Add callbackHandle.cancel unittest

Author: dcarp

src/asynchronous/events.d

@@ -83,9 +83,15 @@ if (isDelegate!Dg)
         }
     }
 
+    /**
+     * Cancel the call. If the callback is already canceled or executed, this
+     * method has no effect.
+     */
     override void cancel()
     {
         this.cancelled = true;
+        this.dg = null;
+        this.args = Args.init;
     }
 
     protected override void opCallImpl()

test/events.d

@@ -17,6 +17,42 @@ unittest
     assert(fooCalled);
 }
 
+unittest
+{
+    auto loop = getEventLoop;
+    int bar = 0;
+
+    void foo(int i)
+    {
+        bar = i;
+        loop.stop;
+    }
+
+    loop.callSoon(&foo, 10);
+    assert(bar == 0);
+    loop.runForever;
+    assert(bar == 10);
+}
+
+unittest
+{
+    auto loop = getEventLoop;
+    int bar = 0;
+
+    void foo(int i)
+    {
+        bar += i;
+        loop.stop;
+    }
+
+    auto h1 = loop.callSoon(&foo, 10);
+    auto h2 = loop.callSoon(&foo, 15);
+    assert(bar == 0);
+    h1.cancel;
+    loop.runForever;
+    assert(bar == 15);
+}
+
 unittest
 {
     import std.datetime : msecs;