Port cairo_threads example to use the new GLib MainContext channel in…

…stead of polling

This ensures that the image is redrawn only if it actually changed and
more or less immediately after it has changed instead of waiting up to
100ms.

Cargo.toml

@@ -60,6 +60,7 @@ name = "cairo_png"
 
 [[bin]]
 name = "cairo_threads"
+required-features = ["glib/v2_36"]
 
 [[bin]]
 name = "cairotest"
@@ -134,3 +135,6 @@ name = "treeview"
 [[bin]]
 name = "status_icon"
 required-features = ["gio_2_40"]
+
+[patch."https://github.com/gtk-rs/glib"]
+glib = { path = "../glib" }

src/bin/cairo_threads.rs

@@ -121,10 +121,10 @@ impl Image {
 // The GUI thread holds an image per image part at all times and these images are painted on a
 // DrawingArea in its 'draw' signal handler whenever needed.
 //
-// Additionally the GUI thread checks periodically every 100ms if any worker has sent a freshly
-// rendered image. If there is a new image, the old image stored by the GUI thread is replaced with
-// the new one and the old image is sent back to the worker thread. Then the appropriate part of
-// the DrawingArea is invalidated prompting a redraw.
+// Additionally the GUI thread has a channel for receiving the images from the worker threads. If
+// there is a new image, the old image stored by the GUI thread is replaced with the new one and
+// the old image is sent back to the worker thread. Then the appropriate part of the DrawingArea is
+// invalidated prompting a redraw.
 //
 // The two images per thread are allocated and initialized once and sent back and forth repeatedly.
 
@@ -139,7 +139,8 @@ fn build_ui(application: &gtk::Application) {
     let initial_image = draw_initial();
 
     // This is the channel for sending results from the worker thread to the main thread
-    let (ready_tx, ready_rx) = mpsc::channel();
+    // For every received image, queue the corresponding part of the DrawingArea for redrawing
+    let (ready_tx, ready_rx) = glib::MainContext::channel(glib::PRIORITY_DEFAULT);
 
     let mut images = Vec::new();
     let mut origins = Vec::new();
@@ -215,20 +216,17 @@ fn build_ui(application: &gtk::Application) {
         Inhibit(false)
     }));
 
-    // Check every 100ms if there are new images from any worker threads and if so queue the
-    // corresponding part of the DrawingArea for redrawing
-    gtk::timeout_add(100, move || {
-        while let Ok((thread_num, image)) = ready_rx.try_recv() {
-            let (ref images, ref origins, ref workers) = &*workspace;
+    ready_rx.attach(None, move |(thread_num, image)| {
+        let (ref images, ref origins, ref workers) = &*workspace;
 
-            // Swap the newly received image with the old stored one and send the old one back to
-            // the worker thread
-            let tx = &workers[thread_num];
-            let image = images[thread_num].replace(image);
-            let _ = tx.send(image);
+        // Swap the newly received image with the old stored one and send the old one back to
+        // the worker thread
+        let tx = &workers[thread_num];
+        let image = images[thread_num].replace(image);
+        let _ = tx.send(image);
+
+        area.queue_draw_area(origins[thread_num].0, origins[thread_num].1, WIDTH, HEIGHT);
 
-            area.queue_draw_area(origins[thread_num].0, origins[thread_num].1, WIDTH, HEIGHT);
-        }
         Continue(true)
     });