How debugging works today
To start debugging, we open the developer menu (using Cmd+D) and select "Debug with Chrome". This action tells the Metro Packager to open the Chrome browser and the JavaScript code now runs on Chrome, instead of running on the device. A web socket connection between Chrome and the device/emulator via metro is responsible delivering the JS commands to Chrome, and sending the results of the statements back to the emulator.Issues with Chrome as a Debugger
The entire debugger setup is pretty clever as it brings the classic web development workflow to mobile. However, there are a few issues with this setup- The JS VM running on the device is JavaScript Core (JSC), and is different from V8/Chrome during debug mode. These JS VM differences could lead to hard to fix bugs.
- Communication during debug mode is over a web socket, which is inherently asynchronous. This poses a problem for cases when native modules expose synchronous APIs. The issue only gets larger Fabric and TurboModules, which have many more synchronous methods.
- Many developers currently using the JS Profiler in Chrome to understand the performance of their React Native app. The profiles are not accurate since they have that web socket layer, introducing a level of network latency, which is not present in the real app.
Using Safari
Turns out, Safari can be used to connect to apps that run JSC. For React Native apps, we can use Safari to debug JavaScript and use workflows like setting breakpoints, inspecting variables, etc. However, the JSC debugger in Safari does not use the network stack, which means that the sourcemap URL in the index.bundle file is not evaluated, leaving us to debug one giant JS file.To work around this, we an use inline sourcemap that can be enabled by a single line change in the Appdelegate.m file.
In the file sourceURLForBridge, replace return [[RCTBundleURLProvider sharedSettings]... with the following
[NSURL URLWithString:[[[[RCTBundleURLProvider sharedSettings] jsBundleURLForBundleRoot:@"index" fallbackResource:nil] absoluteString] stringByAppendingString:@"&inlineSourceMap=true" ]];
This line simply appends a inlineSourceMap=true, making metro return the sourecmaps with the index.bundle file.
Once this is setup, Safari can successfully recognize the sourcecmaps and open the source files correctly.
Next Steps
Note that this method works with emulators, it does not work with the device. Now that we have Safari working, I would also like to get the JS Profiler in Safari, to give us accurate JS execution information, which should help improving performance of the apps.
