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 {
+  // 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;
-
 // 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
   // doesn't really matter at that point.
@@ skipping 66 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);

[robliao, 2016/02/19 02:33:43]

Can this go in a test only utils file?
https://code.google.com/p/chromium/codesearch#chromium/src/ui/gfx/test/gfx_ut...
is an example of this.

[fdoray, 2016/02/19 14:12:15]

It was in a test only file, but gab asked me to move it here to match
string16.h. A benefit of having this here is that we don't have to think about
including the test-only file to get pretty-printing. What should I do now?

[gab, 2016/02/19 16:50:47]

Keep it here is my vote. string16 does it, it won't affect non-test binary size
as it won't be linked in, and we can just use TaskPriority comparison checks
without thinking about an extra include. IMO this goes alongside the type.

The only other option IMO is to define it in the test cc that needs it if there
is only one (and even then I still prefer this), but a separate header I say no.

[fdoray, 2016/02/19 22:28:29]

ok, keeping the declaration here.

+
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_TRAITS_H_