问题
Let's say that I want to connect one view to another with app:layout_constraintTop_toTopOf (it can be any other constraint).
If I will do it manually from the XML display it will be something like this:
app:layout_constraintTop_toTopOf="@id/something"
But - If I will do the same thing with the design option and not the XML option I will get extra + (auto generated) char for some reason and it will look like this :
//notice the extra + in @+id
app:layout_constraintTop_toTopOf="@+id/something"`
Now the funny thing is that the app will run with or without the + char.
So - looks like I don't really need that extra + char while referring to other views, my question is why this is happening and how can I prevent android studio layout editor to add this +.
回答1:
The +id is only needed the first time an id is mentioned in the xml.
It can still be useful to have the + every time in case you want to reorder the xml in the future.
I would not worry too much about this. Doing +id likely has an extremely low build-time performance hit. (See if you can measure the difference. I doubt you can.)
来源：https://stackoverflow.com/questions/55536227/constraintlayout-weird-attributes-behavior-when-constraining-2-views