Flashlight, a Lighthouse for Mobile Apps

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The promise of Lighthouse is pretty cool, right? Visit any website, and with only a few clicks, you can get a performance score for it!

Wouldn’t it be grand if we had the same thing for React Native apps? 


Well, you’re in luck! let me present our new open source tool called Flashlight 🔦 

(cause you know, a Flashlight is basically a very small “mobile” lighthouse, right?)

Alexandre Moureaux
Alexandre Moureaux
7 min
23 Oct, 2023

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Video Summary and Transcription

Today's Talk introduces Flashlight, a tool for measuring mobile app performance. Flashlight provides a performance report that highlights issues like high CPU usage on low-end devices. To fix these issues, the Shopify component Flashlist can be used. Flashlight can also be used locally to measure performance changes in React Native apps, with the ability to see the score rapidly changing and improved performance after implementing fixes. More information can be found in the documentation and an article comparing scrolling performance in React Native.

1. Introduction to Flashlight

Short description:

Hi, everyone. Today, I'm happy to talk about measuring mobile app performance and introduce our new tool called Flashlight. It's like a mobile Lighthouse. Just upload your Android app on app.flashlight.dev, configure the measurement, and get a performance report.

Hi, everyone. I'm Alex. I've been working with React Native for the past eight years at BAM, and today I'm very happy to talk to you about measuring mobile app's performance. I've always wanted to have an easy way to know whether the performance of my app was good or not. And I've always been jealous of web developers for Lighthouse. Have you ever used Lighthouse? You can go to any website and with it, with a simple few clicks, just generate a performance report and get a performance score for the web page you visited, which is pretty neat, right?

Wouldn't it be nice if we had something similar, but for mobile apps? Well, today I'm very proud to present you our new tool called Flashlight. It's meant as a mobile Lighthouse. So let's see how it works. You can go right now, if you like, on app.flashlight.dev and upload any Android app, iOS support is on the way. So let's say, for example, this is sharing my phone. We have an app like this, like I have sort of a Netflix clone. I can generate a release APK for it and upload it right here. And then I can configure what I want Flashlight to measure. So by default, you can measure app start performance quite easily. You just need to input here some text that will appear when the app has loaded. For example, here when my app has loaded, I know that family and history will appear so I can just input family here, and then click send me my performance report.

2. Analyzing Performance Report and Fixing Issues

Short description:

Now a new page opens, and after roughly 10 minutes, Flashlight provides a performance report. We can see that the app has high CPU usage, particularly in the JS Thread. This issue is highlighted by Flashlight, especially on low-end devices. To fix it, we can switch to using the Shopify component called Flashlist, which significantly improves performance.

Now a new page opens, depending on traffic, it should take roughly 10 minutes to finish and Flashlight to get the report. So I'm going to split this up a little bit. So when Flashlight is finally done, after roughly 10 minutes, this is what you would see. You would see that essentially what happened is that Flashlight has started the app 10 times and measured performance and what's really nice is that you have this see report button and if you click it, boom, you have your performance report. And in our case, we can see that it's pretty bad. Luckily, Flashlight gives us some performance metrics to understand why it's that bad.

Here we can see, for example, that this one here is the one that seems to be problematic. High CPU usage. We see that we have a thread using a lot of CPU for a lot of time, a thread called mqtjs. We can check the graphs. So we have frame rates, we have total CPU usage and we have CPU usage per thread and we can see again mqtjs in fact, basically for the whole duration of measure it's really really high. It uses way too much CPU. Okay, we can ask ourselves, what is mqtjs? You guessed it, it's the JS Thread. You never want this guy to be close to 100% of CPU usage because then that's the same as having zero JSFPS, your app would not respond to anything. The JS Thread is blocked. So, that's what happens when we open our app, basically, and flashlight is telling us we have a big issue. Note that actually Flashlight is running on a real low-end android device, a Samsung Galaxy A10s by default. So on a low-end device yeah, performance is really bad. What can we do to fix? Well, our app is basically a vertical list composed of horizontal nested lists. And we're using flat lists for both of those, horizontal and vertical. But let's see what happens if we change to using the Shopify component called Flashlist. Then we can just regenerate our APK and do exactly the same thing as before to get a new report with Flashlist. And boom! Ah, this is much better. We get a score of 75 out of 100. But what's really nice is we can actually go onto the history section here. We can click both reports, so the first one, MyAppWithFlatlist and the second one called Flashlist. And we can open the comparison view. We have the video as well. I'm just going to close them. And for example, if I jump to the CPU usage per thread, we can see quite easily that yeah, in green with Flashlist, CPU usage of the Jez thread is in fact much better with Flashlist.

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