signalTracker: Add TransientSignalHolder class

There are cases where we want to connect to a number of signals
for the lifetime of an object, but also other signals for a
limited period (say: between show and hide).

It is currently not possible to use disconnectObject() for the
latter, because it will disconnect all signals.

To address this use case, add a small class that can be used as
a transient signal holder, while still benefiting from autocleanup
by proxying the real owner.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2221>

tests/unit/signalTracker.js

@@ -8,6 +8,8 @@ const JsUnit = imports.jsUnit;
 const Signals = imports.signals;
 
 const Environment = imports.ui.environment;
+const { TransientSignalHolder } = imports.misc.signalTracker;
+
 Environment.init();
 
 const Destroyable = GObject.registerClass({
@@ -77,3 +79,39 @@ emitter1.emit('signal');
 emitter2.emit('signal');
 
 JsUnit.assertEquals(count, 10);
+
+emitter1.connectObject('signal', handler, tracked1);
+emitter2.connectObject('signal', handler, tracked1);
+
+transientHolder = new TransientSignalHolder(tracked1);
+
+emitter1.connectObject('signal', handler, transientHolder);
+emitter2.connectObject('signal', handler, transientHolder);
+
+emitter1.emit('signal');
+emitter2.emit('signal');
+
+JsUnit.assertEquals(count, 14);
+
+transientHolder.destroy();
+
+emitter1.emit('signal');
+emitter2.emit('signal');
+
+JsUnit.assertEquals(count, 16);
+
+transientHolder = new TransientSignalHolder(tracked1);
+
+emitter1.connectObject('signal', handler, transientHolder);
+emitter2.connectObject('signal', handler, transientHolder);
+
+emitter1.emit('signal');
+emitter2.emit('signal');
+
+JsUnit.assertEquals(count, 20);
+
+tracked1.emit('destroy');
+emitter1.emit('signal');
+emitter2.emit('signal');
+
+JsUnit.assertEquals(count, 20);