What are the rules and limitations of startTransition in React 18? - javascript

React has just released the startTransition API in v18. While it seems like a "magic spell" to bring more responsiveness, the doc is quite vague about its behavior under the hood. The only sentence related to its behavior in the doc is "Updates in a transition yield to more urgent updates such as clicks."
As a React programmer, I am concerned about whether it will cause glitches to my components. E.g.:
Will the state split into two branches during transition? Let's assume we have a current state. When a transition is turning it into slow_update(state), another event occurs that turns the state into fast_update(state). We will have both slow_update(state) and fast_update(state) calculated in this process. This never happens in the old React when state updates are done in order.
If some useEffect hooks are triggered by the state update, will the callback execute twice if the transition is interrupted? Will the callback receive some inconsistent data since the state is branched during transition?
I am also curious about will it will make my app any more responsive at all. E.g.:
Does startTransition apply to render phase, or it only optimizes reconciliation? Does it apply to just the callback itself, or also the follow-up renders, useMemos and useEffects on the updated state? It seems that CPU-heavy logic blocks the main thread anyway so React has no way to interrupt it. Therefore, if my component filters through a long list in a startTransition callback, it is optimized at all?
If urgent updates occur repeatedly, will the slow update be blocked forever? For example, if the component uses requestAnimationFrame to update its state every 16ms, and the slow transition takes longer time, will the slow transition be interrupted again and again?
I have read the doc, the release note, the reactwg discussion and the official example, but they do not answer the questions above. Since that concurrent-related bugs are hard to fix or reproduce, it would be great if there are some thumb rules (like the rules of hooks) that keep the transition code always correct.
In general, what code will benefit from startTransition, and what is needed to avoid glitches?

Related

How to measure a large number of components in React Native without causing an error?

I'm trying to implement a generic Drag-and-Drop module that I may or may not release on NPM.
I took inspiration from an already existing DnD module which consisted of essentially two main components: Draggables and DropZones.
The Draggable components would enable any content that they enclosed to be dragged around, and DropZones were the components that would react to when a Draggable component was dragged in/out of them and/or dropped onto them.
The module's functionality was too basic for what I needed, so I set off on making my own.
Everything has been going fine except for one aspect: measuring the absolute position of a large number of components in order to find their corresponding bounding shapes for collision detection.
Currently, I've been using the View's measure method to measure the absolute position and size of my components, however, if you call this method too many times within a short period of time, React Native will throw this error: Warning: Please report: Excessive number of pending callbacks: 501. Some pending callbacks that might have leaked by never being called from native code and it lists the offending method as {"module":"UIManager","method":"measure"}.
If I make a basic for loop that repeatedly calls the measure method, I can get it up to just over 500 iterations before React Native complains (this number may be hardware-dependent; I'm not sure).
Also, the same problem happens when using the View's other measuring methods (e.g. measureInWindow, measureLayout).
What's even worse is that up until now, I have been calling the measure method in a View's onLayout callback, however, I recently discovered that onLayout does not get called when a style changes that doesn't directly affect the given View (such as when a parent component's margin changes).
This means that the given components can be re-positioned (relative to the screen) without them knowing about it.
Because of this I will have to use a different means of notifying my components that they need to remeasure themselves.
I thought about using some of the life-cycle methods that pertain to component updates, but that would most likely be even worse, because most of them get called every time the component updates which is far more often than a layout happens.
Edit: One other problem with using the life-cycle methods that pertain to component updates is if a parent component prevents it's child components from being updated (such as through shouldComponentUpdate).

What is the best way to handle data-intensive requests with React and Redux?

The bulk of my page is essentially a list of 40-100~ish components, each containing about 11 KB of data (in JSON format). The problem is that 100 times 11 KB makes 1.1 MB, which seems to be a little too memory-intensive to store in the Redux state for browsers on older mobile devices. It makes my gorgeous CSS animations look choppy and buttons take about a second to toggle on/off state.
Because each component is exactly 148px tall, my first thought was to use a virtual list (Virtuoso) which only renders as many items at a time as can fit on the screen (which is like 5-8 tops). This made the first time render much faster, but did nothing to make animations smoother, which definitely confirmed it's mostly a memory issue.
So, if I can't store and keep all my data in the state object, then I need to do something similar to Virtuoso and only keep as much data as I need to render the current screen. Now, I'm not exactly sure how Redux works internally, but if the state is immutable, doesn't that mean that everything that ever goes there stays there? And wouldn't that mean I'm trying to do something impossible, or at the very least anti-pattern?
Oh, and to make things worse, the data will need to be updated every 3 seconds, which means a component will sometimes disappear, only to reappear with the next update. I haven't tested how this would affect Virtuoso scrolling yet, but I don't exactly expect perfect plug-and-play behaviour.
I would appreciate ideas on how to solve this with redux and (possibly) its middleware, since it's the only architecture I'm familiar with atm and switching to e.g. Flux would mean having to learn it from scratch AND rewrite about 2000 lines of redux code.
I have never used Virtuoso, react-virtualized or react-window, but this looks like an issue that you can tackle with one of those libraries.
Do other parts of your React application care about the data you are rendering in the list?
If not, put the data in this component's local state and not in Redux.
If they do, maybe try to set this huge list in localStorage when the component mounts, unset it when it unmounts, and use the component's local state to select start / end indexes to pick a slice of the data.
See also: https://blog.jakoblind.no/is-using-a-mix-of-redux-state-and-react-local-component-state-ok/

React and CSS animations

When making a state change, React will run a reconciliation algorithm and may completely re-create parts of the DOM.
If I have a CSS animation or transition being applied to a DOM node, how do React developers typically avoid interfering with the DOM during the animation (which might stop the animation)?
Does this "just work", because developers know to only start the animation (e.g. by adding or removing a class) when a specific part of the component lifecycle has completed (e.g. onComponentDidUpdate) - i.e. after they can be sure no further major DOM subtree changes will occur?
Usually this should "just work" with Reacts dom diffing. A more controlled approach would be to move whatever dom that is being animated into it's own component and using the shouldComponentUpdate method to control when you let React perform a re-render.

How to optimize the rendering of components in Ember JS

There are around 3 hundred components rendered inside the wrapper component and its taking to much of time to render. But I need thousand of components to get rendered inside the wrapper container. How can I achieve this without any performance issues while rendering the components
Image shows the rendering time taken by 300 components which is too much
If you have a scroll and all of your components are not in the viewport at the same time, you may use the Proxy Pattern.
There is an ember addon called ember-in-viewport to detect whether your component is in viewport or not. By using it, you may implement the proxy pattern.
Here is a sample twiddle. At the application.hbs, if you use my-proxy-component instead of my-component, page rendering would be nearly 3 times faster.
These tips are kinda hacky but might help:
You might want to lazy load some of the components, like for example load the most critical ones and then load the rest changing a computed property after a fixed timeout on didRender event (during the idle state).
For example:
onDidRender: Ember.on('didRender', function() {
Ember.run.later(() => {
this.set('displayLazyItems', true);
}, 2000);
})
Another thing you might want to do is inline code instead of creating a new component for small items. Rendering those might take some serious time.
Record a timeline and ensure that the performance is actually coming from render time, sometimes is actually some javascript execution or loading that is messing up.
Lastly, ember 2.10 includes glimmer 2, this new render engine can optimize render time up to 50%, you might want to consider using it.

VirtualScroll rowRenderer method is called many times while scrolling

I have a react-virtualized's infinite scroll list, (Most of the setup is copied from this example). I'm providing it with a rowRenderer function like the specs requires. this works fine if the rowRenderer function is very lightweight (i.e returns a very basic component as row).
but the rendering of my RowComponent includes some Array.map over some properties. this shouldn't cause any problem except that the rowRenderer functions is being called tens or even hundreds of times while scrolling. this causes a performance issue, making the scroll not smooth enough.
So far I tried:
Caching my rowRenderer this works, but I don't like this solution as it may cause problems in the future.
Make my RowComponent's render function pure and implement shouldComponentUpdate using react-addons-shallow-compare. this slightly improved the performance but not enough.
In this example, the rowRenderer function is also being called many times per scroll (no perf issues there as the function is very lightweight), which makes me believe this behavior is by design. so:
Is caching a good solution? any advice in how to sync it with my app's state (I use redux for state management)? is there something I missed in the docs that can reduce calls to rowRenderer (there's no reason for my rows to change while scrolling)?
Author of react-virtualized here.
Your rowRenderer methods should be lightweight because, as you've found, they may be called rapidly when a user is scrolling. The good news is that- since browsers manage scrolling in a separate thread from the UI, this usually doesn't cause any performance problems. If anything, you may notice some empty/white space at the edge of the list in the direction you're scrolling- indicating that your renderers aren't able to keep up with the user's scrolling speed.
One caveat to be aware of though is that if you attach touch or wheel event handlers to a react-virtualized component or one of its DOM ancestors, this will force the browser to scroll in the main/UI thread. That can definitely cause slowness.
I'm currently in the middle of a major update (version 7) which, among other things, will passed named arguments to user-functions like rowRenderer. This will enable me to pass meta information (like whether or not the list is currently scrolling) which could enable you to defer "heavy" logic while a scroll is in progress. Unfortunately this isn't possible in version 6 unless you're willing to use a timeout as doron-zavelevsky mentions.
Edit: You may be happy to learn that with this commit cell caching has made its way into the upcoming version 7 release.
From my experience with this library (I'm not using the latest version though) - this is by design.
It makes sense - in order to avoid rendering all the list at once - and to allow you infinite scroll - it asks you every time to render the currently viewed item.
Your goal is to optimize the render function - as you yourself mentioned.
One more thing that can improve your overall experience is to check and see if your item contains some complex code in its componentDidMount life-cycle method - or any other code that runs post-render. If that's the case - you can optimize for fast scrolling by delaying these calculations with a timeout - and only let them run if the component is still mounted when the timeout passes.
Consider the case that you fast scroll over items to get to the bottom - there's no sense in fully populating all the items you scroll past on the way there. So you return the render result as fast as you can - and inside the item you wait ~200ms - and then you check whether the component is still mounted and do the real work.
Since isMounted is obsolete you can simply set a variable to true during componentDidMount and back to false of componentWillUnmount.

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