Add JUPYTER_WIDGETS_ECHO env variable for echo_update

include/xwidgets/xcommon.hpp

@@ -13,8 +13,10 @@
 #include <utility>
 #include <vector>
 
+#include <xeus/xcomm.hpp>
+#include <xtl/xoptional.hpp>
+
 #include "xbinary.hpp"
-#include "xeus/xcomm.hpp"
 #include "xwidgets_config.hpp"
 
 namespace xw
@@ -85,6 +87,13 @@ namespace xw
 
     private:
 
+        /**
+         * Indicate whether a global setting activates or deactivates ``echo_update`` messages.
+         *
+         * If the optional is empty, then no setting is set explicitly.
+         */
+        static xtl::xoptional<bool> global_echo_update();
+
         bool
         same_patch(const std::string&, const nl::json&, const xeus::buffer_sequence&, const nl::json&, const xeus::buffer_sequence&)
             const;
@@ -132,6 +141,14 @@ namespace xw
             auto const it = hold_state.find(name);
             if ((it != hold_state.end()) && same_patch(name, *it, hold_buffers, state[name], buffers))
             {
+                // If the "echo_update" is explicitly deactivated we do not send the message
+                auto const echo_update = global_echo_update();
+                if (echo_update.has_value() && !echo_update.value())
+                {
+                    return;
+                }
+                // If the "echo_update" is explicitly activated or unspecified, we continue and
+                // send message
                 method = "echo_update";
             }
             // On the contrary, the update could differ from the change in the frontend.

src/xcommon.cpp

@@ -9,10 +9,13 @@
 #include "xwidgets/xcommon.hpp"
 
 #include <algorithm>
+#include <cstdlib>
 #include <string>
 #include <utility>
 #include <vector>
 
+#include <xtl/xoptional.hpp>
+
 #include "xeus/xinterpreter.hpp"
 #include "xtarget.hpp"
 
@@ -194,6 +197,69 @@ namespace xw
         m_comm.close(nl::json::object(), nl::json::object(), xeus::buffer_sequence());
     }
 
+    namespace
+    {
+        std::string tolower(std::string s)
+        {
+            auto safe_tolower = [](unsigned char c)
+            {
+                return std::tolower(c);
+            };
+            std::transform(s.begin(), s.end(), s.begin(), safe_tolower);
+            return s;
+        }
+
+        std::string ltrim(std::string s)
+        {
+            auto const safe_isnotspace = [](unsigned char ch)
+            {
+                return !std::isspace(ch);
+            };
+            s.erase(s.begin(), std::find_if(s.begin(), s.end(), safe_isnotspace));
+            return s;
+        }
+
+        std::string rtrim(std::string s)
+        {
+            auto const safe_isnotspace = [](unsigned char ch)
+            {
+                return !std::isspace(ch);
+            };
+            s.erase(std::find_if(s.rbegin(), s.rend(), safe_isnotspace).base(), s.end());
+            return s;
+        }
+
+        std::string trim(std::string s)
+        {
+            rtrim(s);
+            ltrim(s);
+            return s;
+        }
+
+        bool is_true_string(const char* str)
+        {
+            const std::string s = tolower(trim(str));
+            static const auto trues = {"true", "on", "yes", "1"};
+            return std::find(trues.begin(), trues.end(), s) < trues.end();
+        }
+
+        xtl::xoptional<bool> get_tristate_env(const char* name)
+        {
+            const char* const val = std::getenv(name);
+            if (val == nullptr)
+            {
+                return {};
+            }
+            return is_true_string(val);
+        }
+    }
+
+    xtl::xoptional<bool> xcommon::global_echo_update()
+    {
+        static const auto out = get_tristate_env("JUPYTER_WIDGETS_ECHO");
+        return out;
+    }
+
     bool
     xcommon::same_patch(const std::string& name, const nl::json& j1, const xeus::buffer_sequence&, const nl::json& j2, const xeus::buffer_sequence&)
         const