Add layer observer based on raw-window-metal (#7026) (#7466)

Co-authored-by: Mads Marquart <[email protected]>
Co-authored-by: Connor Fitzgerald <[email protected]>

Author: 3 people

CHANGELOG.md

@@ -40,6 +40,9 @@ Bottom level categories:
 
 ## Unreleased
 
+#### Metal
+ - Use resize observers for smoother resizing. By @madsmtm in [#7026](https://github.com/gfx-rs/wgpu/pull/7026).
+
 ### v24.0.3 (2025-03-19)
 
 #### Bug Fixes

wgpu-hal/src/metal/layer_observer.rs

@@ -0,0 +1,190 @@
+//! A rewrite of `raw-window-metal` using `objc` instead of `objc2`.
+//!
+//! See that for details: <https://docs.rs/raw-window-metal/1.1.0/>
+//!
+//! This should be temporary, see <https://github.com/gfx-rs/wgpu/pull/6210>.
+
+use core::ffi::{c_void, CStr};
+use core_graphics_types::base::CGFloat;
+use core_graphics_types::geometry::CGRect;
+use objc::declare::ClassDecl;
+use objc::rc::StrongPtr;
+use objc::runtime::{Class, Object, Sel, BOOL, NO};
+use objc::{class, msg_send, sel, sel_impl};
+use std::sync::OnceLock;
+
+#[link(name = "Foundation", kind = "framework")]
+extern "C" {
+    static NSKeyValueChangeNewKey: &'static Object;
+}
+
+#[allow(non_upper_case_globals)]
+const NSKeyValueObservingOptionNew: usize = 0x01;
+#[allow(non_upper_case_globals)]
+const NSKeyValueObservingOptionInitial: usize = 0x04;
+
+const CONTENTS_SCALE: &CStr = unsafe { CStr::from_bytes_with_nul_unchecked(b"contentsScale\0") };
+const BOUNDS: &CStr = unsafe { CStr::from_bytes_with_nul_unchecked(b"bounds\0") };
+
+/// Create a new custom layer that tracks parameters from the given super layer.
+///
+/// Same as <https://docs.rs/raw-window-metal/1.1.0/src/raw_window_metal/observer.rs.html#74-132>.
+pub unsafe fn new_observer_layer(root_layer: *mut Object) -> StrongPtr {
+    let this: *mut Object = unsafe { msg_send![class(), new] };
+
+    // Add the layer as a sublayer of the root layer.
+    let _: () = unsafe { msg_send![root_layer, addSublayer: this] };
+
+    // Register for key-value observing.
+    let key_path: *const Object =
+        unsafe { msg_send![class!(NSString), stringWithUTF8String: CONTENTS_SCALE.as_ptr()] };
+    let _: () = unsafe {
+        msg_send![
+            root_layer,
+            addObserver: this
+            forKeyPath: key_path
+            options: NSKeyValueObservingOptionNew | NSKeyValueObservingOptionInitial
+            context: context_ptr()
+        ]
+    };
+
+    let key_path: *const Object =
+        unsafe { msg_send![class!(NSString), stringWithUTF8String: BOUNDS.as_ptr()] };
+    let _: () = unsafe {
+        msg_send![
+            root_layer,
+            addObserver: this
+            forKeyPath: key_path
+            options: NSKeyValueObservingOptionNew | NSKeyValueObservingOptionInitial
+            context: context_ptr()
+        ]
+    };
+
+    // Uncomment when debugging resize issues.
+    // extern "C" {
+    //     static kCAGravityTopLeft: *mut Object;
+    // }
+    // let _: () = unsafe { msg_send![this, setContentsGravity: kCAGravityTopLeft] };
+
+    unsafe { StrongPtr::new(this) }
+}
+
+/// Same as <https://docs.rs/raw-window-metal/1.1.0/src/raw_window_metal/observer.rs.html#74-132>.
+fn class() -> &'static Class {
+    static CLASS: OnceLock<&'static Class> = OnceLock::new();
+
+    CLASS.get_or_init(|| {
+        let superclass = class!(CAMetalLayer);
+        let class_name = format!("WgpuObserverLayer@{:p}", &CLASS);
+        let mut decl = ClassDecl::new(&class_name, superclass).unwrap();
+
+        // From NSKeyValueObserving.
+        let sel = sel!(observeValueForKeyPath:ofObject:change:context:);
+        let method: extern "C" fn(
+            &Object,
+            Sel,
+            *mut Object,
+            *mut Object,
+            *mut Object,
+            *mut c_void,
+        ) = observe_value;
+        unsafe { decl.add_method(sel, method) };
+
+        let sel = sel!(dealloc);
+        let method: extern "C" fn(&Object, Sel) = dealloc;
+        unsafe { decl.add_method(sel, method) };
+
+        decl.register()
+    })
+}
+
+/// The unique context pointer for this class.
+fn context_ptr() -> *mut c_void {
+    let ptr: *const Class = class();
+    ptr.cast_mut().cast()
+}
+
+/// Same as <https://docs.rs/raw-window-metal/1.1.0/src/raw_window_metal/observer.rs.html#74-132>.
+extern "C" fn observe_value(
+    this: &Object,
+    _cmd: Sel,
+    key_path: *mut Object,
+    object: *mut Object,
+    change: *mut Object,
+    context: *mut c_void,
+) {
+    // An unrecognized context must belong to the super class.
+    if context != context_ptr() {
+        // SAFETY: The signature is correct, and it's safe to forward to
+        // the superclass' method when we're overriding the method.
+        return unsafe {
+            msg_send![
+                super(this, class!(CAMetalLayer)),
+                observeValueForKeyPath: key_path
+                ofObject: object
+                change: change
+                context: context
+            ]
+        };
+    }
+
+    assert!(!change.is_null());
+
+    let key = unsafe { NSKeyValueChangeNewKey };
+    let new: *mut Object = unsafe { msg_send![change, objectForKey: key] };
+    assert!(!new.is_null());
+
+    let to_compare: *const Object =
+        unsafe { msg_send![class!(NSString), stringWithUTF8String: CONTENTS_SCALE.as_ptr()] };
+    let is_equal: BOOL = unsafe { msg_send![key_path, isEqual: to_compare] };
+    if is_equal != NO {
+        // `contentsScale` is a CGFloat, and so the observed value is always a NSNumber.
+        let scale_factor: CGFloat = if cfg!(target_pointer_width = "64") {
+            unsafe { msg_send![new, doubleValue] }
+        } else {
+            unsafe { msg_send![new, floatValue] }
+        };
+
+        // Set the scale factor of the layer to match the root layer.
+        let _: () = unsafe { msg_send![this, setContentsScale: scale_factor] };
+        return;
+    }
+
+    let to_compare: *const Object =
+        unsafe { msg_send![class!(NSString), stringWithUTF8String: BOUNDS.as_ptr()] };
+    let is_equal: BOOL = unsafe { msg_send![key_path, isEqual: to_compare] };
+    if is_equal != NO {
+        // `bounds` is a CGRect, and so the observed value is always a NSNumber.
+        let bounds: CGRect = unsafe { msg_send![new, rectValue] };
+
+        // Set `bounds` and `position` to match the root layer.
+        //
+        // This differs from just setting the `bounds`, as it also takes into account any
+        // translation that the superlayer may have that we'd want to preserve.
+        let _: () = unsafe { msg_send![this, setFrame: bounds] };
+        return;
+    }
+
+    panic!("unknown observed keypath {key_path:?}");
+}
+
+extern "C" fn dealloc(this: &Object, _cmd: Sel) {
+    // Load the root layer if it still exists, and deregister the observer.
+    //
+    // This is not entirely sound, as the ObserverLayer _could_ have been
+    // moved to another layer; but Wgpu doesn't do that, so it should be fine.
+    //
+    // `raw-window-metal` uses a weak instance variable to do it correctly:
+    // https://docs.rs/raw-window-metal/1.1.0/src/raw_window_metal/observer.rs.html#74-132
+    // (but that's difficult to do with `objc`).
+    let root_layer: *mut Object = unsafe { msg_send![this, superlayer] };
+    if !root_layer.is_null() {
+        let key_path: *const Object =
+            unsafe { msg_send![class!(NSString), stringWithUTF8String: CONTENTS_SCALE.as_ptr()] };
+        let _: () = unsafe { msg_send![root_layer, removeObserver: this forKeyPath: key_path] };
+
+        let key_path: *const Object =
+            unsafe { msg_send![class!(NSString), stringWithUTF8String: BOUNDS.as_ptr()] };
+        let _: () = unsafe { msg_send![root_layer, removeObserver: this forKeyPath: key_path] };
+    }
+}

wgpu-hal/src/metal/mod.rs

@@ -22,6 +22,7 @@ mod adapter;
 mod command;
 mod conv;
 mod device;
+mod layer_observer;
 mod surface;
 mod time;