Bug 1495386 - emit event when target scoped front is created; r=ochameau

this change only impacts the target front, and emits an event every time a target is
created. No listners yet.

Differential Revision: https://phabricator.services.mozilla.com/D8084

--HG--
extra : moz-landing-system : lando

devtools/client/framework/target.js

@@ -388,6 +388,16 @@ Target.prototype = {
     return this._inspector;
   },
 
+  // Run callback on every front of this type that currently exists, and on every
+  // instantiation of front type in the future.
+  onFront(typeName, callback) {
+    const front = this.fronts.get(typeName);
+    if (front) {
+      return callback(front);
+    }
+    return this.on(typeName, callback);
+  },
+
   // Get a Front for a target-scoped actor.
   // i.e. an actor served by RootActor.listTabs or RootActorActor.getTab requests
   getFront(typeName) {
@@ -397,6 +407,7 @@ Target.prototype = {
       return front;
     }
     front = getFront(this.client, typeName, this.form);
+    this.emit(typeName, front);
     this.fronts.set(typeName, front);
     return front;
   },

devtools/client/framework/test/browser.ini

@@ -77,6 +77,7 @@ skip-if = os == 'win' || debug # Bug 1282269, 1448084
 [browser_source_map-late-script.js]
 [browser_target_from_url.js]
 [browser_target_events.js]
+[browser_target_listeners.js]
 [browser_target_remote.js]
 [browser_target_support.js]
 [browser_toolbox_custom_host.js]

devtools/client/framework/test/browser_target_listeners.js

@@ -0,0 +1,26 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+add_task(async function() {
+  gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
+  await BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser);
+
+  const target = await TargetFactory.forTab(gBrowser.selectedTab);
+  await target.attach();
+
+  info("Test applying onFront to a front that will be created");
+  const promise = new Promise(resolve => {
+    target.onFront("accessibility", resolve);
+  });
+  const getFrontFront = target.getFront("accessibility");
+  const onFrontFront = await promise;
+  is(getFrontFront, onFrontFront, "got the front instantiated in the future and it's the same");
+
+  info("Test applying onFront to an existing front");
+  await new Promise(resolve => {
+    target.onFront("accessibility", front => {
+      is(front, getFrontFront, "got the already instantiated front and it's the same");
+      resolve();
+    });
+  });
+});