Adding support to replace drawer and content within AppLayout.

[vlipovetskii]

Purpose: To have several drawers and contents for AppLayout and switch between them.

For example, navbar contains several tabs.
When a tab is selected, it set its specific drawer and re-populates AppLayout content.
If a tab doesn't have a drawer, removeDrawer() can be called to by its own to clear the previous assigned drawer.

From AppLayout.content the restriction has been removed.
check([email protected] == null) { "The content has already been initialized!" }

[vlipovetskii]

@mvysny

@VaadinDsl
public fun (@VaadinDsl Tab).onSelected(handler: OnTabSelectedHandler) : OnTabSelectedHandler {
    val tabs = parent.get() as Tabs
    tabs.onSelectedHandlers[this] = handler
    if (tabs.selectedTab == this) {
        handler()
    }
    return handler
}

    if (tabs.selectedTab == this) {
        handler()
    }

instead of defaultOnSelectedHandler is a brilliant solution!

[vlipovetskii]

@mvysny
Please explain, why to leverage provideSingleComponentOrNull in TabSheet.tab and componentColumn and componentColumn instead of provideSingleComponent ? What are the cases covered by returing null instead of component?

[mvysny]

Sure! The ComponentRenderer is free to return null instead of a Component instance; Grid renders an empty cell in such case. This is handy e.g. when the column displays an "edit" button, but the user may not have rights to edit certain rows; for such rows the edit button will be null. In other words, your column looks like this:

componentColumn({
  if (canEdit(it)) {
    button("Edit") { ... }
  }
})

The TabSheet.tab is similar: it's possible to create a tab without the component body; at least I think - if that's not the case, then we should definitely use provideSingleComponent instead.

[vlipovetskii]

Now it's clear. Thank you for explanation!