arc: Revamp the ArcBridgeService interface

components/arc/instance_holder.h

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef COMPONENTS_ARC_INSTANCE_HOLDER_H_
+#define COMPONENTS_ARC_INSTANCE_HOLDER_H_
+
+#include <utility>
+
+#include "base/bind.h"
+#include "base/macros.h"
+#include "base/memory/weak_ptr.h"
+#include "base/observer_list.h"
+#include "base/threading/thread_checker.h"
+#include "mojo/public/cpp/bindings/interface_ptr.h"
+
+namespace arc {
+
+// Holds a Mojo instance+version pair. This also allows for listening for state
+// changes for the particular instance. T should be a Mojo interface type
+// (arc::mojom::XxxInstance).
+template <typename T>
+class InstanceHolder {
+ public:
+  // Notifies about connection events for individual instances.
+  class Observer {
+   public:
+    // Called once the instance is ready.
+    virtual void OnInstanceReady() {}
+
+    // Called when the connection to the instance is closed.
+    virtual void OnInstanceClosed() {}
+
+   protected:
+    virtual ~Observer() = default;
+  };
+
+  InstanceHolder() : weak_factory_(this) {}
+
+  // Gets the Mojo interface for all the instance services. This will return
+  // nullptr if that particular service is not ready yet. Use an Observer if you
+  // want to be notified when this is ready. This can only be called on the
+  // thread that this class was created on.
+  T* instance() const { return raw_ptr_; }
+  uint32_t version() const { return version_; }
+
+  // Adds or removes observers. This can only be called on the thread that this
+  // class was created on. RemoveObserver does nothing if |observer| is not in
+  // the list.
+  void AddObserver(Observer* observer) {
+    DCHECK(thread_checker_.CalledOnValidThread());
+    observer_list_.AddObserver(observer);
+
+    if (instance())
+      observer->OnInstanceReady();
+  }
+
+  void RemoveObserver(Observer* observer) {
+    DCHECK(thread_checker_.CalledOnValidThread());
+    observer_list_.RemoveObserver(observer);
+  }
+
+  // Called when the channel is closed.
+  void CloseChannel() {
+    if (!ptr_)
+      return;
+
+    ptr_.reset();
+    raw_ptr_ = nullptr;
+    version_ = 0;
+    if (observer_list_.might_have_observers()) {
+      typename base::ObserverList<Observer>::Iterator it(&observer_list_);
+      Observer* obs;
+      while ((obs = it.GetNext()) != nullptr)
+        obs->OnInstanceClosed();
+    }
+  }
+
+  // Sets the interface pointer to |ptr|, once the version is determined. This
+  // will eventually invoke SetInstance(), which will notify the observers.
+  void OnInstanceReady(mojo::InterfacePtr<T> ptr) {
+    temporary_ptr_ = std::move(ptr);
+    temporary_ptr_.QueryVersion(base::Bind(&InstanceHolder<T>::OnVersionReady,
+                                           weak_factory_.GetWeakPtr()));
+  }
+
+  // This method is not intended to be called directly. Normally it is called by
+  // OnInstanceReady once the version of the instance is determined, but it is
+  // also exposed so that tests can directly inject a raw pointer+version
+  // combination.
+  void SetInstance(T* raw_ptr, uint32_t raw_version = T::Version_) {
+    raw_ptr_ = raw_ptr;
+    version_ = raw_version;
+    if (observer_list_.might_have_observers()) {
+      typename base::ObserverList<Observer>::Iterator it(&observer_list_);
+      Observer* obs;
+      while ((obs = it.GetNext()) != nullptr)
+        obs->OnInstanceReady();
+    }
+  }
+
+ private:
+  void OnVersionReady(uint32_t version) {
+    ptr_ = std::move(temporary_ptr_);
+    ptr_.set_connection_error_handler(base::Bind(
+        &InstanceHolder<T>::CloseChannel, weak_factory_.GetWeakPtr()));
+    SetInstance(ptr_.get(), version);
+  }
+
+  // These two are copies of the contents of ptr_. They are provided here just
+  // so that tests can provide non-mojo implementations.
+  T* raw_ptr_ = nullptr;
+  uint32_t version_ = 0;
+
+  mojo::InterfacePtr<T> ptr_;
+
+  // Temporary Mojo interfaces.  After a Mojo interface pointer has been
+  // received from the other endpoint, we still need to asynchronously query its
+  // version.  While that is going on, we should still return nullptr on the
+  // instance() function.
+  // To keep the instance() functions being trivial, store the instance pointer
+  // in a temporary variable to avoid losing its reference.
+  mojo::InterfacePtr<T> temporary_ptr_;
+
+  base::ThreadChecker thread_checker_;
+  base::ObserverList<Observer> observer_list_;
+
+  // This needs to be the last member in order to cancel all inflight callbacks
+  // before destroying any other members.
+  base::WeakPtrFactory<InstanceHolder<T>> weak_factory_;
+
+  DISALLOW_COPY_AND_ASSIGN(InstanceHolder<T>);
+};
+
+}  // namespace arc
+
+#endif  // COMPONENTS_ARC_INSTANCE_HOLDER_H_