2.6. Add text

This chapter shows how to add a text to a game.

This chapter introduces:

• Working with Rust Strings
• The Bevy Text2d
• Using app.update() in the wrong place causing problems

2.6.1. First test: an App has no text

Similar to earlier chapters, we'll start our game development for counting the number of texts to be zero:

fn test_empty_app_has_text() {
    let mut app = App::new();
    assert_eq!(count_n_texts(&mut app), 0);
}

Like in earlier chapters, we already make App mutable, as querying requires the App to be so.

2.6.2. First fix

Part of the Bevy Text2d documentation

Taking a look at the Bevy Text2d documentation, one can see that there is a field called text of data type Text. We'll use that -and only that- for our query:

fn count_n_texts(app: &mut App) -> usize {
    let mut query = app.world_mut().query::<&Text2d>();
    return query.iter(app.world()).len();
}

This query is probably (and indeed is!) good enough, as it will not conflict with the Bevy entities that are added by the default plugins. This was different from when we were querying for a player's Transform, as a default camera also has a Transform Component, hence we had to use a Player marker component to get access to the right Transform.

2.6.3. Second test: can create an App with text

As our game will show a text, we'll have to be able to pass our desired text to it. We do so using a create_app function:

fn test_can_create_app_from_str() {
    create_app(String::from("irrelevant"));
}

The String used by Bevy is the standard Rust String.

We could just as well have picked to use a string slice instead, as it would have made our test look cleaner:

fn test_can_create_app_from_str_slice() {
    create_app("irrelevant");
}

When trying out if the string slice resulted in cleaner code over using a String, the answer turned out to be no: due to the string slice lifespans it was needed to create Strings for them.

One could argue that the cleanest looking test should be chosen and, hence, the string slice. However, having a String construction in a test is reasonable enough and because it resulted in a cleaner implementation, using a String was chosen.

2.6.4. Second fix

All that this test forces us to do, is to write a create_app function that accepts a String. Here is an example stub that will pass the test:

pub fn create_app(_text: String) -> () {}

2.6.5. Third test: an App has text

Now we force our game to actually store the text in an entity:

fn test_app_has_text() {
    let mut app = create_app(String::from("irrelevant"));
    app.update();
    assert_eq!(count_n_texts(&mut app), 1);
}

Where the previous chapter needed to have app.update() to prevent a panic, here we have a different reason: would we add app.update() in the create_app function, our text will not be shown in a regular run:

This game when app.update() is added to create_app: no text is visible

The reason is, again, that calling app.update() finalizes the App's currents state. When then, in the main function, the default plugins are added, something happens to the already initialized text making it not appear.

2.6.6. Third fix

To make this test pass, we need to:

• Write create_app to pass on the String to a add_text function
• Write an add_text function to add a component with that String

The create_app is quite similar to versions is earlier chapters:

pub fn create_app(text: String) -> App {
    let mut app = App::new();
    let add_text_fn = move |commands: Commands| add_text(commands, &text);
    app.add_systems(Startup, add_text_fn);
    app
}

Also in this incarnation of create_app, we use a closure to be able to fit our add_text function in the app.add_systems member function.

The add_text function may look like this:

fn add_text(mut commands: Commands, str: &String) {
    commands.spawn(Text2d::new(str));
}

Most of this function is similar to adding a Sprite. It may come as a surprise that the text field is more than just a String: a Text contains multiple TextSections, of which each TextSection can have its own text and style.

2.6.7. Fourth test: an App has the correct text

In the final test we assure that our desired text is actually stored by our application:

fn test_app_uses_text() {
    let text = String::from("some random text");
    let mut app = create_app(text.clone());
    app.update();
    assert_eq!(get_text(&mut app), text);
}

In this test, our text string needs to be cloned and the Rust compiler will remind us of that.

2.6.8. Fourth fix

If your implementations matched the earlier fixes, this test would already pass. In case you've cut a corner, this is the test that will force you to write a non-stub implementation.

2.6.9. main.rs

The resulting main function is not much different than we are used to:

fn main() {
    let text = String::from("Hello from main");
    let mut app = create_app(text);
    let add_camera_fn = |mut commands: Commands| {
        commands.spawn(Camera2d);
    };
    app.add_systems(Startup, add_camera_fn);
    app.add_plugins(DefaultPlugins);
    app.run();
}

2.6.10. Conclusion

We can now create an App with a text. We have tested everything that the App does!

Full code can be found at https://github.com/richelbilderbeek/bevy_tdd_book_add_text.