Function current_partial 

pub fn current_partial(
    scope: &Scope<'_>,
    entity: Entity,
    name: &str,
) -> Result<Partial<'static, false>, ComponentError>

Available on crate feature reflect only.

A partial value over the entity’s current component, ready to be filled and handed back to set.

The fields a fill leaves alone keep the value the entity holds, which is what setting one property at a time means.