setstate

问题 If you do an asynchronous action that updates the state in componentWillMount (like the docs say), but the component is unmounted (the user navigates away) before that async call is complete, you end up with the async callback trying to set the state on a now unmounted component, and an "Invariant Violation: replaceState(...): Can only update a mounted or mounting component." error. What's the best way around this? Thanks. 回答1: update 2016 Don't start using isMounted because it will be

问题 I'm working with a deeply nested state object in React. My code base dictates that we try to stick with function components and so every time I want to update a key/value pair inside that nested object, I have to use a hook to set the state. I can't seem to get at the deeper nested items, though. I have a drop down menu w/ an onChange handler. . .inside the onChange handler is an inline function to directly setValue of whatever key/val pair is changing. The syntax I'm using after the spread

问题 Thanks in advance. I have a state array as below. I need to add an item to state array, I came across that we need not do state mutation. How do i set state with prevState. const [messages, setMessages] = React.useState( [ { _id: 12, createdAt: new Date(), text: 'All good', user: { _id: 1, name: 'Sian Pol', } }, { _id: 21, createdAt: "2019-11-10 22:21", text: 'Hello user', user: { _id: 2, name: 'User New' } }] ) How to to i call the set State to append this state array. Something like this?

问题 Thanks in advance. I have a state array as below. I need to add an item to state array, I came across that we need not do state mutation. How do i set state with prevState. const [messages, setMessages] = React.useState( [ { _id: 12, createdAt: new Date(), text: 'All good', user: { _id: 1, name: 'Sian Pol', } }, { _id: 21, createdAt: "2019-11-10 22:21", text: 'Hello user', user: { _id: 2, name: 'User New' } }] ) How to to i call the set State to append this state array. Something like this?

问题 I use setState(() {}) for assigning a value to a variable. But it's printing again and again. Why does it react like this? And how can I fix it? Here is my code: class Sample extends StatefulWidget { @override _SampleState createState() => _SampleState(); } class _SampleState extends State<Sample> { String _message; String _appLink; Firestore db = Firestore.instance; @override Widget build(BuildContext context) { db.collection('share').document('0').get().then((value) { var message = value

问题 My question is setstate is not working with the statefulbuilder and list of widgets I want to update color of the container. Although color of button is updating. I am not sure what is the problem Color currentColor = Colors.grey; void changecolor(Color color) { setState(() { currentColor = color; }); } List<Widget> list = []; @override Widget build(BuildContext context) { return Container( child: Stack( children: [ Container( height: MediaQuery.of(context).size.height, width: MediaQuery.of

问题 My question is setstate is not working with the statefulbuilder and list of widgets I want to update color of the container. Although color of button is updating. I am not sure what is the problem Color currentColor = Colors.grey; void changecolor(Color color) { setState(() { currentColor = color; }); } List<Widget> list = []; @override Widget build(BuildContext context) { return Container( child: Stack( children: [ Container( height: MediaQuery.of(context).size.height, width: MediaQuery.of

问题 I need to capture cmd button up and down events, in order to choose, whether to use concatenation or not in setState. How do i get these events, for example, on table element? 回答1: You have to capture the keypress then in body/window level. Table element doesn't have input focus so you can't capture the keys from table (without input element). var cmdDown = false; document.body.addEventListener('keydown', function(event) { var key = event.keyCode || event.charCode || 0; if ([91,93,224,17]

问题 I need to capture cmd button up and down events, in order to choose, whether to use concatenation or not in setState. How do i get these events, for example, on table element? 回答1: You have to capture the keypress then in body/window level. Table element doesn't have input focus so you can't capture the keys from table (without input element). var cmdDown = false; document.body.addEventListener('keydown', function(event) { var key = event.keyCode || event.charCode || 0; if ([91,93,224,17]

问题 I need to capture cmd button up and down events, in order to choose, whether to use concatenation or not in setState. How do i get these events, for example, on table element? 回答1: You have to capture the keypress then in body/window level. Table element doesn't have input focus so you can't capture the keys from table (without input element). var cmdDown = false; document.body.addEventListener('keydown', function(event) { var key = event.keyCode || event.charCode || 0; if ([91,93,224,17]