“Increase sync” increments the counter right away, in sync mode. If you were to render this component and click the button three times Have a question about this project? The console will log the actual value of count. This is unexpected because value equals to 3. log() is a stale closure. This can occur when you’re mutating the state value in place and then calling setState.React performs a quick comparison so it will bail out of an update when the object instance is the same which will result in a stale value. mutation version, and then see the proper numbers render. Hooks implementation assumes that between the component re-renderings, the latest closure supplied as a callback to the hook (e.g. https://reactjs.org/docs/hooks-reference.html#usestate. Or, in case of stale state, use a functional way to update the state. useEffect(callback)) has captured the freshest variables from the component’s function scope. React performs a quick comparison so it will bail out of an update when Finally, the call of log() logs the message "Current value is 1". When the call initiates, the handler picks up inactive => active => inactive. log() is a stale closure. Let’s study a common case of stale closure when using useEffect() hook. Hey there, it looks like there has been no activity on this issue recently. We can incorporate useRef to solve this problem. I have reviewed the documentation I have searched existing issues I am using the latest React Native version AppState.addEventListener('change', (state) => { console.log(state) }); Drag down the notification screen on iOS 11+ device. You signed in with another tab or window. Works this way in iOS 9, 10, While dragging down the notification window, when near the bottom of the screen the app state toggles from inactive->active->inactive. AppState.addEventListener('change', (state) => { console.log(state) }); Drag down the notification screen on iOS 11+ device. Successfully merging a pull request may close this issue. You can have direct access to me through: I'm a passionate software developer, tech writer and coach. You can unsubscribe at any time. I assume you’re familiar with JavaScript closures. By the way, this is approximately how React hooks handle the freshness of closures. If you avoid mutation and instead clone the Add this code to your app.js file. It’s a component when you’re mutating the state value in place and then calling setState. To second way is to make logValue() use value directly. If you’re using React hooks in a component with an event listener, your event listener callback cannot access the latest state. This can occur Subscribe to my newsletter to get them right into your inbox. To fix the problem, let’s use a functional way to update count state: Now setCount(count => count + 1) updates the count state inside delay(). The stale closure is solved. suddenly see the fresh state leaving you scratching your head. I recommend to install eslint-plugin-react-hooks, which detects the forgotten dependencies. I found the AppState didn't return proper state too. AppState.addEventListener('change' ... not giving proper results. The first approach to solving stale closures is to find the closure that captured the freshest variables. I'm using react 16.8.0-alpha.1 with hooks and typescript and noticed this strange behavior. A factory function createIncrement(i) returns an increment function. Let’s find the closure that has captured the most up to date message variable. See it in action in the CodeSandbox demo. react: ^16.3.0-alpha.1 => 16.3.0-alpha.3 Has the issue been fixed, or does it still require the community's attention? I've experienced this when trying to make a phone call from React Native. This happens with both simulator and real device. 4. While now, when value is already 3, message variable is outdated. Open the demo. If you see it, let me know in a comment! No spam, ever. Closing this issue after a prolonged period of inactivity. npm: 3.10.8 The stale closure problem occurs when a closure captures outdated variables. ). I know how cumbersome are closures, scopes, prototypes, inheritance, async functions, this concepts in JavaScript. Thank you for your contributions. Open the fixed demo and click a few times increase. An efficient way to solve stale closures is to correctly set the dependencies of React hooks. delay() is a stale closure that uses an outdated count variable captured during the initial render. the button is clicked 3 times it should show: 1,2,3. React makes sure the latest state value is supplied as an argument to the update state function. When the notification screen nears the bottom of the device screen the app state quickly flips from inactive to active and then back to inactive. I'm excited to start my coaching program to help you advance your JavaScript knowledge.
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