[CacheStorage] Simpler way to wrap callbacks to run pending operation completions

content/browser/cache_storage/cache_storage_scheduler.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 CONTENT_BROWSER_CACHE_STORAGE_CACHE_STORAGE_SCHEDULER_H_
 #define CONTENT_BROWSER_CACHE_STORAGE_CACHE_STORAGE_SCHEDULER_H_
 
 #include <list>
 
+#include "base/bind.h"
 #include "base/callback.h"
 #include "base/macros.h"
+#include "base/memory/weak_ptr.h"
 #include "content/common/content_export.h"
 
 namespace content {
 
 // TODO(jkarlin): Support readers and writers so operations can run in parallel.
 // TODO(jkarlin): Support operation identification so that ops can be checked in
 // DCHECKs.
 
 // CacheStorageScheduler runs the scheduled callbacks sequentially. Add an
 // operation by calling ScheduleOperation() with your callback. Once your
 // operation is done be sure to call CompleteOperationAndRunNext() to schedule
 // the next operation.
 class CONTENT_EXPORT CacheStorageScheduler {
  public:
   CacheStorageScheduler();
   virtual ~CacheStorageScheduler();
 
   // Adds the operation to the tail of the queue and starts it if the scheduler
   // is idle.
   void ScheduleOperation(const base::Closure& closure);
 
   // Call this after each operation completes. It cleans up the current
   // operation and starts the next.
   void CompleteOperationAndRunNext();
 
   // Returns true if there are any running or pending operations.
   bool ScheduledOperations() const;
 
+  // Wraps |callback| to first call CompleteOperationAndRunNext before running.
+  template <typename R, typename... Args>
+  base::Callback<R(Args...)> WrapCallbackToRunNext(
+      const base::Callback<R(Args...)>& callback) {
+    return base::Bind(&CacheStorageScheduler::RunNextContinuation<R, Args...>,
+                      weak_ptr_factory_.GetWeakPtr(), callback);
+  }
+
  private:
   void RunOperationIfIdle();
 
+  template <typename R, typename... Args>
+  R RunNextContinuation(const base::Callback<R(Args...)>& callback,

[michaeln, 2016/07/19 23:45:25]

should this function return a value of type 'R'? how does this compile? R must
always be void. seems like it from the callsiites.

would this template work with just parametrizing Args?

template <typename... Args>
void RunNextContinuation(const base::Callback<void(Args...)>& ...

ditto for the WrapCallback method?

[jkarlin, 2016/07/20 00:51:50]

Good catch on specifying void. Done.

+                        Args... args) {
+    // Grab a weak ptr to guard against the scheduler being deleted during the
+    // callback.
+    base::WeakPtr<CacheStorageScheduler> scheduler =
+        weak_ptr_factory_.GetWeakPtr();
+
+    callback.Run(std::forward<Args>(args)...);
+    if (scheduler)
+      CompleteOperationAndRunNext();
+  }
+
   // The list of operations waiting on initialization.
   std::list<base::Closure> pending_operations_;
   bool operation_running_;
+  base::WeakPtrFactory<CacheStorageScheduler> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(CacheStorageScheduler);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_CACHE_STORAGE_CACHE_STORAGE_SCHEDULER_H_