[Input] Should determine if the component is controlled at the mount time

• I have searched the issues of this repository and believe that this is not a duplicate.

Expected Behavior

The controlled/uncontrolled behavior is determined at the mount time.

Current Behavior

The controlled/uncontrolled behavior is determined at each render.

Steps to Reproduce (for bugs)

Look at the source code.

Context

@kgregory Raised this issue in #9523. It’s important to notice that the Input component is the only component behaving this way. It’s no the case for: input, Checkbox, Switch, Radio, ExpansionPanel and Tooltip.

Your Environment

Tech	Version
Material-UI	v1.0.0-beta.24
React	v16.2.0

[Hivemind9000]

@oliviertassinari Fair enough, it’s your call - I agree about not making it more complicated (it was more a thought experiment about explicit intention vs implicit behavior).

I dug around to try and understand why React have frowned upon controlled state switching. It appears to only relate to components wrapping the underlying DOM elements of <input> <textarea> and <select>, as they are either directly controlling these elements, or handing control over to the underlying engine. So I’m not convinced it’s a convention - more so a pragmatic wariness (possibly due to performance and/or potential stability issues?) for those specific elements.

One thing to be aware of is that users are already get the warning from react, but it still seems to work (switch control). So I don’t think they’ll notice anything except their code suddenly not working. Perhaps a new warning in Input should be added? Might be nice to have similar warnings if you’re not allowing control switching in other components (or just a note in the docs relating to those specific components/props could be equally effective).

[oliviertassinari]

Actually, let’s use != null to save some bytes. We have the tests to cover our back.