Add Hello World explanation

CODE_OF_CONDUCT.md

@@ -60,7 +60,7 @@ representative at an online or offline event.
 
 Instances of abusive, harassing, or otherwise unacceptable behavior may be
 reported to the community leaders responsible for enforcement at
-rjcbilderbeek@gmail.com.
+<rjcbilderbeek@gmail.com>.
 All complaints will be reviewed and investigated promptly and fairly.
 
 All community leaders are obligated to respect the privacy and security of the
@@ -116,13 +116,13 @@ the community.
 
 This Code of Conduct is adapted from the [Contributor Covenant][homepage],
 version 2.0, available at
-https://www.contributor-covenant.org/version/2/0/code_of_conduct.html.
+<https://www.contributor-covenant.org/version/2/0/code_of_conduct.html>.
 
 Community Impact Guidelines were inspired by [Mozilla's code of conduct
 enforcement ladder](https://github.com/mozilla/diversity).
 
 [homepage]: https://www.contributor-covenant.org
 
 For answers to common questions about this code of conduct, see the FAQ at
-https://www.contributor-covenant.org/faq. Translations are available at
-https://www.contributor-covenant.org/translations.
+<https://www.contributor-covenant.org/faq>. Translations are available at
+<https://www.contributor-covenant.org/translations>.

CONTRIBUTING.md

@@ -12,7 +12,7 @@ Here we discuss how to contribute:
 
 Spoken text suggestions are ideas, feedback, etc. and are written in English.
 
-For ideas or feedback, 
+For ideas or feedback,
 create an [Issue](https://github.com/richelbilderbeek/bevy_tdd_book/issues).
 These Issues will be discussed publicly in that Issue
 and will help shape the development of the book.
@@ -21,13 +21,13 @@ and will help shape the development of the book.
 
 Book text suggestions are suggested textual changes to the book.
 
-To suggest these, either 
+To suggest these, either
 create an [Issue](https://github.com/richelbilderbeek/bevy_tdd_book/issues)
 or submit a Pull Request.
 
 These Issues and Pull Requests will be discussed publicly
 and accepted if they are judged to improve the book.
-Of course, 'to improve the book' is subjective. 
+Of course, 'to improve the book' is subjective.
 
 Here are things that are likely to be accepted:
 
@@ -44,13 +44,13 @@ Here are things that are unlikely to be accepted:
 
 Code suggestions are suggested changes to the Rust code.
 
-To suggest these, either 
+To suggest these, either
 create an [Issue](https://github.com/richelbilderbeek/bevy_tdd_book/issues)
 or submit a Pull Request.
 
 These Issues and Pull Requests will be discussed publicly
 and accepted if they are judged to improve the code.
-Of course, 'to improve the code' is subjective. 
+Of course, 'to improve the code' is subjective.
 
 Here are things that are likely to be accepted:
 
@@ -60,4 +60,4 @@ Here are things that are unlikely to be accepted:
 
 - Anything that breaks a continuous integration test
 - Anything that reduces code coverage below 100%
-- Using a faster, yet less readable alternative 
+- Using a faster, yet less readable alternative

README.md

@@ -44,9 +44,9 @@ Filename                              |Descriptions
 
 ## Footnotes
 
- * [1] 'book' is an overstatement for now: it is just a collection of notes
- * [2] There is no chapter numbering yet
-   
+* [1] 'book' is an overstatement for now: it is just a collection of notes
+* [2] There is no chapter numbering yet
+
 ## Links
 
- * [Blog post 'Test-Driven Development in Rust Game Development with Bevy' by Edgardo Carreras](https://edgardocarreras.com/blog/tdd-in-rust-game-engine-bevy/)
+* [Blog post 'Test-Driven Development in Rust Game Development with Bevy' by Edgardo Carreras](https://edgardocarreras.com/blog/tdd-in-rust-game-engine-bevy/)

add_player.md

@@ -8,4 +8,4 @@ This chapter shows how to add a player to a game.
 
 ## Links
 
- * [https://github.com/richelbilderbeek/bevy_tdd_book_add_player](https://github.com/richelbilderbeek/bevy_tdd_book_add_player)
+* [https://github.com/richelbilderbeek/bevy_tdd_book_add_player](https://github.com/richelbilderbeek/bevy_tdd_book_add_player)

faq.md

@@ -10,19 +10,19 @@ This book tries to start from scratch and build up gradually.
 ## What is the intended audience of this book?
 
 Intermediate Rust developers: people that have read parts of
-'The Rust programming language' [Klabnik & Nichols, 2018][Klabnik & Nichols, 2023].
+'The Rust programming language' `[Klabnik & Nichols, 2018]``[Klabnik & Nichols, 2023]`.
 
-This book does not teach Rust, nor Bevy. 
+This book does not teach Rust, nor Bevy.
 Instead, it shows Test-Driven Development in Rust with Bevy.
 
 ## What is the goal of this book?
 
-The goal is to demonstrate how to do Test-Driven Development ('TDD') 
+The goal is to demonstrate how to do Test-Driven Development ('TDD')
 in Rust with Bevy.
 
 ## What are the subgoals of this book?
 
-* Always achieve 100% code coverage when ignoring 
+* Always achieve 100% code coverage when ignoring
   the `main` function in `src/main.rs`.
 
 ## What are the non-goals of this book?
@@ -38,7 +38,7 @@ TDD needs tests that do not require user input.
 
 ## References
 
-* [Klabnik & Nichols, 2018] Klabnik, Steve, and Carol Nichols. 
+* `[Klabnik & Nichols, 2018]` Klabnik, Steve, and Carol Nichols.
   The Rust programming language. No Starch Press, 2023.
-* [Klabnik & Nichols, 2023] Klabnik, Steve, and Carol Nichols. 
+* `[Klabnik & Nichols, 2023]` Klabnik, Steve, and Carol Nichols.
   The Rust programming language. No Starch Press, 2023.

hello_world.md

@@ -2,9 +2,59 @@
 
 This chapter is a minimal Bevy program.
 
+The minimal program will create a Bevy `App`
+and then run it ([main.rs](https://github.com/richelbilderbeek/bevy_tdd_book_hello_world/blob/master/src/main.rs)): 
+
+```rust
+use crate::app::create_app;
+mod app;
+
+fn main() {
+    let mut app = create_app();
+    app.run();
+}
+```
+
+The `main` function will not be used in automated testing,
+as it starts our game: starting the game 
+will require that a user needs to do something to close it.
+Hence, the `main` function 'just' runs the `App`.
+
+The testing takes place in the `create_app` function.
+The first two lines of `main.rs` allow us to call the `create_app`
+function in the file [app.rs](https://github.com/richelbilderbeek/bevy_tdd_book_hello_world/blob/master/src/app.rs).
+
+Our first trivial test will be if `create_app` does something,
+i.e. it does not crash:
+
 ```
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_can_create_app() {
+        create_app();
+    }
+}
 ```
 
+Writing this test will break the code, as the function `create_app`
+does not exist yet.
+
+Here is the `create_app` function definition that fixes all tests:
+
+```rust
+use bevy::prelude::*;
+
+pub fn create_app() -> App {
+    return App::new();
+}
+```
+
+The full code of [app.rs](https://github.com/richelbilderbeek/bevy_tdd_book_hello_world/blob/master/src/app.rs)
+can be found at [app.rs](https://github.com/richelbilderbeek/bevy_tdd_book_hello_world/blob/master/src/app.rs).
+
 ## Links
 
- * [https://github.com/richelbilderbeek/bevy_tdd_book_hello_world](https://github.com/richelbilderbeek/bevy_tdd_book_hello_world)
+* [https://github.com/richelbilderbeek/bevy_tdd_book_hello_world](https://github.com/richelbilderbeek/bevy_tdd_book_hello_world)