Introduce 'NullCompositor'.

src/components/main/compositing/mod.rs

@@ -197,7 +197,8 @@ impl CompositorTask {
                                           profiler_chan);
             }
             Headless => {
-                run_headless::run_compositor(&constellation_chan, port);
+                run_headless::NullCompositor::create(port,
+                                                     constellation_chan.clone());
             }
         }
 

src/components/main/compositing/run_headless.rs

@@ -8,16 +8,36 @@ use geom::size::Size2D;
 use servo_msg::constellation_msg::{ConstellationChan, ResizedWindowMsg};
 use std::comm::Port;
 
+
 /// Starts the compositor, which listens for messages on the specified port.
 ///
 /// This is the null compositor which doesn't draw anything to the screen.
 /// It's intended for headless testing.
-pub fn run_compositor(constellation_chan: &ConstellationChan, port: Port<Msg>) {
+pub struct NullCompositor {
+    /// The port on which we receive messages.
+    port: Port<Msg>,
+}
+
+impl NullCompositor {
+
+    fn new(port: Port<Msg>) -> NullCompositor {
+
+        NullCompositor {
+            port: port
+        }
+    }
+
+    pub fn create(port: Port<Msg>, constellation_chan: ConstellationChan) {
+        let compositor = NullCompositor::new(port);
+
         // Tell the constellation about the initial fake size.
         constellation_chan.send(ResizedWindowMsg(Size2D(640u, 480u)));
+        compositor.handle_message();
+    }
 
+    fn handle_message(&self) {
         loop {
-        match port.recv() {
+            match self.port.recv() {
                 Exit => break,
 
                 GetGraphicsMetadata(chan) => {
@@ -38,4 +58,5 @@ pub fn run_compositor(constellation_chan: &ConstellationChan, port: Port<Msg>) {
                     => ()
             }
         }
+    }
 }