React after render code

Question

I have an app where I need to set the height of an element  lets say  app-content   dynamically  It takes the height of the  chrome  of the app and subtracts it and then sets the height of the  app-content  to fit 100  within those constraints  This is super simple with vanilla JS  jQuery  or Backbone views  but I m struggling to figure out what the right process would be for doing this in React   Below is an example component  I want to be able to set app-content s height to be 100  of the window minus the size of the ActionBar and BalanceBar  but how do I know when everything is rendered and where would I put the calculation stuff in this React Class        jsx React DOM    var List   require     list    var ActionBar   require     action-bar    var BalanceBar   require     balance-bar    var Sidebar   require     sidebar    var AppBase   React createClass     render  function          return          lt div className  wrapper  gt           lt Sidebar   gt           lt div className  inner-wrapper  gt             lt ActionBar title  Title Here    gt             lt BalanceBar balance  balance    gt             lt div className  app-content  gt               lt List items  items    gt             lt  div gt           lt  div gt         lt  div gt                  module exports   AppBase

User · Answer

There is actually a lot simpler and cleaner version than using request animationframe or timeouts. Iam suprised no one brought it up: the vanilla-js onload handler. If you can, use component did mount, if not, simply bind a function on the onload hanlder of the jsx component. If you want the function to run every render, also execute it before returning you results in the render function. the code would look like this:

_x000D_

runAfterRender = () => _x000D_
{_x000D_
  const myElem = document.getElementById("myElem")_x000D_
  if(myElem)_x000D_
  {_x000D_
    //do important stuff_x000D_
  }_x000D_
}_x000D_
_x000D_
render()_x000D_
{_x000D_
  this.runAfterRender()_x000D_
  return (_x000D_
    <div_x000D_
      onLoad = {this.runAfterRender}_x000D_
    >_x000D_
      //more stuff_x000D_
    </div>_x000D_
  )_x000D_
}

_x000D_

}

User · Answer

I m actually having a trouble with similar behaviour  I render a video element in a Component with it s id attribute so when RenderDOM render   ends it loads a plugin that needs the id to find the placeholder and it fails to find it   The setTimeout with 0ms inside the componentDidMount   fixed it      componentDidMount         if  this props onDidMount instanceof Function            setTimeout      gt                this props onDidMount               0

User · Answer

You can change the state and then do your calculations in the setState callback  According to the React documentation  this is  guaranteed to fire after the update has been applied    This should be done in componentDidMount or somewhere else in the code  like on a resize event handler  rather than in the constructor   This is a good alternative to window requestAnimationFrame and it does not have the issues some users have mentioned here  needing to combine it with setTimeout or call it multiple times   For example   class AppBase extends React Component       state             showInProcess  false          size  null             componentDidMount             this setState   showInProcess  true         gt                this setState                   showInProcess  false                  size  this calculateSize                                          render             const appStyle   this state showInProcess     visibility   hidden      null           return                lt div className  wrapper  gt                                       lt div className  app-content  style  appStyle  gt                       lt List items  items    gt                   lt  div gt                                   lt  div gt

User · Answer

I am not going to pretend I know why this particular function works  however window getComputedStyle works 100  of the time for me whenever I need to access DOM elements with a Ref in a useEffect     I can only presume it will work with componentDidMount as well  I put it at the top of the code in a useEffect and it appears as if it forces the effect to wait for the elements to be painted before it continues with the next line of code  but without any noticeable delay such as using a setTimeout or an async sleep function  Without this  the Ref element returns as undefined when I try to access it  const ref   useRef null    useEffect     gt       window getComputedStyle ref current          Next lines of code to get element and do something after getComputedStyle         return  lt div ref  ref  gt  lt  div gt

User · Answer

I had weird situation when i need to print react component which receives big amount of data and paint in on canvas  I ve tried all mentioned approaches  non of them worked reliably for me  with requestAnimationFrame inside setTimeout i get empty canvas in 20  of the time  so i did the following   nRequest   n   gt  range 0 n  reduce   acc val    gt       gt  requestAnimationFrame acc        gt  requestAnimationFrame this save       Basically i made a chain of requestAnimationFrame s  not sure is this good idea or not but this works in 100  of the cases for me so far  i m using 30 as a value for n variable

User · Answer

For me  no combination of window requestAnimationFrame or setTimeout produced consistent results  Sometimes it worked  but not always   or sometimes it would be too late   I fixed it by looping window requestAnimationFrame as many times as necessary   Typically 0 or 2-3 times   The key is diff  gt  0  here we can ensure exactly when the page updates      Ensure new image was loaded before scrolling if  oldH  gt  0  amp  amp  images length  gt  prevState images length         function scroll             const newH   ref scrollHeight          const diff   newH - oldH           if  diff  gt  0                const newPos   top   diff              window scrollTo 0  newPos             else               window requestAnimationFrame scroll

User · Answer

componentDidMount    This method is called once after your component is rendered  So your code would look like so   var AppBase   React createClass     componentDidMount  function         var  this     ReactDOM findDOMNode this           set el height and width etc          render  function          return          lt div className  wrapper  gt           lt Sidebar   gt             lt div className  inner-wrapper  gt               lt ActionBar title  Title Here    gt               lt BalanceBar balance  balance    gt               lt div className  app-content  gt                 lt List items  items    gt             lt  div gt           lt  div gt         lt  div gt

User · Answer

In my experience window requestAnimationFrame wasn t enough to ensure that the DOM had been fully rendered   reflow-complete from componentDidMount  I have code running that accesses the DOM immediately after a componentDidMount call and using solely window requestAnimationFrame would result in the element being present in the DOM  however  updates to the element s dimensions aren t reflected yet since a reflow hasn t yet occurred   The only truly reliable way for this to work was to wrap my method in a setTimeout and a window requestAnimationFrame to ensure React s current call stack gets cleared before registering for the next frame s render   function onNextFrame callback        setTimeout function              requestAnimationFrame callback             If I had to speculate on why this is occurring   necessary I could see React batching DOM updates and not actually applying the changes to the DOM until after the current stack is complete   Ultimately  if you re using DOM measurements in the code you re firing after the React callbacks you ll probably want to use this method

User · Answer

From the ReactDOM render   documentation      If the optional callback is provided  it will be executed after the   component is rendered or updated

User · Answer

After render  you can specify the height like below and can specify the height to corresponding react components   render  function          var style1    height   100px        var style2     height   100px          window  height actually will get the height of the window     var hght     window  height       var style3    hght -  style1   style2          return          lt div className  wrapper  gt           lt Sidebar   gt           lt div className  inner-wrapper  gt             lt ActionBar style  style1  title  Title Here    gt             lt BalanceBar style  style2  balance  balance    gt             lt div className  app-content  style  style3  gt               lt List items  items    gt             lt  div gt           lt  div gt         lt  div gt                or you can specify the height of the each react component using sass  Specify first 2 react component main div s with fixed width and then the third component main div s height with auto  So based on the third div s content the height will be assigned

User · Answer

I ran into the same problem    In most scenarios using the hack-ish setTimeout      gt       0  in componentDidMount   worked   But not in a special case  and I didn t want to use the ReachDOM findDOMNode since the documentation says      Note  findDOMNode is an escape hatch used to access the underlying DOM   node  In most cases  use of this escape hatch is discouraged because   it pierces the component abstraction     Source  findDOMNode   So in that particular component I had to use the componentDidUpdate   event  so my code ended up being like this   componentDidMount            feel this a little hacky  check this  http   stackoverflow com questions 26556436 react-after-render-code     setTimeout      gt           window addEventListener  resize   this updateDimensions bind this           this updateDimensions           0       And then   componentDidUpdate         this updateDimensions        Finally  in my case  I had to remove the listener created in componentDidMount   componentWillUnmount         window removeEventListener  resize   this updateDimensions bind this

User · Answer

React has few lifecycle methods which help in these situations  the lists including but not limited to getInitialState  getDefaultProps  componentWillMount  componentDidMount etc   In your case and the cases which needs to interact with the DOM elements  you need to wait till the dom is ready  so use componentDidMount as below        jsx React DOM    var List   require     list    var ActionBar   require     action-bar    var BalanceBar   require     balance-bar    var Sidebar   require     sidebar    var AppBase   React createClass     componentDidMount  function         ReactDOM findDOMNode this  height      whatever HEIGHT            render  function          return          lt div className  wrapper  gt           lt Sidebar   gt           lt div className  inner-wrapper  gt             lt ActionBar title  Title Here    gt             lt BalanceBar balance  balance    gt             lt div className  app-content  gt               lt List items  items    gt             lt  div gt           lt  div gt         lt  div gt                  module exports   AppBase    Also for more information about lifecycle in react you can have look the below link  https   facebook github io react docs state-and-lifecycle html

User · Answer

I am currently using hooks  Something like this  import React    useEffect   from  react    const AppBase           gt         useEffect      gt               set el height and width etc                  return            lt div className  quot wrapper quot  gt               lt Sidebar   gt               lt div className  quot inner-wrapper quot  gt                   lt ActionBar title  quot Title Here quot    gt                   lt BalanceBar balance  balance    gt                   lt div className  quot app-content quot  gt                       lt List items  items    gt                   lt  div gt               lt  div gt           lt  div gt            export default AppBase

User · Answer

I feel that this solution is dirty  but here we go   componentDidMount         this componentDidUpdate      componentDidUpdate            A whole lotta functions here  fired after every render      Now I am just going to sit here and wait for the down votes

User · Answer

One drawback of using componentDidUpdate  or componentDidMount is that they are actually executed before the dom elements are done being drawn  but after they ve been passed from React to the browser s DOM   Say for example if you needed set node scrollHeight to the rendered node scrollTop  then React s DOM elements may not be enough  You need to wait until the elements are done being painted to get their height   Solution   Use requestAnimationFrame to ensure that your code is run after the painting of  your newly rendered object   scrollElement  function          Store a  this  ref  and   var  this   this       wait for a paint before running scrollHeight dependent code    window requestAnimationFrame function         var node    this getDOMNode        if  node     undefined          node scrollTop   node scrollHeight                 componentDidMount  function       this scrollElement          and or componentDidUpdate  function       this scrollElement          and or render  function       this scrollElement     return

User · Answer

Just to update a bit this question with the new Hook methods  you can simply use the useEffect hook   import React    useEffect   from  react   export default function App props          useEffect      gt                your post layout code  or  effect   here                               array of variables that can trigger an update if they change  Pass an         an empty array if you just want to run it once after component mounted                Also if you want to run before the layout paint use the useLayoutEffect hook   import React    useLayoutEffect   from  react   export default function App props          useLayoutEffect      gt                your pre layout code  or  effect   here

User · Answer

A little bit of update with ES6 classes instead of React createClass  import React    Component   from  react    class SomeComponent extends Component     constructor props        super props          this code might be called when there is no element avaliable in  document  yet  eg  initial render         componentDidMount            this code will be always called when component is mounted in browser DOM   after render          render         return          lt div className  component  gt          Some Content        lt  div gt                 Also - check React component lifecycle methods The Component Lifecycle    Every component have a lot of methods similar to componentDidMount eg    componentWillUnmount   - component is about to be removed from browser DOM

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