TaskScheduler [3/9] Task and Sequence

base/task_scheduler/task_traits.h

Index: base/task_scheduler/task_traits.h
diff --git a/base/task_scheduler/task_traits.h b/base/task_scheduler/task_traits.h
index 8b564f44f5a5cdb8477ac9e2f4966d807e1e2204..fb9ac280da32ebb4b2f0fba671bfa7a2401aa84c 100644
--- a/base/task_scheduler/task_traits.h
+++ b/base/task_scheduler/task_traits.h
@@ -5,32 +5,42 @@
 #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;
-
-// 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,
+namespace internal {
+using TaskPriorityUnderlyingType = uint8_t;
+}  // namespace internal
+
+// 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
@@ -117,6 +127,9 @@ enum class ExecutionMode {
   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_