[notifier] add Update method (#4274)

This commit adds a new `Notifier::Update()` method which performs a
commonly used pattern in the code where a variable is updated with a
new given value and if the variable value does change a related
notifier `OT_CHANGED_<NAME>` is signaled.

src/core/common/notifier.hpp

@@ -176,6 +176,40 @@ public:
      */
     bool HasSignaled(otChangedFlags aFlags) const { return (mSignaledFlags & aFlags) == aFlags; }
 
+    /**
+     * This template method updates a variable of a type `Type` with a new value and signals the given changed flags.
+     *
+     * If the variable is already set to the same value, this method returns `OT_ERROR_ALREADY` and the changed flags
+     * is signaled using `SignalIfFirst()` (i.e. signal is scheduled only if the flag has not been signaled before).
+     *
+     * The template `Type` should support comparison operator `==` and assignment operator `=`.
+     *
+     * @param[inout] aVariable    A reference to the variable to update.
+     * @param[in]    aNewValue    The new value.
+     * @param[in]    aFlags       The changed flags to signal.
+     *
+     * @retval OT_ERROR_NONE      The variable was update successfully and @p aFlags was signaled.
+     * @retval OT_ERROR_ALREADY   The variable was already set to the same value.
+     *
+     */
+    template <typename Type> otError Update(Type &aVariable, const Type &aNewValue, otChangedFlags aFlags)
+    {
+        otError error = OT_ERROR_NONE;
+
+        if (aVariable == aNewValue)
+        {
+            SignalIfFirst(aFlags);
+            error = OT_ERROR_ALREADY;
+        }
+        else
+        {
+            aVariable = aNewValue;
+            Signal(aFlags);
+        }
+
+        return error;
+    }
+
 private:
     enum
     {

src/core/mac/mac.cpp

@@ -373,14 +373,11 @@ otError Mac::SetPanChannel(uint8_t aChannel)
 
     VerifyOrExit(mSupportedChannelMask.ContainsChannel(aChannel), error = OT_ERROR_INVALID_ARGS);
 
-    VerifyOrExit(mPanChannel != aChannel, Get<Notifier>().SignalIfFirst(OT_CHANGED_THREAD_CHANNEL));
+    SuccessOrExit(Get<Notifier>().Update(mPanChannel, aChannel, OT_CHANGED_THREAD_CHANNEL));
 
-    mPanChannel = aChannel;
     mCcaSuccessRateTracker.Reset();
-    Get<Notifier>().Signal(OT_CHANGED_THREAD_CHANNEL);
 
     VerifyOrExit(!mRadioChannelAcquisitionId);
-
     mRadioChannel = mPanChannel;
 
     UpdateIdleMode();
@@ -441,13 +438,7 @@ void Mac::SetSupportedChannelMask(const ChannelMask &aMask)
     ChannelMask newMask = aMask;
 
     newMask.Intersect(ChannelMask(Get<Radio>().GetSupportedChannelMask()));
-    VerifyOrExit(newMask != mSupportedChannelMask, Get<Notifier>().SignalIfFirst(OT_CHANGED_SUPPORTED_CHANNEL_MASK));
-
-    mSupportedChannelMask = newMask;
-    Get<Notifier>().Signal(OT_CHANGED_SUPPORTED_CHANNEL_MASK);
-
-exit:
-    return;
+    Get<Notifier>().Update(mSupportedChannelMask, newMask, OT_CHANGED_SUPPORTED_CHANNEL_MASK);
 }
 
 otError Mac::SetNetworkName(const char *aNameString)
@@ -485,10 +476,8 @@ exit:
 
 void Mac::SetPanId(PanId aPanId)
 {
-    VerifyOrExit(mPanId != aPanId, Get<Notifier>().SignalIfFirst(OT_CHANGED_THREAD_PANID));
-    mPanId = aPanId;
+    SuccessOrExit(Get<Notifier>().Update(mPanId, aPanId, OT_CHANGED_THREAD_PANID));
     mSubMac.SetPanId(mPanId);
-    Get<Notifier>().Signal(OT_CHANGED_THREAD_PANID);
 
 exit:
     return;
@@ -496,13 +485,7 @@ exit:
 
 void Mac::SetExtendedPanId(const ExtendedPanId &aExtendedPanId)
 {
-    VerifyOrExit(mExtendedPanId != aExtendedPanId, Get<Notifier>().SignalIfFirst(OT_CHANGED_THREAD_EXT_PANID));
-
-    mExtendedPanId = aExtendedPanId;
-    Get<Notifier>().Signal(OT_CHANGED_THREAD_EXT_PANID);
-
-exit:
-    return;
+    Get<Notifier>().Update(mExtendedPanId, aExtendedPanId, OT_CHANGED_THREAD_EXT_PANID);
 }
 
 otError Mac::RequestDirectFrameTransmission(void)

src/core/thread/key_manager.cpp

@@ -102,11 +102,7 @@ void KeyManager::Stop(void)
 #if OPENTHREAD_MTD || OPENTHREAD_FTD
 void KeyManager::SetPskc(const Pskc &aPskc)
 {
-    VerifyOrExit(mPskc != aPskc, Get<Notifier>().SignalIfFirst(OT_CHANGED_PSKC));
-    mPskc = aPskc;
-    Get<Notifier>().Signal(OT_CHANGED_PSKC);
-
-exit:
+    Get<Notifier>().Update(mPskc, aPskc, OT_CHANGED_PSKC);
     mIsPskcSet = true;
 }
 #endif // OPENTHREAD_MTD || OPENTHREAD_FTD
@@ -116,9 +112,9 @@ otError KeyManager::SetMasterKey(const MasterKey &aKey)
     otError error = OT_ERROR_NONE;
     Router *parent;
 
-    VerifyOrExit(mMasterKey != aKey, Get<Notifier>().SignalIfFirst(OT_CHANGED_MASTER_KEY));
+    SuccessOrExit(
+        Get<Notifier>().Update(mMasterKey, aKey, OT_CHANGED_MASTER_KEY | OT_CHANGED_THREAD_KEY_SEQUENCE_COUNTER));
 
-    mMasterKey   = aKey;
     mKeySequence = 0;
     ComputeKey(mKeySequence, mKey);
 
@@ -144,8 +140,6 @@ otError KeyManager::SetMasterKey(const MasterKey &aKey)
         iter.GetChild()->SetMleFrameCounter(0);
     }
 
-    Get<Notifier>().Signal(OT_CHANGED_THREAD_KEY_SEQUENCE_COUNTER | OT_CHANGED_MASTER_KEY);
-
 exit:
     return error;
 }

src/core/thread/mle.cpp

@@ -316,12 +316,11 @@ exit:
 
 void Mle::SetRole(otDeviceRole aRole)
 {
-    VerifyOrExit(aRole != mRole, Get<Notifier>().SignalIfFirst(OT_CHANGED_THREAD_ROLE));
+    otDeviceRole oldRole = mRole;
 
-    otLogNoteMle("Role %s -> %s", RoleToString(mRole), RoleToString(aRole));
+    SuccessOrExit(Get<Notifier>().Update(mRole, aRole, OT_CHANGED_THREAD_ROLE));
 
-    mRole = aRole;
-    Get<Notifier>().Signal(OT_CHANGED_THREAD_ROLE);
+    otLogNoteMle("Role %s -> %s", RoleToString(oldRole), RoleToString(mRole));
 
     switch (mRole)
     {
@@ -354,7 +353,7 @@ void Mle::SetRole(otDeviceRole aRole)
 #endif
 
 exit:
-    return;
+    OT_UNUSED_VARIABLE(oldRole);
 }
 
 void Mle::SetAttachState(AttachState aState)