[LeftNav] move unnecessary state to instance properties.

[maoziliang]

For performance.
I think the properties that not trigger component's render should not belong to component's state.
This can reduce many render cycles.

[hai-cea]

@pomerantsev What do you think about this change?

[pomerantsev]

Actually, I was just used to storing any local state properties on the state object, that's why I did it that way.
@maoziliang: correct me if I'm wrong, but I think one of the points of introducing the state object in React was to have a single place to store a component's state.
Also, you claim there's a performance gain. I don't see any difference in performance in both cases where you've eliminated setState (tested your branch on two devices, and there's no visual difference, but I didn't do any measurements). Is there anything to prove your approach is better? Some articles maybe? Or maybe you ran some performance tests?
I'm curious to know if people do it differently.

[cgestes]

@pomerantsev use React.tools.Perfs to measure performance.

From what I understand about reading the code, the changeset make a lot of sense.

State should only include props that trigger a rerender or a lot of rerender are done for nothing.

(and there are many many useless rerender in mui at the moment, it would be nice to lower them)

Optimally when the render method is called it must produced a different output than the previous call.

[cgestes]

to say it in the reverse way, state should only includes properties used in the render function.

[pomerantsev]

@cgestes: cool, I'll take a look at the Perf stuff. I totally understand what you're saying about the render method.
Anyway, this PR seems to be safe, it doesn't change the logic.
If you guys feel that it will make the code cleaner, I'm all for it.

[oliviertassinari]

I agree with @cgestes the state should only contains properties used in the render method.
👍

[maoziliang]

Agree with @cgestes.
I'll upload chrome dev Timeline screenshots here later. It improves so much performance and has obvious different user experience in not well implemented(pool performance) webview and browser.React render cycle is the bottleneck of performance.

[maoziliang]

• All properties in state. The timeline screenshot.
  
• Some properties move out. touchstart and touchmove timeline screenshots.
  
  

The difference of JS Frame between these two version tell us the truth.

[pomerantsev]

Great. @maoziliang, thanks for the optimization!

[hai-cea]

Thanks @maoziliang @cgestes @pomerantsev @oliviertassinari - you guys are awesome! 👍