order of commands.with(...) matters

[fopsdev]

Bevy version
0.4
The release number or commit hash of the version you're using.
master 3b2c6ce
Operating system & version
Win 10

What you did
Adding a Component (Empty Struct) to Example sprite,rs

...
commands
        .with(Sprite)
        .spawn(Camera2dBundle::default())
        ...

What you expected to happen
Sprite Component should be added

What actually happened
Panic

Additional information
It works when adding the

.with(...)

at the end like:

    commands
        .spawn(Camera2dBundle::default())
        .spawn(SpriteBundle {
            material: materials.add(texture_handle.into()),

            ..Default::default()
        })
        .with(Sprite);

[cart]

Anything that uses with_x naming is a modifier on the "current" thing (in this case, an entity). This fails because there is no "current" entity. spawn is the command that creates a new entity / entity id, so we can't create the "with" command because the entity id hasn't been created yet. And semantically it has no meaning because the "X" in X.with() is undefined.

We could consider adopting a "typed builder" pattern to only allow the correct call pattern, but this has implications for ergonomics because it requires the typed builder to "terminate" (which would mean adding a .finish() function call).

I'll leave this open for a bit in case anyone wants to respond, but I consider this to be the "expected / desirable" behavior.

[smokku]

We could add a more meaningful panic message though, suggesting what to do.

[cart]

Good call!

[smokku]

Current message is "Cannot add component because the 'current entity' is not set. You should spawn an entity first." which is fine I think.

Maybe "Cannot add component because the 'current entity' is not set. You should use .spawn() method to create an entity first."?

[fopsdev]

Or maybe adding a hint to the "Warning" Section which already exists and is discoverable by rust analyzer:

[fopsdev]

though there are some more places where a method expects an existing entity...
not sure how to add this meta information to the method signatures without a lot of duplication

my vote: carts mentioned "typed builder" - pattern. i wouldn't mind adding a .finish() method to those candidates

On the other hand the panic msg is quite ok. together with the stack trace myself as a beginner was able to locate the issue.
so for sure not high prio :)

[smokku]

This panic is a small price to pay for not having to use .finish() every time you spawn an entity. And since this engine is ECS based, you deal with entity spawning a lot.

Like you mentioned, panic explains what you need to do, and in a moment it will be habit.

[fopsdev]

yes, i think we can close this. current behaviour is ok