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src/CalcViewModel/Common/DelegateCommand.h

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+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+#pragma once
+
+namespace CalculatorApp
+{
+    namespace Common
+    {
+        template <typename TTarget>
+        ref class DelegateCommand: public Windows::UI::Xaml::Input::ICommand
+        {
+        internal:
+
+            typedef void (TTarget::*CommandHandlerFunc)(Platform::Object^);
+
+            DelegateCommand(TTarget^ target, CommandHandlerFunc func):
+                m_weakTarget(target),
+                m_function(func)
+            { }
+
+        private:
+
+            // Explicit, and private, implementation of ICommand, this way of programming makes it so 
+            // the ICommand methods will only be available if the ICommand interface is requested via a dynamic_cast
+            // The ICommand interface is meant to be consumed by Xaml and not by the app, this is a defensive measure against
+            // code in the app calling Execute. 
+            virtual void ExecuteImpl(Platform::Object^ parameter) sealed = Windows::UI::Xaml::Input::ICommand::Execute
+            {
+                TTarget^ target = m_weakTarget.Resolve<TTarget>();
+                if (target)
+                {
+                    (target->*m_function)(parameter);
+                }
+            }
+
+            virtual bool CanExecuteImpl(Platform::Object^ parameter) sealed = Windows::UI::Xaml::Input::ICommand::CanExecute
+            {
+                return true;
+            }
+
+            virtual event Windows::Foundation::EventHandler<Platform::Object^>^ CanExecuteChangedImpl
+            {
+                virtual Windows::Foundation::EventRegistrationToken add(Windows::Foundation::EventHandler<Platform::Object^>^ handler) sealed = Windows::UI::Xaml::Input::ICommand::CanExecuteChanged::add
+                {
+                    return m_canExecuteChanged += handler;
+                }
+                virtual void remove(Windows::Foundation::EventRegistrationToken token) sealed = Windows::UI::Xaml::Input::ICommand::CanExecuteChanged::remove
+                {
+                    m_canExecuteChanged -= token;
+                }
+            }
+
+        private:
+
+            event Windows::Foundation::EventHandler<Platform::Object^>^ m_canExecuteChanged;
+
+            CommandHandlerFunc m_function;
+            Platform::WeakReference m_weakTarget;
+
+        };
+
+        template <typename TTarget, typename TFuncPtr>
+        DelegateCommand<TTarget>^ MakeDelegate(TTarget^ target, TFuncPtr&& function)
+        {
+            return ref new DelegateCommand<TTarget>(target, std::forward<TFuncPtr>(function));
+        }
+
+    }
+}