[tasklet] declare Scheduler as nested type of Tasklet (#6815)

This commit also inlines simple constructors.

src/core/api/tasklet_api.cpp

@@ -47,7 +47,7 @@ void otTaskletsProcess(otInstance *aInstance)
     Instance &instance = *static_cast<Instance *>(aInstance);
 
     VerifyOrExit(otInstanceIsInitialized(aInstance));
-    instance.Get<TaskletScheduler>().ProcessQueuedTasklets();
+    instance.Get<Tasklet::Scheduler>().ProcessQueuedTasklets();
 
 exit:
     return;
@@ -59,7 +59,7 @@ bool otTaskletsArePending(otInstance *aInstance)
     Instance &instance = *static_cast<Instance *>(aInstance);
 
     VerifyOrExit(otInstanceIsInitialized(aInstance));
-    retval = instance.Get<TaskletScheduler>().AreTaskletsPending();
+    retval = instance.Get<Tasklet::Scheduler>().AreTaskletsPending();
 
 exit:
     return retval;

src/core/common/instance.hpp

@@ -333,7 +333,7 @@ private:
     // Tasklet and Timer Schedulers are first to ensure other
     // objects/classes can use them from their constructors.
 
-    TaskletScheduler    mTaskletScheduler;
+    Tasklet::Scheduler  mTaskletScheduler;
     TimerMilliScheduler mTimerMilliScheduler;
 #if OPENTHREAD_CONFIG_PLATFORM_USEC_TIMER_ENABLE
     TimerMicroScheduler mTimerMicroScheduler;
@@ -945,7 +945,7 @@ template <> inline Mac::SubMac &Instance::Get(void)
 
 #endif // OPENTHREAD_RADIO || OPENTHREAD_CONFIG_LINK_RAW_ENABLE
 
-template <> inline TaskletScheduler &Instance::Get(void)
+template <> inline Tasklet::Scheduler &Instance::Get(void)
 {
     return mTaskletScheduler;
 }

src/core/common/tasklet.cpp

@@ -34,34 +34,19 @@
 #include "tasklet.hpp"
 
 #include "common/code_utils.hpp"
-#include "common/debug.hpp"
-#include "common/instance.hpp"
 #include "common/locator_getters.hpp"
-#include "net/ip6.hpp"
 
 namespace ot {
 
-Tasklet::Tasklet(Instance &aInstance, Handler aHandler)
-    : InstanceLocator(aInstance)
-    , mHandler(aHandler)
-    , mNext(nullptr)
-{
-}
-
 void Tasklet::Post(void)
 {
     if (!IsPosted())
     {
-        Get<TaskletScheduler>().PostTasklet(*this);
+        Get<Scheduler>().PostTasklet(*this);
     }
 }
 
-TaskletScheduler::TaskletScheduler(void)
-    : mTail(nullptr)
-{
-}
-
-void TaskletScheduler::PostTasklet(Tasklet &aTasklet)
+void Tasklet::Scheduler::PostTasklet(Tasklet &aTasklet)
 {
     // Tasklets are saved in a circular singly linked list.
 
@@ -79,7 +64,7 @@ void TaskletScheduler::PostTasklet(Tasklet &aTasklet)
     }
 }
 
-void TaskletScheduler::ProcessQueuedTasklets(void)
+void Tasklet::Scheduler::ProcessQueuedTasklets(void)
 {
     Tasklet *tail = mTail;
 

src/core/common/tasklet.hpp

@@ -63,9 +63,46 @@ class TaskletScheduler;
  */
 class Tasklet : public InstanceLocator
 {
-    friend class TaskletScheduler;
-
 public:
+    /**
+     * This class implements the tasklet scheduler.
+     *
+     */
+    class Scheduler : private NonCopyable
+    {
+        friend class Tasklet;
+
+    public:
+        /**
+         * This constructor initializes the object.
+         *
+         */
+        Scheduler(void)
+            : mTail(nullptr)
+        {
+        }
+
+        /**
+         * This method indicates whether or not there are tasklets pending.
+         *
+         * @retval TRUE   If there are tasklets pending.
+         * @retval FALSE  If there are no tasklets pending.
+         *
+         */
+        bool AreTaskletsPending(void) const { return mTail != nullptr; }
+
+        /**
+         * This method processes all tasklets queued when this is called.
+         *
+         */
+        void ProcessQueuedTasklets(void);
+
+    private:
+        void PostTasklet(Tasklet &aTasklet);
+
+        Tasklet *mTail; // A circular singly linked-list
+    };
+
     /**
      * This function reference is called when the tasklet is run.
      *
@@ -81,7 +118,12 @@ public:
      * @param[in]  aHandler    A pointer to a function that is called when the tasklet is run.
      *
      */
-    Tasklet(Instance &aInstance, Handler aHandler);
+    Tasklet(Instance &aInstance, Handler aHandler)
+        : InstanceLocator(aInstance)
+        , mHandler(aHandler)
+        , mNext(nullptr)
+    {
+    }
 
     /**
      * This method puts the tasklet on the tasklet scheduler run queue.
@@ -145,42 +187,6 @@ private:
     void *mContext;
 };
 
-/**
- * This class implements the tasklet scheduler.
- *
- */
-class TaskletScheduler : private NonCopyable
-{
-    friend class Tasklet;
-
-public:
-    /**
-     * This constructor initializes the object.
-     *
-     */
-    TaskletScheduler(void);
-
-    /**
-     * This method indicates whether or not there are tasklets pending.
-     *
-     * @retval TRUE   If there are tasklets pending.
-     * @retval FALSE  If there are no tasklets pending.
-     *
-     */
-    bool AreTaskletsPending(void) const { return mTail != nullptr; }
-
-    /**
-     * This method processes all tasklets queued when this is called.
-     *
-     */
-    void ProcessQueuedTasklets(void);
-
-private:
-    void PostTasklet(Tasklet &aTasklet);
-
-    Tasklet *mTail; // A circular singly linked-list
-};
-
 /**
  * @}
  *