TaskScheduler [3/9] Task and Sequence

base/task_scheduler/task_traits.h

 // Copyright 2016 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 BASE_TASK_SCHEDULER_TASK_TRAITS_H_
 #define BASE_TASK_SCHEDULER_TASK_TRAITS_H_
 
+#include <stdint.h>
+
+#include <iosfwd>
+
 #include "base/base_export.h"
 #include "build/build_config.h"
 
 namespace base {
 
-using TaskPriorityUnderlyingType = char;
+namespace internal {
+using TaskPriorityUnderlyingType = uint8_t;
+}  // namespace internal
 
-// Valid priorities supported by the task scheduler.
-enum class TaskPriority : TaskPriorityUnderlyingType {
-  // This task affects UI immediately after a user interaction.
-  // Example: Generating data shown in the UI immediately after a click.
-  USER_BLOCKING = 2,
+// Valid priorities supported by the task scheduler. Note: internal algorithms
+// depend on priorities being expressed as a continuous zero-based list from
+// lowest to highest priority. Users of this API shouldn't otherwise care about
+// nor use the underlying values.
+enum class TaskPriority : internal::TaskPriorityUnderlyingType {

[danakj, 2016/03/15 23:16:32]

Is there a reason why you need to define the UnderlyingType and use that
explicitly, instead of just static_cast to "int" when you want to use it that
way?

[fdoray, 2016/03/16 19:11:44]

static_cast to int is fine.

+  // This will always be equal to the lowest priority available.
+  LOWEST = 0,
+  // User won't notice if this task takes an arbitrarily long time to complete.
+  BACKGROUND = LOWEST,
   // This task affects UI or responsiveness of future user interactions. It is
   // not an immediate response to a user interaction.
   // Examples:
   // - Updating the UI to reflect progress on a long task.
   // - Loading data that might be shown in the UI after a future user
   //   interaction.
-  USER_VISIBLE = 1,
-  // Everything else (user won't notice if this takes an arbitrarily long time
-  // to complete).
-  BACKGROUND = 0,
+  USER_VISIBLE,
+  // This task affects UI immediately after a user interaction.
+  // Example: Generating data shown in the UI immediately after a click.
+  USER_BLOCKING,
+  // This will always be equal to the highest priority available.
+  HIGHEST = USER_BLOCKING,
 };
 
-const TaskPriorityUnderlyingType kNumTaskPriorities = 3;
+namespace internal {
+const size_t kNumTaskPriorities =
+    static_cast<TaskPriorityUnderlyingType>(TaskPriority::HIGHEST) + 1;

[danakj, 2016/03/15 23:16:32]

use an enum to put a static const in a header file until we get constexptr (to
avoid weird storage concerns - see chromium-dev about it, or chromium style
guide)

[fdoray, 2016/03/16 19:11:44]

I removed this constant. I use static_cast<int>(TaskPriority::HIGHEST) + 1
directly when needed now.

+}  // namespace internal
 
 // Valid shutdown behaviors supported by the task scheduler.
 enum class TaskShutdownBehavior {
   // Tasks posted with this mode which have not started executing before
   // shutdown is initiated will never run. Tasks with this mode running at
   // shutdown will be ignored (the worker thread will not be joined).
   //
   // This option provides a nice way to post stuff you don't want blocking
   // shutdown. For example, you might be doing a slow DNS lookup and if it's
   // blocked on the OS, you may not want to stop shutdown, since the result
@@ skipping 67 matching lines @@
   PARALLEL,
 
   // Executes one task at a time in posting order. The sequence’s priority is
   // equivalent to the highest priority pending task in the sequence.
   SEQUENCED,
 
   // Executes one task at a time on a single thread in posting order.
   SINGLE_THREADED,
 };
 
+// Pretty Printer for Google Test.
+void BASE_EXPORT PrintTo(const TaskPriority& task_priority, std::ostream* os);
+
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_TRAITS_H_