Add a Buffer::age() method

Fixes https://github.com/rust-windowing/softbuffer/issues/90.

src/cg.rs

@@ -87,6 +87,10 @@ impl<'a> BufferImpl<'a> {
         &mut self.buffer
     }
 
+    pub fn age(&self) -> u8 {
+        0
+    }
+
     pub fn present(self) -> Result<(), SoftBufferError> {
         let data_provider = CGDataProvider::from_buffer(Arc::new(Buffer(self.buffer)));
         let image = CGImage::new(

src/lib.rs

@@ -124,6 +124,15 @@ macro_rules! make_dispatch {
                 }
             }
 
+            pub fn age(&self) -> u8 {
+                match self {
+                    $(
+                        $(#[$attr])*
+                        Self::$name(inner) => inner.age(),
+                    )*
+                }
+            }
+
             pub fn present(self) -> Result<(), SoftBufferError> {
                 match self {
                     $(
@@ -326,7 +335,7 @@ impl Surface {
 
     /// Return a [`Buffer`] that the next frame should be rendered into. The size must
     /// be set with [`Surface::resize`] first. The initial contents of the buffer may be zeroed, or
-    /// may contain a previous frame.
+    /// may contain a previous frame. Call [`Buffer::age`] to determine this.
     pub fn buffer_mut(&mut self) -> Result<Buffer, SoftBufferError> {
         Ok(Buffer {
             buffer_impl: self.surface_impl.buffer_mut()?,
@@ -377,6 +386,16 @@ pub struct Buffer<'a> {
 }
 
 impl<'a> Buffer<'a> {
+    /// Is age is the number of frames ago this buffer was last presented. So if the value is
+    /// `1`, it is the same as the last frame, and if it is `2`, it is the same as the frame
+    /// before that (for backends using double buffering). If the value is `0`, it is a new
+    /// buffer that has unspecified contents.
+    ///
+    /// This can be used to update only a portion of the buffer.
+    pub fn age(&self) -> u8 {
+        self.buffer_impl.age()
+    }
+
     /// Presents buffer to the window.
     ///
     /// # Platform dependent behavior

src/orbital.rs

@@ -57,6 +57,7 @@ pub struct OrbitalImpl {
     handle: OrbitalWindowHandle,
     width: u32,
     height: u32,
+    presented: bool,
 }
 
 impl OrbitalImpl {
@@ -65,12 +66,18 @@ impl OrbitalImpl {
             handle,
             width: 0,
             height: 0,
+            presented: false,
         })
     }
 
     pub fn resize(&mut self, width: NonZeroU32, height: NonZeroU32) -> Result<(), SoftBufferError> {
-        self.width = width.get();
-        self.height = height.get();
+        let width = width.get();
+        let height = height.get();
+        if width != self.width && height != self.height {
+            self.presented = false;
+            self.width = width;
+            self.height = height;
+        }
         Ok(())
     }
 
@@ -172,11 +179,19 @@ impl<'a> BufferImpl<'a> {
         }
     }
 
+    pub fn age(&self) -> u8 {
+        match self.pixels {
+            Pixels::Mapping(_) if self.imp.presented => 1,
+            _ => 0,
+        }
+    }
+
     pub fn present(self) -> Result<(), SoftBufferError> {
         match self.pixels {
             Pixels::Mapping(mapping) => {
                 drop(mapping);
                 syscall::fsync(self.imp.window_fd()).expect("failed to sync orbital window");
+                self.imp.presented = true;
             }
             Pixels::Buffer(buffer) => {
                 self.imp

src/wayland/buffer.rs

@@ -90,6 +90,7 @@ pub(super) struct WaylandBuffer {
     width: i32,
     height: i32,
     released: Arc<AtomicBool>,
+    pub age: u8,
 }
 
 impl WaylandBuffer {
@@ -125,6 +126,7 @@ impl WaylandBuffer {
             width,
             height,
             released,
+            age: 0,
         }
     }
 

src/wayland/mod.rs

@@ -125,9 +125,14 @@ impl WaylandImpl {
             ));
         };
 
-        Ok(BufferImpl(util::BorrowStack::new(self, |buffer| {
-            Ok(unsafe { buffer.buffers.as_mut().unwrap().1.mapped_mut() })
-        })?))
+        let age = self.buffers.as_mut().unwrap().1.age;
+
+        Ok(BufferImpl {
+            stack: util::BorrowStack::new(self, |buffer| {
+                Ok(unsafe { buffer.buffers.as_mut().unwrap().1.mapped_mut() })
+            })?,
+            age,
+        })
     }
 
     fn present_with_damage(&mut self, damage: &[Rect]) -> Result<(), SoftBufferError> {
@@ -138,6 +143,11 @@ impl WaylandImpl {
             .dispatch_pending(&mut State);
 
         if let Some((front, back)) = &mut self.buffers {
+            front.age = 1;
+            if back.age != 0 {
+                back.age += 1;
+            }
+
             // Swap front and back buffer
             std::mem::swap(front, back);
 
@@ -170,25 +180,32 @@ impl WaylandImpl {
     }
 }
 
-pub struct BufferImpl<'a>(util::BorrowStack<'a, WaylandImpl, [u32]>);
+pub struct BufferImpl<'a> {
+    stack: util::BorrowStack<'a, WaylandImpl, [u32]>,
+    age: u8,
+}
 
 impl<'a> BufferImpl<'a> {
     #[inline]
     pub fn pixels(&self) -> &[u32] {
-        self.0.member()
+        self.stack.member()
     }
 
     #[inline]
     pub fn pixels_mut(&mut self) -> &mut [u32] {
-        self.0.member_mut()
+        self.stack.member_mut()
+    }
+
+    pub fn age(&self) -> u8 {
+        self.age
     }
 
     pub fn present_with_damage(self, damage: &[Rect]) -> Result<(), SoftBufferError> {
-        self.0.into_container().present_with_damage(damage)
+        self.stack.into_container().present_with_damage(damage)
     }
 
     pub fn present(self) -> Result<(), SoftBufferError> {
-        let imp = self.0.into_container();
+        let imp = self.stack.into_container();
         let (width, height) = imp
             .size
             .expect("Must set size of surface before calling `present()`");

src/web.rs

@@ -41,8 +41,14 @@ pub struct WebImpl {
     /// The buffer that we're drawing to.
     buffer: Vec<u32>,
 
+    /// Buffer has been presented.
+    buffer_presented: bool,
+
     /// The current width of the canvas.
     width: u32,
+
+    /// The current height of the canvas.
+    height: u32,
 }
 
 impl WebImpl {
@@ -70,7 +76,9 @@ impl WebImpl {
             canvas,
             ctx,
             buffer: Vec::new(),
+            buffer_presented: false,
             width: 0,
+            height: 0,
         })
     }
 
@@ -83,10 +91,15 @@ impl WebImpl {
         let width = width.get();
         let height = height.get();
 
-        self.buffer.resize(total_len(width, height), 0);
-        self.canvas.set_width(width);
-        self.canvas.set_height(height);
-        self.width = width;
+        if width != self.width || height != self.height {
+            self.buffer_presented = false;
+            self.buffer.resize(total_len(width, height), 0);
+            self.canvas.set_width(width);
+            self.canvas.set_height(height);
+            self.width = width;
+            self.height = height;
+        }
+
         Ok(())
     }
 
@@ -109,6 +122,14 @@ impl<'a> BufferImpl<'a> {
         &mut self.imp.buffer
     }
 
+    pub fn age(&self) -> u8 {
+        if self.imp.buffer_presented {
+            1
+        } else {
+            0
+        }
+    }
+
     /// Push the buffer to the canvas.
     pub fn present(self) -> Result<(), SoftBufferError> {
         // Create a bitmap from the buffer.
@@ -151,6 +172,8 @@ impl<'a> BufferImpl<'a> {
         // This can only throw an error if `data` is detached, which is impossible.
         self.imp.ctx.put_image_data(&image_data, 0.0, 0.0).unwrap();
 
+        self.imp.buffer_presented = true;
+
         Ok(())
     }
 

src/win32.rs

@@ -27,6 +27,7 @@ struct Buffer {
     pixels: NonNull<u32>,
     width: NonZeroI32,
     height: NonZeroI32,
+    presented: bool,
 }
 
 impl Drop for Buffer {
@@ -101,6 +102,7 @@ impl Buffer {
             width,
             height,
             pixels,
+            presented: false,
         }
     }
 
@@ -203,8 +205,8 @@ impl Win32Impl {
         Ok(BufferImpl(self))
     }
 
-    fn present_with_damage(&self, damage: &[Rect]) -> Result<(), SoftBufferError> {
-        let buffer = self.buffer.as_ref().unwrap();
+    fn present_with_damage(&mut self, damage: &[Rect]) -> Result<(), SoftBufferError> {
+        let buffer = self.buffer.as_mut().unwrap();
         unsafe {
             for Rect {
                 x,
@@ -229,6 +231,7 @@ impl Win32Impl {
             // Validate the window.
             Gdi::ValidateRect(self.window, ptr::null_mut());
         }
+        buffer.presented = true;
 
         Ok(())
     }
@@ -247,6 +250,13 @@ impl<'a> BufferImpl<'a> {
         self.0.buffer.as_mut().unwrap().pixels_mut()
     }
 
+    pub fn age(&self) -> u8 {
+        match self.0.buffer.as_ref() {
+            Some(buffer) if buffer.presented => 1,
+            _ => 0,
+        }
+    }
+
     pub fn present(self) -> Result<(), SoftBufferError> {
         let imp = self.0;
         let buffer = imp.buffer.as_ref().unwrap();

src/x11.rs

@@ -107,6 +107,9 @@ pub struct X11Impl {
     /// The buffer we draw to.
     buffer: Buffer,
 
+    /// Buffer has been presented.
+    buffer_presented: bool,
+
     /// The current buffer width.
     width: u16,
 
@@ -223,6 +226,7 @@ impl X11Impl {
             gc,
             depth: geometry_reply.depth,
             buffer,
+            buffer_presented: false,
             width: 0,
             height: 0,
         })
@@ -255,6 +259,7 @@ impl X11Impl {
             }))?;
 
         if width != self.width || height != self.height {
+            self.buffer_presented = false;
             self.buffer
                 .resize(&self.display.connection, width, height)
                 .swbuf_err("Failed to resize X11 buffer")?;
@@ -294,13 +299,21 @@ impl<'a> BufferImpl<'a> {
         unsafe { self.0.buffer.buffer_mut() }
     }
 
+    pub fn age(&self) -> u8 {
+        if self.0.buffer_presented {
+            1
+        } else {
+            0
+        }
+    }
+
     /// Push the buffer to the window.
     pub fn present_with_damage(self, damage: &[Rect]) -> Result<(), SoftBufferError> {
         let imp = self.0;
 
         log::trace!("present: window={:X}", imp.window);
 
-        let result = match imp.buffer {
+        match imp.buffer {
             Buffer::Wire(ref wire) => {
                 // This is a suboptimal strategy, raise a stink in the debug logs.
                 log::debug!("Falling back to non-SHM method for window drawing.");
@@ -368,9 +381,12 @@ impl<'a> BufferImpl<'a> {
                     Ok(())
                 }
             }
-        };
+        }
+        .swbuf_err("Failed to draw image to window")?;
 
-        result.swbuf_err("Failed to draw image to window")
+        imp.buffer_presented = true;
+
+        Ok(())
     }
 
     pub fn present(self) -> Result<(), SoftBufferError> {