Support error isolation/capture.

src/alia/flow/events.cpp

@@ -102,7 +102,8 @@ on_init(context ctx, action<> on_init)
     on_refresh(ctx, [&](auto) {
         if (!data.initialized && on_init.is_ready())
         {
-            perform_action(on_init);
+            isolate_errors(ctx, [&] { perform_action(on_init); });
+            mark_dirty_component(ctx);
             data.initialized = true;
         }
     });
@@ -116,7 +117,8 @@ on_activate(context ctx, action<> on_activate)
     on_refresh(ctx, [&](auto) {
         if (!data.initialized && on_activate.is_ready())
         {
-            perform_action(on_activate);
+            isolate_errors(ctx, [&] { perform_action(on_activate); });
+            mark_dirty_component(ctx);
             data.initialized = true;
         }
     });

src/alia/flow/events.hpp

@@ -5,6 +5,7 @@
 #include <alia/flow/components.hpp>
 #include <alia/flow/data_graph.hpp>
 #include <alia/flow/macros.hpp>
+#include <alia/flow/try_catch.hpp>
 #include <alia/system/interface.hpp>
 
 // This file implements utilities for routing events through an alia content
@@ -262,7 +263,7 @@ on_value_change(context ctx, Signal signal, action<> on_change)
         [&](auto const& new_value) {
             if (!data->has_value || data->value != new_value)
             {
-                perform_action(on_change);
+                isolate_errors(ctx, [&] { perform_action(on_change); });
                 mark_dirty_component(ctx);
                 data->has_value = true;
                 data->value = new_value;
@@ -271,7 +272,7 @@ on_value_change(context ctx, Signal signal, action<> on_change)
         [&] {
             if (data->has_value)
             {
-                perform_action(on_change);
+                isolate_errors(ctx, [&] { perform_action(on_change); });
                 mark_dirty_component(ctx);
                 data->has_value = false;
             }
@@ -290,7 +291,7 @@ on_value_gain(context ctx, Signal signal, action<> on_gain)
         bool current_state = signal_has_value(signal);
         if (current_state && !*saved_state)
         {
-            perform_action(on_gain);
+            isolate_errors(ctx, [&] { perform_action(on_gain); });
             mark_dirty_component(ctx);
         }
         *saved_state = current_state;
@@ -309,7 +310,7 @@ on_value_loss(context ctx, Signal signal, action<> on_loss)
         bool current_state = signal_has_value(signal);
         if (!current_state && *saved_state)
         {
-            perform_action(on_loss);
+            isolate_errors(ctx, [&] { perform_action(on_loss); });
             mark_dirty_component(ctx);
         }
         *saved_state = current_state;

src/alia/flow/try_catch.hpp

@@ -148,6 +148,28 @@ struct catch_block
 #define alia_catch(pattern) ALIA_CATCH(pattern)
 #endif
 
+template<class Function>
+void
+isolate_errors(system& sys, Function&& function)
+{
+    try
+    {
+        std::forward<Function>(function)();
+    }
+    catch (...)
+    {
+        if (sys.error_handler)
+            sys.error_handler(std::current_exception());
+    }
+}
+
+template<class Function>
+void
+isolate_errors(dataless_context ctx, Function&& function)
+{
+    isolate_errors(get<system_tag>(ctx), std::forward<Function>(function));
+}
+
 } // namespace alia
 
 #endif

src/alia/system/interface.cpp

@@ -30,4 +30,10 @@ refresh_system(system& sys)
     };
 }
 
+void
+set_error_handler(system& sys, std::function<void(std::exception_ptr)> handler)
+{
+    sys.error_handler = handler;
+}
+
 } // namespace alia

src/alia/system/interface.hpp

@@ -1,6 +1,8 @@
 #ifndef ALIA_SYSTEM_INTERFACE_HPP
 #define ALIA_SYSTEM_INTERFACE_HPP
 
+#include <functional>
+
 namespace alia {
 
 struct system;
@@ -11,6 +13,10 @@ system_needs_refresh(system const& sys);
 void
 refresh_system(system& sys);
 
+void
+set_error_handler(
+    system& sys, std::function<void(std::exception_ptr)> handler);
+
 } // namespace alia
 
 #endif

src/alia/system/internals.hpp

@@ -86,6 +86,7 @@ struct system : noncopyable
     std::unique_ptr<external_interface> external;
     timer_event_scheduler scheduler;
     component_container_ptr root_component;
+    std::function<void(std::exception_ptr)> error_handler;
 };
 
 void

unit_tests/flow/events.cpp

@@ -275,3 +275,36 @@ TEST_CASE("on_value events", "[flow][events]")
     REQUIRE(change_count == 4);
     REQUIRE(change_shadow == 4);
 }
+
+TEST_CASE("error isolation", "[flow][events]")
+{
+    int n = 0;
+
+    alia::system sys;
+    initialize_system(sys, [&](context ctx) {
+        on_value_change(ctx, value(n), lambda_action([&] {
+                            static_cast<void>(std::string("abc").at(n));
+                        }));
+    });
+
+    int error_count = 0;
+    set_error_handler(sys, [&](std::exception_ptr) { ++error_count; });
+
+    refresh_system(sys);
+    REQUIRE(error_count == 0);
+
+    n = 2;
+    refresh_system(sys);
+    REQUIRE(error_count == 0);
+
+    ++n;
+    refresh_system(sys);
+    REQUIRE(error_count == 1);
+
+    refresh_system(sys);
+    REQUIRE(error_count == 1);
+
+    ++n;
+    refresh_system(sys);
+    REQUIRE(error_count == 2);
+}