bevy/docs-template/EXAMPLE_README.md.tpl

<!-- MD024 - The Headers from the Platform-Specific Examples should be identical  -->
<!-- Use 'cargo run -p build-templated-pages -- build-example-page' to generate the final example README.md -->
<!-- markdownlint-disable-file MD024 -->

# Examples

These examples demonstrate the main features of Bevy and how to use them.
To run an example, use the command `cargo run --example <Example>`, and add the option `--features x11` or `--features wayland` to force the example to run on a specific window compositor, e.g.

```sh
cargo run --features wayland --example hello_world
```

**⚠️ Note: for users of releases on crates.io!**

There are often large differences and incompatible API changes between the latest [crates.io](https://crates.io/crates/bevy) release and the development version of Bevy in the git main branch!

If you are using a released version of bevy, you need to make sure you are viewing the correct version of the examples!

- Latest release: [https://github.com/bevyengine/bevy/tree/latest/examples](https://github.com/bevyengine/bevy/tree/latest/examples)
- Specific version, such as `0.4`: [https://github.com/bevyengine/bevy/tree/v0.4.0/examples](https://github.com/bevyengine/bevy/tree/v0.4.0/examples)

When you clone the repo locally to run the examples, use `git checkout` to get the correct version:

```bash
# `latest` always points to the newest release
git checkout latest
# or use a specific version
git checkout v0.4.0
```

---

## Table of Contents

- [Examples](#examples)
  - [Table of Contents](#table-of-contents)
- [The Bare Minimum](#the-bare-minimum)
  - [Hello, World!](#hello-world)
- [Cross-Platform Examples](#cross-platform-examples)
{% for category, _ in all_examples %}  - [{{ category }}](#{{ category | slugify }})
{% endfor %}
- [Tests](#tests)
- [Platform-Specific Examples](#platform-specific-examples)
  - [Android](#android)
    - [Setup](#setup)
    - [Build & Run](#build--run)
    - [Old phones](#old-phones)
  - [iOS](#ios)
    - [Setup](#setup-1)
    - [Build & Run](#build--run-1)
  - [WASM](#wasm)
    - [Setup](#setup-2)
    - [Build & Run](#build--run-2)
    - [WebGL2 and WebGPU](#webgl2-and-webgpu)
    - [Audio in the browsers](#audio-in-the-browsers)
    - [Optimizing](#optimizing)
    - [Loading Assets](#loading-assets)

# The Bare Minimum

<!-- MD026 - Hello, World! looks better with the ! -->
<!-- markdownlint-disable-next-line MD026 -->
## Hello, World!

Example | Description
--- | ---
[`hello_world.rs`](./hello_world.rs) | Runs a minimal example that outputs "hello world"

# Cross-Platform Examples
{% for category, details in all_examples %}
## {{ category }}

{% if details.description is string %}{{ details.description }}
{% endif %}Example | Description
--- | ---
{% for example in details.examples %}[{{ example.name }}](../{{ example.path }}) | {{ example.description }}
{% endfor %}{% endfor %}
# Tests

Example | Description
--- | ---
[How to Test Systems](../tests/how_to_test_systems.rs) | How to test systems with commands, queries or resources

# Platform-Specific Examples

## Android

### Setup

```sh
rustup target add aarch64-linux-android armv7-linux-androideabi
cargo install cargo-apk
```

The Android SDK must be installed, and the environment variable `ANDROID_SDK_ROOT` set to the root Android `sdk` folder.

When using `NDK (Side by side)`, the environment variable `ANDROID_NDK_ROOT` must also be set to one of the NDKs in `sdk\ndk\[NDK number]`.

### Build & Run

To run on a device setup for Android development, run:

```sh
cargo apk run -p bevy_mobile_example
```

When using Bevy as a library, the following fields must be added to `Cargo.toml`:

```toml
[package.metadata.android]
build_targets = ["aarch64-linux-android", "armv7-linux-androideabi"]

[package.metadata.android.sdk]
target_sdk_version = 31
```

Please reference `cargo-apk` [README](https://crates.io/crates/cargo-apk) for other Android Manifest fields.

### Debugging

You can view the logs with the following command:

```sh
adb logcat | grep 'RustStdoutStderr\|bevy\|wgpu'
```

In case of an error getting a GPU or setting it up, you can try settings logs of `wgpu_hal` to `DEBUG` to get more information.

Sometimes, running the app complains about an unknown activity. This may be fixed by uninstalling the application:

```sh
adb uninstall org.bevyengine.example
```

### Old phones

Bevy by default targets Android API level 31 in its examples which is the <!-- markdown-link-check-disable -->
[Play Store's minimum API to upload or update apps](https://developer.android.com/distribute/best-practices/develop/target-sdk). <!-- markdown-link-check-enable -->
Users of older phones may want to use an older API when testing.

To use a different API, the following fields must be updated in Cargo.toml:

```toml
[package.metadata.android.sdk]
target_sdk_version = >>API<<
min_sdk_version = >>API or less<<
```

Example | File | Description
--- | --- | ---
`android` | [`mobile/src/lib.rs`](./mobile/src/lib.rs) | A 3d Scene with a button and playing sound

## iOS

### Setup

You need to install the correct rust targets:

- `aarch64-apple-ios`: iOS devices
- `x86_64-apple-ios`: iOS simulator on x86 processors
- `aarch64-apple-ios-sim`: iOS simulator on Apple processors

```sh
rustup target add aarch64-apple-ios x86_64-apple-ios aarch64-apple-ios-sim
```

### Build & Run

Using bash:

```sh
cd examples/mobile
make run
```

In an ideal world, this will boot up, install and run the app for the first
iOS simulator in your `xcrun simctl devices list`. If this fails, you can
specify the simulator device UUID via:

```sh
DEVICE_ID=${YOUR_DEVICE_ID} make run
```

If you'd like to see xcode do stuff, you can run

```sh
open bevy_mobile_example.xcodeproj/
```

which will open xcode. You then must push the zoom zoom play button and wait
for the magic.

Example | File | Description
--- | --- | ---
`ios` | [`mobile/src/lib.rs`](./mobile/src/lib.rs) | A 3d Scene with a button and playing sound

## WASM

### Setup

```sh
rustup target add wasm32-unknown-unknown
cargo install wasm-bindgen-cli
```

### Build & Run

Following is an example for `lighting`. For other examples, change the `lighting` in the
following commands.

```sh
cargo build --release --example lighting --target wasm32-unknown-unknown --features webgl
wasm-bindgen --out-name wasm_example \
  --out-dir examples/wasm/target \
  --target web target/wasm32-unknown-unknown/release/examples/lighting.wasm
```

The first command will build the example for the wasm target, creating a binary. Then,
[wasm-bindgen-cli](https://rustwasm.github.io/wasm-bindgen/reference/cli.html) is used to create
javascript bindings to this wasm file, which can be loaded using this
[example HTML file](./wasm/index.html).

Then serve `examples/wasm` directory to browser. i.e.

```sh
# cargo install basic-http-server
basic-http-server examples/wasm

# with python
python3 -m http.server --directory examples/wasm

# with ruby
ruby -run -ehttpd examples/wasm
```

#### WebGL2 and WebGPU

Bevy support for WebGPU is being worked on, but is currently experimental.

To build for WebGPU, you'll need to disable default features and add all those you need, making sure to omit the `webgl2` feature.

Bevy has an helper to build its examples:

- Build for WebGL2: `cargo run -p build-wasm-example -- --api webgl2 load_gltf`
- Build for WebGPU: `cargo run -p build-wasm-example -- --api webgpu load_gltf`

This helper will log the command used to build the examples.

### Audio in the browsers

For the moment, everything is single threaded, this can lead to stuttering when playing audio in browsers. Not all browsers react the same way for all games, you will have to experiment for your game.

In browsers, audio is not authorized to start without being triggered by an user interaction. This is to avoid multiple tabs all starting to auto play some sounds. You can find more context and explanation for this on [Google Chrome blog](https://developer.chrome.com/blog/web-audio-autoplay/). This page also describes a JS workaround to resume audio as soon as the user interact with your game.

### Optimizing

On the web, it's useful to reduce the size of the files that are distributed.
With rust, there are many ways to improve your executable sizes.
Here are some.

#### 1. Tweak your `Cargo.toml`

Add a new [profile](https://doc.rust-lang.org/cargo/reference/profiles.html)
to your `Cargo.toml`:

```toml
[profile.wasm-release]
# Use release profile as default values
inherits = "release"

# Optimize with size in mind, also try "s", sometimes it is better.
# This doesn't increase compilation times compared to -O3, great improvements
opt-level = "z"

# Do a second optimization pass removing duplicate or unused code from dependencies.
# Slows compile times, marginal improvements
lto = "fat"

# When building crates, optimize larger chunks at a time
# Slows compile times, marginal improvements
codegen-units = 1
```

Now, when building the final executable, use the `wasm-release` profile
by replacing `--release` by `--profile wasm-release` in the cargo command.

```sh
cargo build --profile wasm-release --example lighting --target wasm32-unknown-unknown
```

Make sure your final executable size is smaller, some of those optimizations
may not be worth keeping, due to compilation time increases.

#### 2. Use `wasm-opt` from the binaryen package

Binaryen is a set of tools for working with wasm. It has a `wasm-opt` CLI tool.

First download the `binaryen` package,
then locate the `.wasm` file generated by `wasm-bindgen`.
It should be in the `--out-dir` you specified in the command line,
the file name should end in `_bg.wasm`.

Then run `wasm-opt` with the `-Oz` flag. Note that `wasm-opt` is _very slow_.

Note that `wasm-opt` optimizations might not be as effective if you
didn't apply the optimizations from the previous section.

```sh
wasm-opt -Oz --output optimized.wasm examples/wasm/target/lighting_bg.wasm
mv optimized.wasm examples/wasm/target/lighting_bg.wasm
```

For a small project with a basic 3d model and two lights,
the generated file sizes are, as of Jully 2022 as following:

|profile                           | wasm-opt | no wasm-opt |
|----------------------------------|----------|-------------|
|Default                           | 8.5M     | 13.0M       |
|opt-level = "z"                   | 6.1M     | 12.7M       |
|"z" + lto = "thin"                | 5.9M     | 12M         |
|"z" + lto = "fat"                 | 5.1M     | 9.4M        |
|"z" + "thin" + codegen-units = 1  | 5.3M     | 11M         |
|"z" + "fat"  + codegen-units = 1  | 4.8M     | 8.5M        |

There are more advanced optimization options available,
check the following pages for more info:

- <https://rustwasm.github.io/book/reference/code-size.html>
- <https://rustwasm.github.io/docs/wasm-bindgen/reference/optimize-size.html>
- <https://rustwasm.github.io/book/game-of-life/code-size.html>

### Loading Assets

To load assets, they need to be available in the folder examples/wasm/assets. Cloning this
repository will set it up as a symlink on Linux and macOS, but you will need to manually move
the assets on Windows.