[FEATURE] Documentation of View::required_size() should mention when it is called. about cursive

[gyscos]

Hi, and thanks for the report!

I agree, the documentation could absolutely be improved. In the meantime, here's a small description.

In theory, both required_size and layout can be called at any times. Note that after calling layout(), we should only call draw() with a printer of the same size as the last call to layout.

In practice, this is up to the various views in the tree to respect that. The current views builtin cursive itself mostly follow the following rules:

• layout() is called on the parent view once per layout cycle. Most view implement this by first asking the child view about its requirements (required_size), then calling layout with this requirement (or a variation thereof).
• Most views implement required_size by calling required_size ~once on their children.
• From this, it follows that a view could receive many calls to required_size in a layout cycle, and one call to layout at the end of this cycle (usually followed by a call to draw()).

There are variations to this rule: some views disregard their children's requirements and will not even call required_size on them, but will call layout(). Some views will call required_size multiple times to find out a correct size to give the children (LinearLayout). Some views may try to cache the requirements and reduce the number of calls to required_size - though that's still a work in progress.

In the end, I think view implementations should be robust to random required_size calls, each with potentially different requirements. View::layout() is where the real commitment is.