Tapping into componentWillUpdate()

Once we have determined that we do need to re-render in our Update phase, the componentWillUpdate() will be called. The method is passed in two arguments: nextProps and nextState. The method componentWillUpdate() is similar to componentWillMount(), and many of the same considerations and tasks are the same. The difference being that componentWillUpdate() is called every time a re-render is required, such as when this.setState() is called. Unlike componentWillMount() we get access to the next props and state.

Just like componentWillMount(), this method is called before render(). Because we have not rendered yet, our Component's access to the Native UI (DOM, etc.) will reflect the old rendered UI. Unlike componentWillMount(), we can access refs but in general this is not recommended because the refs will soon be out of date. There are use cases for accessing the Native UI here, such as starting animations.

The componentWillUpdate() is a chance for us to handle configuration changes and prepare for the next render. If we want to access the old props or state, we can call this.props or this.state. We can then compare them to the new values and make changes/calculations as required.

Unlike componentWillMount(), we should not call this.setState() here. The reason we do not call this.setState() is that the method triggers another componentWillUpdate(). If we trigger a state change in componentWillUpdate() we will end up in an infinite loop ¹.

Some of the more common uses for componentWillUpdate() is to set a variable based on state changes (not using this.setState()), dispatching events or starting animations ².

// dispatching an action based on state change
componentWillUpdate(nextProps, nextState) {
  if (nextState.open == true && this.state.open == false) {
    this.props.onWillOpen();
  }
}

Up Next: Re-rendering and Children Updates

¹ In the previous version of this section we mistakenly said that you can safely call setState() in this method. Our assumption at the time was that a dirty flag was tracking the current state of the render pass, but this is not the case. It is technically possible to call setState() behind a conditional (such as when a prop/state changes) but it is not recommended and should be considered a no go. Special thanks to Robin Venneman for catching this error and calling it to our attention!

² An example of triggering CSS transitions in componentWillUpdate() and other discussions around this method's usages is over at this StackOverflow response.