example useful command lines with adb logcat:

  • adb logcat -T 100 ReactNativeJS:V *:S

    This filters out logs unrelated to the app (along with many things that are related), but includes anything you print with console.debug. It starts with the last 100 matching log lines from before you run the command (so it can be helpful for seeing something that just happened), and then it keeps running, printing any new log messages that come through. To quit, hit Ctrl-C.

  • adb logcat -t 100 *:W

    This filters out logs at levels V, D, and I (verbose, debug, info), leaving only W, E, and F (warning, error, fatal). It includes errors at these levels from anything on the system -- often good because it isn't always predictable what tag an important message will come with. It prints the last 100 matching messages, and exits.

  • adb logcat -T $(date +%s.%N -d "2 minutes ago") *:W

    This prints messages since a certain time, then keeps running to print new log messages that come through. The date command is there in order to turn a nice human-formatted time into the format adb logcat expects.

    (On macOS, your date command may not have this feature, because it's a version whose UI hasn't changed since the '80s. You want GNU date. brew install coreutils will install it, with the name gdate.)

Debugging with Sentry

We use Sentry to get alerts about things going wrong in the app.

Testing Sentry itself, with testing-only project

Normally, debug builds of the app (and others that aren't meant to become published release builds) don't send any data to Sentry. In development one sees errors in a more direct way anyway, and we don't want to get confusing noise into our production Sentry data. (For full discussion, see src/sentryConfig.js.)

For testing our Sentry reporting itself, though, one wants to send events to Sentry after all. To avoid polluting our production Sentry data, do this by sending them to a Sentry project that's specifically for testing. For core developers, we have a project called "testing" in our Sentry team, but any project will work.

The "DSN" or "client key" is how we tell the Sentry client where to send events. For Zulip's "testing" project, members of the team can find the DSN at https://sentry.io/settings/zulip/projects/testing/keys/ .

To send events from the JavaScript layer, paste the DSN into src/sentryConfig.js as the value of sentryKey.

To send events from the native Android layer, edit android/app/src/main/AndroidManifest.xml similarly, following the comment around io.sentry.dsn.

Troubleshooting

Remote JS Debugging ("Debug") opens a webpage that never loads.

For some reason, React Native may try to open a browser tab for you at http://10.0.2.2:8081/debugger-ui . Instead, it should be http://localhost:8081/debugger-ui .

To fix this, simply open http://localhost:8081/debugger-ui in your browser. This should load the web debugger you expected. Also, if the app was showing a blank screen before, it should now behave normally.

If you're still experiencing this issue, open the Developer menu in your app. Then, go to "Debug server host & port for device". Here, enter localhost:8081 and click OK. Now, try to open http://localhost:8081/debugger-ui again and see if it works.