Add remove attribute to mount and mount_all

[willmcgugan]

It's common to remove some widgets and add some other widgets. To make this easier, I think we should add a remove attribute to the mount methods which accepts a selector and removes those widgets prior to mounting.

The remove + mount should be atomic, to avoid flicker (might need to be wrapped in batch_update).

API would look something like this:

# Replace all MyWidgets with a single MyWidget
await self.mount(MyWidget("Foo"), remove="MyWidget")

# Remove all children and replace with a label
await self.mount(Label("Goodbye"), remove="*")

[rodrigogiraoserrao]

I think of the method mount as a method that “builds”, “adds”, “constructs”.
Adding a kwd argument that goes in the opposite direction feels a bit clunky.

Wouldn't it make more sense to grow a method replace(selector, new_widget)?

[willmcgugan]

The thing is that it is not replacing. It's not like we can guarantee the widgets will be mounted in the same place as the "remove" selector.

[rodrigogiraoserrao]

In that case, call it remove_then_mount.
Or, better yet, let Widget.remove_children grow the selector parameter that is "*" by default but that you can change.
I think

self.remove_children(selector="...")
self.mount(widget)

looks much better than

self.mount(widget, remove="...")

The thing with remove inside mount is that people will think that the parameter remove is somehow related to the mounting when it's completely orthogonal.

[willmcgugan]

Two methods aren't atomic. Which means you would have to wrap both calls in an asyncio Lock, and await them both. You would also need to use batch_update to avoid flicker. So now your code will look like this:

async with self.lock:
    with self.app.batch_update():
        await self.remove_children("*")
        await self.mount(widget)

That's too much to ask of devs.

The remove attribute on mount is still my favorite. People misunderstanding the extra parameter can be alleviated by docs.

[rodrigogiraoserrao]

If you prefer the mount(..., remove=selector) over remove_then_mount, sure.

[willmcgugan]

After a discussion in the office. We could combine the locking and batch update in to a single context manager.

async with self.batch():
    await self.remove_children("*")
    await self.mount(widget)

That feels a little less clumsy, but it has its own flaw: Namely, that it requires async. You wouldn't be able to have optional awaitables.

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