Note: this book contains runnable examples. You can click the play button to see their output!

What is this book about?

Several rust libraries use a pattern something like this (in this example, bevy):

fn main() {

fn foo(query: Query<Foo, With<Bar>>) {
    // some code

fn bar(query: Query<Bar, Without<Foo>>) {
    // some other code

Most users can intuitively grasp that this causes the app to automatically call the systems foo and bar once per frame, but very few are able to easily figure out how this is possible. This book aims to explain how this works, starting from scratch.

What is Dependency Injection?

Dependency injection is a needlessly complicated way to phrase "asking for things instead of providing them".

An easy example of dependency injection would be Iterator::map(); you provide a function/closure which asks for an item to map and maps it, and the iterator itself "injects" that "dependency".

In this case we're mimicking what Bevy Engine does. An App is created, provided with System's, and those System's are called automatically. System's have various parameters which are automatically known and provided by the App. The parameters are the dependencies, and the app is "injecting" them. Dependency injection is a useful pattern, and most people have probably used it at least somewhere even if they don't know it by name.

Is this book exclusively about rust and bevy?

About rust: Yes, but the techniques within can be applied to other languages if those languages have the features to support it.

About bevy: Sort of. This book is heavily inspired by it, but these techniques can certainly be applied to other rust projects, and have already shown up before in libraries like axum. And overall the technique shown will be simpler than what bevy actually does.

How much rust do I need to know to understand this?

I'll try to aim to make this as easily understandable as possible, but understanding of traits, dyn Traits, tuples, a little bit of lifetimes, and other basic rust knowledge will likely be required.