add Flags<T> as a query to get flags of component

[mockersf]

This let user add Flags<MyComponent> in a query to get the component flags for an entity

I have an example, but didn't commit it as I'm not sure it's a feature that need to be put forward in an example

view example

use bevy::prelude::*;
use rand::Rng;

fn main() {
    App::build()
        .add_plugins(DefaultPlugins)
        .add_startup_system(setup.system())
        .add_system(change_component.system())
        .add_system(change_detection.system())
        .run();
}

#[derive(Debug)]
struct MyComponent(f64);

fn setup(commands: &mut Commands) {
    commands.spawn((MyComponent(0.),));
}

fn change_component(time: Res<Time>, mut query: Query<&mut MyComponent>) {
    for mut component in query.iter_mut() {
        if rand::thread_rng().gen_bool(0.1) {
            info!("changing component");
            component.0 = time.seconds_since_startup();
        }
    }
}
fn change_detection(query: Query<(&MyComponent, Flags<MyComponent>)>) {
    for (component, flags) in query.iter() {
        info!(
            "{:?} -> added: {:?} - mutated: {:?} - changed: {:?}",
            component,
            flags.added(),
            flags.mutated(),
            flags.changed()
        );
    }
}

and it logs:

Dec 31 02:28:28.640  INFO flags: MyComponent(0.0) -> added: true - mutated: false - changed: true
Dec 31 02:28:28.773  INFO flags: MyComponent(0.0) -> added: false - mutated: false - changed: false
Dec 31 02:28:28.876  INFO flags: MyComponent(0.0) -> added: false - mutated: false - changed: false
Dec 31 02:28:28.889  INFO flags: changing component
Dec 31 02:28:28.889  INFO flags: MyComponent(0.471366423) -> added: false - mutated: true - changed: true
Dec 31 02:28:28.910  INFO flags: MyComponent(0.471366423) -> added: false - mutated: false - changed: false
Dec 31 02:28:28.979  INFO flags: MyComponent(0.471366423) -> added: false - mutated: false - changed: false
Dec 31 02:28:28.993  INFO flags: changing component
Dec 31 02:28:28.993  INFO flags: MyComponent(0.575871597) -> added: false - mutated: true - changed: true
Dec 31 02:28:29.004  INFO flags: MyComponent(0.575871597) -> added: false - mutated: false - changed: false

[TheRawMeatball]

Personally, I think its best to keep the examples. There isn't much in terms of documentation, so without an example almost no one would even know the feature exists.

[mockersf]

as there wasn't an example on Changed<T>, I added it also

[cart]

This looks good to me!