Add runtime backend selection

src/graphics/d3d12.rs

@@ -33,7 +33,9 @@ use crate::VIEW_TYPE;
 
 pub fn initialize_xr_instance(
     window: Option<RawHandleWrapper>,
+    xr_entry: xr::Entry,
     reqeusted_extensions: XrExtensions,
+    available_extensions: XrExtensions,
     prefered_blend_mode: XrPreferdBlendMode,
     app_info: XrAppInfo,
 ) -> eyre::Result<(
@@ -46,12 +48,9 @@ pub fn initialize_xr_instance(
     RenderAdapter,
     Instance,
 )> {
-    let xr_entry = super::xr_entry()?;
-
     #[cfg(target_os = "android")]
     xr_entry.initialize_android_loader()?;
 
-    let available_extensions: XrExtensions = xr_entry.enumerate_extensions()?.into();
     assert!(available_extensions.raw().khr_d3d12_enable);
     //info!("available xr exts: {:#?}", available_extensions);
 

src/graphics/mod.rs

@@ -21,6 +21,8 @@ use crate::resources::{
 };
 use crate::OXrSessionSetupInfo;
 
+use crate::Backend;
+
 use openxr as xr;
 
 use self::extensions::XrExtensions;
@@ -91,6 +93,7 @@ pub fn start_xr_session(
     }
 }
 pub fn initialize_xr_instance(
+    backend_preference: &[Backend],
     window: Option<RawHandleWrapper>,
     reqeusted_extensions: XrExtensions,
     prefered_blend_mode: XrPreferdBlendMode,
@@ -105,18 +108,57 @@ pub fn initialize_xr_instance(
     RenderAdapter,
     Instance,
 )> {
-    #[cfg(feature = "vulkan")]
+    if backend_preference.is_empty() {
-    {
+        eyre::bail!("Cannot initialize with no backend selected");
-        vulkan::initialize_xr_instance(window, reqeusted_extensions, prefered_blend_mode, app_info)
     }
-    #[cfg(feature = "d3d12")]
+    let xr_entry = xr_entry()?;
-    {
+
-        d3d12::initialize_xr_instance(window, reqeusted_extensions, prefered_blend_mode, app_info)
+    #[cfg(target_os = "android")]
+    xr_entry.initialize_android_loader()?;
+
+    let available_extensions: XrExtensions = xr_entry.enumerate_extensions()?.into();
+
+    for backend in backend_preference {
+        match backend {
+            #[cfg(feature = "vulkan")]
+            Backend::Vulkan => {
+                if !available_extensions.raw().khr_vulkan_enable2 {
+                    continue;
+                }
+                return vulkan::initialize_xr_instance(
+                    window,
+                    xr_entry,
+                    reqeusted_extensions,
+                    available_extensions,
+                    prefered_blend_mode,
+                    app_info,
+                );
+            }
+            #[cfg(feature = "d3d12")]
+            Backend::D3D12 => {
+                if !available_extensions.raw().khr_d3d12_enable {
+                    continue;
+                }
+                return d3d12::initialize_xr_instance(
+                    window,
+                    xr_entry,
+                    reqeusted_extensions,
+                    available_extensions,
+                    prefered_blend_mode,
+                    app_info,
+                );
+            }
+        }
     }
+    eyre::bail!(
+        "No selected backend was supported by the runtime. Selected: {:?}",
+        backend_preference
+    );
 }
 
 pub fn try_full_init(
     world: &mut World,
+    backend_preference: &[Backend],
     reqeusted_extensions: XrExtensions,
     prefered_blend_mode: XrPreferdBlendMode,
     app_info: XrAppInfo,
@@ -140,6 +182,7 @@ pub fn try_full_init(
         render_adapter,
         wgpu_instance,
     ) = initialize_xr_instance(
+        backend_preference,
         primary_window.clone(),
         reqeusted_extensions,
         prefered_blend_mode,

src/graphics/vulkan.rs

@@ -28,7 +28,9 @@ use super::{XrAppInfo, XrPreferdBlendMode};
 
 pub fn initialize_xr_instance(
     window: Option<RawHandleWrapper>,
+    xr_entry: xr::Entry,
     reqeusted_extensions: XrExtensions,
+    available_extensions: XrExtensions,
     prefered_blend_mode: XrPreferdBlendMode,
     app_info: XrAppInfo,
 ) -> eyre::Result<(
@@ -41,12 +43,9 @@ pub fn initialize_xr_instance(
     RenderAdapter,
     Instance,
 )> {
-    let xr_entry = super::xr_entry()?;
-
     #[cfg(target_os = "android")]
     xr_entry.initialize_android_loader()?;
 
-    let available_extensions: XrExtensions = xr_entry.enumerate_extensions()?.into();
     assert!(available_extensions.raw().khr_vulkan_enable2);
     //info!("available xr exts: {:#?}", available_extensions);
 

src/lib.rs

@@ -47,6 +47,7 @@ pub const RIGHT_XR_TEXTURE_HANDLE: ManualTextureViewHandle = ManualTextureViewHa
 
 /// Adds OpenXR support to an App
 pub struct OpenXrPlugin {
+    pub backend_preference: Vec<Backend>,
     pub reqeusted_extensions: XrExtensions,
     pub prefered_blend_mode: XrPreferdBlendMode,
     pub app_info: XrAppInfo,
@@ -59,6 +60,7 @@ impl Plugin for OpenXrPlugin {
         app.insert_resource(ExitAppOnSessionExit::default());
         #[cfg(not(target_arch = "wasm32"))]
         match graphics::initialize_xr_instance(
+            &self.backend_preference,
             SystemState::<Query<&RawHandleWrapper, With<PrimaryWindow>>>::new(&mut app.world)
                 .get(&app.world)
                 .get_single()
@@ -169,6 +171,14 @@ impl Plugin for OpenXrPlugin {
     }
 }
 
+#[derive(Debug)]
+pub enum Backend {
+    #[cfg(feature = "vulkan")]
+    Vulkan,
+    #[cfg(feature = "d3d12")]
+    D3D12,
+}
+
 #[derive(Resource)]
 struct DoPipelinedRendering;
 
@@ -230,6 +240,7 @@ fn xr_skip_frame(
 }
 
 pub struct DefaultXrPlugins {
+    pub backend_preference: Vec<Backend>,
     pub reqeusted_extensions: XrExtensions,
     pub prefered_blend_mode: XrPreferdBlendMode,
     pub app_info: XrAppInfo,
@@ -238,6 +249,12 @@ pub struct DefaultXrPlugins {
 impl Default for DefaultXrPlugins {
     fn default() -> Self {
         Self {
+            backend_preference: vec![
+                #[cfg(feature = "vulkan")]
+                Backend::Vulkan,
+                #[cfg(feature = "d3d12")]
+                Backend::D3D12,
+            ],
             reqeusted_extensions: default(),
             prefered_blend_mode: default(),
             app_info: default(),
@@ -263,6 +280,7 @@ impl PluginGroup for DefaultXrPlugins {
             })
             .disable::<RenderPlugin>()
             .add_before::<RenderPlugin, _>(OpenXrPlugin {
+                backend_preference: self.backend_preference,
                 prefered_blend_mode: self.prefered_blend_mode,
                 reqeusted_extensions: self.reqeusted_extensions,
                 app_info: self.app_info.clone(),