Use double-buffering for screencopy

Currently this is unnecesssary, but it's needed if we use subsurfaces
(or import) instead of immedaitely converting to an image.

src/wayland/capture.rs

@@ -8,10 +8,7 @@ use cctk::{
 };
 use cosmic::cctk;
 
-use std::sync::{
+use std::sync::{Arc, Mutex};
-    atomic::{AtomicBool, Ordering},
-    Arc, Mutex,
-};
 
 use super::{AppData, ScreencopySession, SessionData};
 

src/wayland/screencopy.rs

@@ -9,12 +9,15 @@ use cosmic::cctk::{
     },
     wayland_client::{Connection, QueueHandle, WEnum},
 };
-use std::sync::{Arc, Weak};
+use std::{
+    mem,
+    sync::{Arc, Weak},
+};
 
 use super::{AppData, Buffer, Capture, CaptureImage, CaptureSource, Event};
 
 pub struct ScreencopySession {
-    buffer: Option<Buffer>,
+    buffers: Option<(Buffer, Buffer)>,
     session: zcosmic_screencopy_session_v1::ZcosmicScreencopySessionV1,
     first_frame: bool,
 }
@@ -47,22 +50,20 @@ impl ScreencopySession {
         };
 
         Self {
-            buffer: None,
+            buffers: None,
             session,
             first_frame: true,
         }
     }
 
     fn attach_buffer_and_commit(&mut self, capture: &Capture, conn: &Connection) {
-        let Some(buffer) = self.buffer.as_ref() else {
+        let Some((front, back)) = self.buffers.as_mut() else {
             return;
         };
 
-        let node = buffer
+        let node = front.node().and_then(|x| x.to_str().map(|x| x.to_string()));
-            .node()
-            .and_then(|x| x.to_str().map(|x| x.to_string()));
 
-        self.session.attach_buffer(&buffer.buffer, node, 0); // XXX age?
+        self.session.attach_buffer(&back.buffer, node, 0); // XXX age?
         if self.first_frame {
             self.session
                 .commit(zcosmic_screencopy_session_v1::Options::empty());
@@ -115,12 +116,13 @@ impl ScreencopyHandler for AppData {
         };
 
         // Create new buffer if none, or different format
-        if !session
+        if !session.buffers.as_ref().map_or(false, |(front, _)| {
-            .buffer
+            buffer_infos.contains(&front.buffer_info)
-            .as_ref()
+        }) {
-            .map_or(false, |x| buffer_infos.contains(&x.buffer_info))
+            session.buffers = Some((
-        {
+                self.create_buffer(buffer_infos),
-            session.buffer = Some(self.create_buffer(buffer_infos));
+                self.create_buffer(buffer_infos),
+            ));
         }
 
         session.attach_buffer_and_commit(&capture, conn);
@@ -140,11 +142,15 @@ impl ScreencopyHandler for AppData {
             return;
         };
 
-        if session.buffer.is_none() {
+        if session.buffers.is_none() {
-            eprintln!("Error: No capture buffer?");
+            eprintln!("Error: No capture buffers?");
             return;
         }
-        let img = unsafe { session.buffer.as_mut().unwrap().to_image() };
+
+        let (front, back) = session.buffers.as_mut().unwrap();
+        mem::swap(front, back);
+
+        let img = unsafe { front.to_image() };
         let image = CaptureImage { img };
         match &capture.source {
             CaptureSource::Toplevel(toplevel) => {