Does a component know its constrained size before Render()?

[jsholmes]

I have a view into a very large datagrid that I'm writing and it would be nice if it knew how large of a viewport it has to use. What's the best way to get this to happen? The main container is giving it a size() constraint like size(HEIGHT, EQUAL, rows).

But I don't see how the component knows that constraint when it renders.

[ArthurSonzogni]

Hello!

A component is responsible for creating the DOM (aka element) via Render(). Then the element is responsible for drawing on the screen

Hence the following modules of FTXUI.

• component/
• element/
• screen/

So a component can't know the size of a element it created, before drawing it on the screen

However, you can know the size of an element after it has been drawn, using the reflect decorator.
https://arthursonzogni.github.io/FTXUI/namespaceftxui.html#aaff8245861617a3d9e846e99de582a63

Box box;
auto wrapped = element | reflect(&box);

Note that box will be populated after the wrapped has been rendered on the screen.
So you will be able to use the information of the previous frame to build the next one. That's what most components uses to check for mouse.

[ArthurSonzogni]

If your question was about knowing the terminal dimension, you can use Terminal::Size().

[jsholmes]

Awesome thanks. reflect(&box) is great.