1 files changed, 179 insertions, 0 deletions
diff --git a/mobile/android/modules/geckoview/DelayedInit.jsm b/mobile/android/modules/geckoview/DelayedInit.jsm
new file mode 100644
index 0000000000..9465b60892
--- /dev/null
+++ b/mobile/android/modules/geckoview/DelayedInit.jsm
@@ -0,0 +1,179 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+/* globals MessageLoop */
+
+var EXPORTED_SYMBOLS = ["DelayedInit"];
+
+/**
+ * Use DelayedInit to schedule initializers to run some time after startup.
+ * Initializers are added to a list of pending inits. Whenever the main thread
+ * message loop is idle, DelayedInit will start running initializers from the
+ * pending list. To prevent monopolizing the message loop, every idling period
+ * has a maximum duration. When that's reached, we give up the message loop and
+ * wait for the next idle.
+ *
+ * DelayedInit is compatible with lazy getters like those from XPCOMUtils. When
+ * the lazy getter is first accessed, its corresponding initializer is run
+ * automatically if it hasn't been run already. Each initializer also has a
+ * maximum wait parameter that specifies a mandatory timeout; when the timeout
+ * is reached, the initializer is forced to run.
+ *
+ *   DelayedInit.schedule(() => Foo.init(), null, null, 5000);
+ *
+ * In the example above, Foo.init will run automatically when the message loop
+ * becomes idle, or when 5000ms has elapsed, whichever comes first.
+ *
+ *   DelayedInit.schedule(() => Foo.init(), this, "Foo", 5000);
+ *
+ * In the example above, Foo.init will run automatically when the message loop
+ * becomes idle, when |this.Foo| is accessed, or when 5000ms has elapsed,
+ * whichever comes first.
+ *
+ * It may be simpler to have a wrapper for DelayedInit.schedule. For example,
+ *
+ *   function InitLater(fn, obj, name) {
+ *     return DelayedInit.schedule(fn, obj, name, 5000); // constant max wait
+ *   }
+ *   InitLater(() => Foo.init());
+ *   InitLater(() => Bar.init(), this, "Bar");
+ */
+var DelayedInit = {
+  schedule(fn, object, name, maxWait) {
+    return Impl.scheduleInit(fn, object, name, maxWait);
+  },
+
+  scheduleList(fns, maxWait) {
+    for (const fn of fns) {
+      Impl.scheduleInit(fn, null, null, maxWait);
+    }
+  },
+};
+
+// Maximum duration for each idling period. Pending inits are run until this
+// duration is exceeded; then we wait for next idling period.
+const MAX_IDLE_RUN_MS = 50;
+
+var Impl = {
+  pendingInits: [],
+
+  onIdle() {
+    const startTime = Cu.now();
+    let time = startTime;
+    let nextDue;
+
+    // Go through all the pending inits. Even if we don't run them,
+    // we still need to find out when the next timeout should be.
+    for (const init of this.pendingInits) {
+      if (init.complete) {
+        continue;
+      }
+
+      if (time - startTime < MAX_IDLE_RUN_MS) {
+        init.maybeInit();
+        time = Cu.now();
+      } else {
+        // We ran out of time; find when the next closest due time is.
+        nextDue = nextDue ? Math.min(nextDue, init.due) : init.due;
+      }
+    }
+
+    // Get rid of completed ones.
+    this.pendingInits = this.pendingInits.filter(init => !init.complete);
+
+    if (nextDue !== undefined) {
+      // Schedule the next idle, if we still have pending inits.
+      ChromeUtils.idleDispatch(() => this.onIdle(), {
+        timeout: Math.max(0, nextDue - time),
+      });
+    }
+  },
+
+  addPendingInit(fn, wait) {
+    const init = {
+      fn,
+      due: Cu.now() + wait,
+      complete: false,
+      maybeInit() {
+        if (this.complete) {
+          return false;
+        }
+        this.complete = true;
+        this.fn.call();
+        this.fn = null;
+        return true;
+      },
+    };
+
+    if (!this.pendingInits.length) {
+      // Schedule for the first idle.
+      ChromeUtils.idleDispatch(() => this.onIdle(), { timeout: wait });
+    }
+    this.pendingInits.push(init);
+    return init;
+  },
+
+  scheduleInit(fn, object, name, wait) {
+    const init = this.addPendingInit(fn, wait);
+
+    if (!object || !name) {
+      // No lazy getter needed.
+      return;
+    }
+
+    // Get any existing information about the property.
+    let prop = Object.getOwnPropertyDescriptor(object, name) || {
+      configurable: true,
+      enumerable: true,
+      writable: true,
+    };
+
+    if (!prop.configurable) {
+      // Object.defineProperty won't work, so just perform init here.
+      init.maybeInit();
+      return;
+    }
+
+    // Define proxy getter/setter that will call first initializer first,
+    // before delegating the get/set to the original target.
+    Object.defineProperty(object, name, {
+      get: function proxy_getter() {
+        init.maybeInit();
+
+        // If the initializer actually ran, it may have replaced our proxy
+        // property with a real one, so we need to reload he property.
+        const newProp = Object.getOwnPropertyDescriptor(object, name);
+        if (newProp.get !== proxy_getter) {
+          // Set prop if newProp doesn't refer to our proxy property.
+          prop = newProp;
+        } else {
+          // Otherwise, reset to the original property.
+          Object.defineProperty(object, name, prop);
+        }
+
+        if (prop.get) {
+          return prop.get.call(object);
+        }
+        return prop.value;
+      },
+      set(newVal) {
+        init.maybeInit();
+
+        // Since our initializer already ran,
+        // we can get rid of our proxy property.
+        if (prop.get || prop.set) {
+          Object.defineProperty(object, name, prop);
+          prop.set.call(object);
+          return;
+        }
+
+        prop.value = newVal;
+        Object.defineProperty(object, name, prop);
+      },
+      configurable: true,
+      enumerable: true,
+    });
+  },
+};