add example of override the asset IO instance

Cargo.toml

@@ -177,6 +177,10 @@ path = "examples/asset/asset_loading.rs"
 name = "custom_asset"
 path = "examples/asset/custom_asset.rs"
 
+[[example]]
+name = "custom_asset_io"
+path = "examples/asset/custom_asset_io.rs"
+
 [[example]]
 name = "audio"
 path = "examples/audio/audio.rs"

examples/asset/custom_asset_io.rs

@@ -0,0 +1,104 @@
+use bevy::{
+    asset::{AssetIo, AssetIoError},
+    prelude::*,
+    utils::BoxedFuture,
+};
+use std::path::{Path, PathBuf};
+
+/// A custom asset io implementation that simply defers to the platform default
+/// implementation.
+///
+/// This can be used as a starting point for developing a useful implementation
+/// that can defer to the default when needed.
+struct CustomAssetIo(Box<dyn AssetIo>);
+
+impl AssetIo for CustomAssetIo {
+    fn load_path<'a>(&'a self, path: &'a Path) -> BoxedFuture<'a, Result<Vec<u8>, AssetIoError>> {
+        println!("load_path({:?})", path);
+        self.0.load_path(path)
+    }
+
+    fn read_directory(
+        &self,
+        path: &Path,
+    ) -> Result<Box<dyn Iterator<Item = PathBuf>>, AssetIoError> {
+        println!("read_directory({:?})", path);
+        self.0.read_directory(path)
+    }
+
+    fn is_directory(&self, path: &Path) -> bool {
+        println!("is_directory({:?})", path);
+        self.0.is_directory(path)
+    }
+
+    fn watch_path_for_changes(&self, path: &Path) -> Result<(), AssetIoError> {
+        println!("watch_path_for_changes({:?})", path);
+        self.0.watch_path_for_changes(path)
+    }
+
+    fn watch_for_changes(&self) -> Result<(), AssetIoError> {
+        println!("watch_for_changes()");
+        self.0.watch_for_changes()
+    }
+}
+
+/// A plugin used to execute the override of the asset io
+struct CustomAssetIoPlugin;
+
+impl Plugin for CustomAssetIoPlugin {
+    fn build(&self, app: &mut AppBuilder) {
+        // must get a hold of the task pool in order to create the asset server
+
+        let task_pool = app
+            .resources()
+            .get::<bevy::tasks::IoTaskPool>()
+            .expect("`IoTaskPool` resource not found.")
+            .0
+            .clone();
+
+        let asset_io = {
+            // the platform default asset io requires a reference to the app
+            // builder to find its configuration
+
+            let default_io = bevy::asset::create_platform_default_asset_io(app);
+
+            // create the custom asset io instance
+
+            CustomAssetIo(default_io)
+        };
+
+        // the asset server is constructed and added the resource manager
+
+        app.add_resource(AssetServer::new(asset_io, task_pool));
+    }
+}
+
+fn main() {
+    App::build()
+        .add_plugins_with(DefaultPlugins, |group| {
+            // the custom asset io plugin must be inserted in-between the
+            // `CorePlugin' and `AssetPlugin`. It needs to be after the
+            // CorePlugin, so that the IO task pool has already been constructed.
+            // And it must be before the `AssetPlugin` so that the asset plugin
+            // doesn't create another instance of an assert server. In general,
+            // the AssetPlugin should still run so that other aspects of the
+            // asset system are initialized correctly.
+            group.add_before::<bevy::asset::AssetPlugin, _>(CustomAssetIoPlugin)
+        })
+        .add_startup_system(setup)
+        .run();
+}
+
+fn setup(
+    commands: &mut Commands,
+    asset_server: Res<AssetServer>,
+    mut materials: ResMut<Assets<ColorMaterial>>,
+) {
+    let texture_handle = asset_server.load("branding/icon.png");
+    commands
+        .spawn(Camera2dBundle::default())
+        .spawn(SpriteBundle {
+            material: materials.add(texture_handle.into()),
+            ..Default::default()
+        });
+}