When to consider the networking for multiplayergame?

It’s up to you.

My preference is to build for multi-player support from the start. That way you won’t have to rewrite and redesign your code to support it later down the road.

1: make the standalone features or the basic logic firstly and consider to add networking feature in the end?it can

It is can handle the schedule easily, but may have a huge work to add networking in the end

2: considering the networking and Making the BP or C++ at the same time

It is will work fine in the end, but also will spend a lot of time in prototype design

Can anyone tell me how to organize time or schedule ,thx

As a general rule if you are producing a title for network play - you should always begin with the networking and add to that, rather than the other way around.

If you’re building them together or adding networking to an existing game - you can “see” a broken animation, missing collision allowing things to pass where they shouldn’t, scores being calculated incorrectly, etc.
But without solid error handling/reporting for the networking - all of that can be taking place while the network has died in the background and you don’t even know it.

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Simply put, it is much easier to debug the networking on its own than it is to do after you have started developing the ‘visual’ game elements.