Don't call callbacks from this.setState() to avoid double rendering

[eldargab]

Hello, I noticed that react-calendar notifies parent component about various events via .setState() callback.

E.g.

https://github.com/wojtekmaj/react-calendar/blob/be7c443fbe39ce90036380dab727c1ca7072a073/src/Calendar.jsx#L310

The problem is that.setState() callback is called after react finishes real DOM updates. That means, changes from parent component will cause another update cycle. Also parent component might remove calendar in response to event entirely making previous update completely useless.

You can verify such behaviour via DevTools profiler

In the screenshot above all the work before Lifecycle hook scheduled a cascading update is completely useless and causes substantial delay.

Another reason not to use .setState() for parent callbacks is to avoid crashing entire app in case parent event handler erroneously raises exception.

[wojtekmaj]

Hmmm, if we moved this to the beginning of the function then we would potentially start two asynchronous updates - one via onChange & props, one internal via state. That would be even worse, I think?

[eldargab]

No, there would be only one update incorporating both props and state changes.

This is what currently happens with react-calendar/sample/parcel app.

On other hand, immediate invocation of onChange callback yields the following picture

PR #302 illustrates proposed changes.