`getDerivedStateFromProps` not called before each `render`

[jiangyijie27]

Hi,
I wrote a component Switch: https://codesandbox.io/s/m5oy50k649

When I use React and ReactDom@16.4.1, getDerivedStateFromProps will be called everytime before the render method.

but when I use React and ReactDom@15.0.0, and with this polyfill, getDerivedStateFromProps is called just before the first render, as my example shows. This is not what I expected.

React Doc:

getDerivedStateFromProps is invoked right before calling the render method, both on the initial mount and on subsequent updates

Here's what I want:
Switch has a state checked, and a prop checked as well.
if <Switch />, the component can work because it has it's own state checked;
if <Switch checked />, the component will never change state, because before every render, getDerivedStateFromProps will reset the components' checked.

But when React and ReactDom@15.0.0, It doesn't work as I expected.

Thanks for helping.

[bvaughn]

Hey @jiangyijie27. Sorry for the difficult here. 😦 Unfortunately it's pretty difficult to create a polyfill that fully adheres to the 16.4+ semantics of getDerivedState (see the discussion in PR #27).

For what it's worth, it looks like the Switch component you linked to is using derived state in one of the ways that we've identified as an anti-pattern. Maybe you could avoid this polyfill problem (and the ones mentioned in the blog post I linked to) by changing the component's approach slightly?