Use Observable instead of Subject

[Brooooooklyn]

I'm submitting a...

[ ] Bug report  
[x] Feature request
[ ] Documentation issue or request
[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Support request => Please do not submit support request here

Current behavior

in some components, ng-zorro use subject to emit values when event callback triggered.
But this is not a good practice when writing Angular component.

Expected behavior

Use Observable.fromEvent / Observable.create to wrap eventListener， and dispose the eventListener in teardown logic when unsubscribe in ngOnDestroy.

Minimal reproduction of the problem with instructions

see #148

What is the motivation / use case for changing the behavior?

performance imporve

[Brooooooklyn]

@vthinkxie @trotyl what do you think about this issue. if you are approving my suggestion, I will help you to change all Subject to Observable just like Dropdown.

[vthinkxie]

@trotyl what do you think about it

[trotyl]

Dropdown is a special case which has dynamic event trigger, I don't think it would apply to all components, is there any specific reason for that?

[lock]

This thread has been automatically locked because it has not had recent activity. Please open a new issue for related bugs and link to relevant comments in this thread.