Bug 1508660 - introduce getCachedFront; r=ochameau

Depends on D12439

Differential Revision: https://phabricator.services.mozilla.com/D13110

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

devtools/client/definitions.js

@@ -593,10 +593,17 @@ function createHighlightButton(highlighterName, id) {
       return highlighter.show({});
     },
     isChecked(toolbox) {
-      if (!toolbox.inspector) {
+      // if the inspector doesn't exist, then the highlighter has not yet been connected
+      // to the front end.
+      const inspectorFront = toolbox.target.getCachedFront("inspector");
+      if (!inspectorFront) {
+        // initialize the inspector front asyncronously. There is a potential for buggy
+        // behavior here, but we need to change how the buttons get data (have them
+        // consume data from reducers rather than writing our own version) in order to
+        // fix this properly.
         return false;
       }
-      const highlighter = toolbox.inspector.getKnownHighlighter(highlighterName);
+      const highlighter = inspectorFront.getKnownHighlighter(highlighterName);
       return highlighter && highlighter.isShown();
     },
   };

devtools/client/framework/target.js

@@ -398,11 +398,24 @@ Target.prototype = {
       return front;
     }
     front = getFront(this.client, typeName, this.form);
+    this.fronts.set(typeName, front);
+    // replace the placeholder with the instance of the front once it has loaded
+    front = await front;
     this.emit(typeName, front);
     this.fronts.set(typeName, front);
     return front;
   },
 
+  getCachedFront(typeName) {
+    // do not wait for async fronts;
+    const front = this.fronts.get(typeName);
+    // ensure that the front is a front, and not async front
+    if (front && front.actorID) {
+      return front;
+    }
+    return null;
+  },
+
   get client() {
     return this._client;
   },

devtools/client/framework/test/browser.ini

@@ -76,6 +76,7 @@ skip-if = os == 'win' || debug # Bug 1282269, 1448084
 [browser_source_map-reload.js]
 [browser_source_map-late-script.js]
 [browser_target_from_url.js]
+[browser_target_cached-front.js]
 [browser_target_events.js]
 [browser_target_remote.js]
 [browser_target_support.js]

devtools/client/framework/test/browser_target_cached-front.js

@@ -0,0 +1,24 @@
+/* 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("Cached front when getFront has not been called");
+  let getCachedFront = target.getCachedFront("accessibility");
+  ok(!getCachedFront, "no front exists");
+
+  info("Cached front when getFront has been called but has not finished");
+  const asyncFront = target.getFront("accessibility");
+  getCachedFront = target.getCachedFront("accessibility");
+  ok(!getCachedFront, "no front exists");
+
+  info("Cached front when getFront has been called and has finished");
+  const front = await asyncFront;
+  getCachedFront = target.getCachedFront("accessibility");
+  is(getCachedFront, front, "front is the same as async front");
+});