How to access the IOS's memory usage in runtime!?

As above, i wanna to check whether there is enough memory for my new allocation!

You need to use native code through a plugin to do so.

But thats not good practice on iOS as you can not assume anything basing on the data you get as iOS has the control, not your app.
On iOS you normally listen to the memory warnings (The AppController.mm has the corresponding function defined - it leads to the log outputs - but its not hooked to call anything in unity until you use UnitySendMessage to call something specific) and reduce memory when you hit the warning