Core: Prepare de-virtualization of IContext::contextHelp

... by providing a way to specify modified implementation
by setting a functor.

Change-Id: Ib0907dd00fdb2ef15242a7ec2e25a5f8a9cc6979
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/icontext.cpp

@@ -9,6 +9,24 @@
 
 namespace Core {
 
+void IContext::contextHelp(const HelpCallback &callback) const
+{
+    if (m_contextHelpProvider) {
+        m_contextHelpProvider(callback);
+    } else {
+        // This is important as this triggers the continued iteration
+        // through other contexts that may provide items.
+        callback({});
+    }
+}
+
+void IContext::setContextHelp(const HelpItem &item)
+{
+    m_contextHelpProvider = [item](const HelpCallback &callback) {
+        callback(item);
+    };
+}
+
 void IContext::attach(QWidget *widget, const Context &context, const HelpItem &help)
 {
     auto icontext = new IContext(widget); // As QObject parent.

src/plugins/coreplugin/icontext.h

@@ -60,9 +60,10 @@ public:
     void setContext(const Context &context) { m_context = context; }
 
     using HelpCallback = std::function<void(const HelpItem &item)>;
-    virtual void contextHelp(const HelpCallback &callback) const { callback(m_contextHelp); }
+    using HelpProvider = std::function<void(const HelpCallback &item)>;
 
-    virtual void setContextHelp(const HelpItem &id) { m_contextHelp = id; }
+    virtual void contextHelp(const HelpCallback &callback) const;
+    void setContextHelp(const HelpItem &id);
 
     static void attach(QWidget *widget,
                        const Context &context,
@@ -71,7 +72,7 @@ public:
 protected:
     Context m_context;
     QPointer<QWidget> m_widget;
-    HelpItem m_contextHelp;
+    HelpProvider m_contextHelpProvider;
 };
 
 } // namespace Core