diff --git a/cwFlow.cpp b/cwFlow.cpp
index b32f2c9..348bb75 100644
--- a/cwFlow.cpp
+++ b/cwFlow.cpp
@@ -754,6 +754,37 @@ namespace cw
       return rc;
     }
 
+    void _make_flow_to_ui_callback( flow_t* p )
+    {
+      
+      // There is no concurrent contention for the linked list when
+      // this function is called and so all accesses use relaxed memory order.
+
+      // Get the first variable to send to the UI
+      variable_t* var = p->ui_var_tail->ui_var_link.load(std::memory_order_relaxed);
+      
+      while( var!=nullptr)
+      {
+        // Send the var to the UI
+        if( p->ui_callback != nullptr )
+          p->ui_callback( p->ui_callback_arg, var->ui_var );
+
+        // Get the next var to send to the UI
+        variable_t* var0 = var->ui_var_link.load(std::memory_order_relaxed);
+
+        // Nullify the list links as they are used
+        var->ui_var_link.store(nullptr,std::memory_order_relaxed);
+        
+        var = var0;
+      }
+
+      // Empty the UI message list.
+      p->ui_var_head.store(&p->ui_var_stub,std::memory_order_relaxed);
+      p->ui_var_tail    = &p->ui_var_stub;
+      p->ui_var_stub.ui_var_link.store(nullptr,std::memory_order_relaxed);
+      
+    }
+    
   }
 }
 
@@ -776,7 +807,9 @@ cw::rc_t cw::flow::create( handle_t&          hRef,
                            const object_t*    classCfg,
                            const object_t*    pgmCfg,
                            const object_t*    udpCfg,
-                           const char*        proj_dir )
+                           const char*        proj_dir,
+                           ui_callback_t      ui_callback,
+                           void*              ui_callback_arg)
 {
   rc_t            rc               = kOkRC;
   bool            printClassDictFl = false;
@@ -810,6 +843,10 @@ cw::rc_t cw::flow::create( handle_t&          hRef,
   p->maxCycleCount  = kInvalidCnt;
   p->proj_dir       = proj_dir;
   p->printLogHdrFl  = true;
+  p->ui_callback    = ui_callback;
+  p->ui_callback_arg= ui_callback_arg;
+  p->ui_var_head.store(&p->ui_var_stub);
+  p->ui_var_tail    = &p->ui_var_stub;
   
   // parse the optional args
   if((rc = pgmCfg->readv("network",               0,      p->networkCfg,
@@ -889,7 +926,8 @@ const char* cw::flow::preset_label( handle_t h, unsigned preset_idx )
   
   if( preset_idx >= p->presetN )
   {
-    
+    cwLogError(kInvalidArgRC,"The preset index %i is invalid.",preset_idx);
+    return nullptr;
   }
   
   return p->presetA[ preset_idx ].label;
@@ -1032,9 +1070,19 @@ cw::rc_t cw::flow::exec_cycle( handle_t h )
   }
   else
   {
-    rc = exec_cycle(*p->net);
+    // Execute one cycle of the network
+    if((rc = exec_cycle(*p->net)) == kOkRC )
+    {
+      // During network execution variables which need to update the UI
+      // are collected in a linked list based on p->ui_var_tail.
+      // Callback to the UI with those variables here.
+      _make_flow_to_ui_callback(p);
+
+    }
+
     p->cycleIndex += 1;
   }
+
     
   return rc;
 }
@@ -1069,6 +1117,8 @@ cw::rc_t cw::flow::apply_preset( handle_t h, const multi_preset_selector_t& mps
   return network_apply_preset(*p->net,mps);
 }
 
+cw::rc_t cw::flow::set_variable_user_id( handle_t h, const ui_var_t* ui_var, unsigned user_id )
+{ return set_variable_user_id( *_handleToPtr(h)->net, ui_var, user_id );  }
 
 
 cw::rc_t cw::flow::set_variable_value( handle_t h, const char* inst_label, const char* var_label, unsigned chIdx, bool value )
diff --git a/cwFlow.h b/cwFlow.h
index 0be164d..1735d40 100644
--- a/cwFlow.h
+++ b/cwFlow.h
@@ -10,7 +10,7 @@ namespace cw
 
     void print_abuf( const struct abuf_str* abuf );
     void print_external_device( const external_device_t* dev );
-    
+
     // Parse the cfg's but don't yet instantiate the network.
     // Upon completion of this function the caller can 
     // query the network for configuration information which can
@@ -18,8 +18,10 @@ namespace cw
     rc_t create(handle_t&          hRef,
                 const object_t*    classCfg,              // processor class dictionary
                 const object_t*    pgmCfg,                // top level program cfg 
-                const object_t*    udpCfg = nullptr,
-                const char*        projDir   = nullptr);
+                const object_t*    udpCfg  = nullptr,
+                const char*        projDir = nullptr,
+                ui_callback_t      ui_callback = nullptr,
+                void*              ui_callback_arg = nullptr);
 
     // Network cfg. information which is available following create().
     bool     is_non_real_time( handle_t h );
@@ -32,6 +34,7 @@ namespace cw
     const char* preset_label( handle_t h, unsigned preset_idx );
     
     // Instantiate the network and prepare for runtime.
+    // The UI is not available until after initialization.
     rc_t initialize( handle_t           handle, 
                      external_device_t* deviceA    = nullptr,
                      unsigned           deviceN    = 0,
@@ -66,6 +69,9 @@ namespace cw
     rc_t get_variable_value( handle_t h, const char* inst_label, const char* var_label, unsigned chIdx, double& valueRef   );
 
 
+    // The 'user_id' shows up as the 'user_id' in the ui_var field.
+    rc_t set_variable_user_id( handle_t h, const ui_var_t* ui_var, unsigned user_id );
+    
     rc_t set_variable_value( handle_t h, const ui_var_t* ui_var, bool value     );
     rc_t set_variable_value( handle_t h, const ui_var_t* ui_var, int value      );
     rc_t set_variable_value( handle_t h, const ui_var_t* ui_var, unsigned value );