Facade class for basic UI, injection of a wxWidgets implementation...

... but none of the methods is defined yet.

The intention is to inject dependencies on wxWidgets (or other) toolkit so that
lower-level files have less build dependency on wxCore classes or on the
event loop.

Original commit: d20cf012556a819e68515d86bb66a2c047007539

Signed-off-by: Panagiotis Vasilopoulos <hello@alwayslivid.com>

libraries/CMakeLists.txt

@@ -7,6 +7,7 @@ set( LIBRARIES
    lib-strings
    lib-utility
    lib-components
+   lib-basic-ui
 )
 
 foreach( LIBRARY ${LIBRARIES} )

libraries/lib-basic-ui/BasicUI.cpp

@@ -0,0 +1,25 @@
+/*!********************************************************************
+
+Audacity: A Digital Audio Editor
+
+@file BasicUI.cpp
+
+Paul Licameli
+
+**********************************************************************/
+#include "BasicUI.h"
+
+namespace  BasicUI {
+Services::~Services() = default;
+
+static Services *theInstance = nullptr;
+
+Services *Get() { return theInstance; }
+
+Services *Install(Services *pInstance)
+{
+   auto result = theInstance;
+   theInstance = pInstance;
+   return result;
+}
+}

libraries/lib-basic-ui/BasicUI.h

@@ -0,0 +1,47 @@
+/*!********************************************************************
+
+Audacity: A Digital Audio Editor
+
+@file BasicUI.h
+@brief Toolkit-neutral facade for basic user interface services
+
+Paul Licameli
+
+**********************************************************************/
+#ifndef __AUDACITY_BASIC_UI__
+#define __AUDACITY_BASIC_UI__
+
+namespace BasicUI {
+
+//! @name Types used in the Services interface
+//! @{
+
+//! @}
+
+//! Abstract class defines a few user interface services, not mentioning particular toolkits
+/*! The intention is that the application supplies a concrete implementation at
+ startup.  Most code will not use this class directly, but call the inline
+ functions that follow. */
+class BASIC_UI_API Services {
+public:
+   virtual ~Services();
+};
+
+//! Fetch the global instance, or nullptr if none is yet installed
+BASIC_UI_API Services *Get();
+
+//! Install an implementation; return the previously installed instance
+BASIC_UI_API Services *Install(Services *pInstance);
+
+/*! @name Functions that invoke global Services
+   These dispatch to the global Services, if supplied.  If none was supplied,
+   they are mostly no-ops, with exceptions as noted.  All should be called on
+   the main thread only, except as noted.
+ */
+//! @{
+
+//! @}
+}
+
+
+#endif

libraries/lib-basic-ui/CMakeLists.txt

@@ -0,0 +1,23 @@
+#[[
+This library defines a facade interface, BasicUI::Services, for a few basic
+interactions with the user, such as showing simple dialog boxes, and for
+enqueuing actions to the event loop, and yielding to to the event dispatcher.
+
+The interface makes no mention of classes in wxWidgets.  Using this library
+instead of making direct use of wxWidgets enlarges the parts of the program
+that are toolkit neutral.
+
+There is a global pointer to an instance of Services, and the main program is
+expected, at startup, to create a static instance of a subclass of Services and
+set the pointer.  If it does not, then calls to the non-member functions in
+namespace BasicUI are no-ops.
+]]#
+
+set( SOURCES
+   BasicUI.cpp
+   BasicUI.h
+)
+audacity_library( lib-basic-ui "${SOURCES}"
+   ""
+   "" ""
+)

src/AudacityApp.cpp

@@ -110,6 +110,7 @@ It handles initialization and termination by subclassing wxApp.
 #include "tracks/ui/Scrubbing.h"
 #include "widgets/FileConfig.h"
 #include "widgets/FileHistory.h"
+#include "widgets/wxWidgetsBasicUI.h"
 
 #ifdef EXPERIMENTAL_EASY_CHANGE_KEY_BINDINGS
 #include "prefs/KeyConfigPrefs.h"
@@ -975,17 +976,22 @@ bool AudacityApp::OnInit() {
     // Ensure we have an event loop during initialization
     wxEventLoopGuarantor eventLoop;
 
-    // Fire up SQLite
-    if (!ProjectFileIO::InitializeSQL())
-        this->CallAfter([] {
-        ::AudacityMessageBox(
-            XO("SQLite library failed to initialize.  Audacity cannot continue."));
-        QuitAudacity(true);
-                        });
+   // Inject basic GUI services behind the facade
+   {
+      static wxWidgetsBasicUI uiServices;
+      (void)BasicUI::Install(&uiServices);
+   }
 
+   // Fire up SQLite
+   if ( !ProjectFileIO::InitializeSQL() )
+      this->CallAfter([]{
+         ::AudacityMessageBox(
+            XO("SQLite library failed to initialize.  Audacity cannot continue.") );
+         QuitAudacity( true );
+      });
 
-    // cause initialization of wxWidgets' global logger target
-    (void)AudacityLogger::Get();
+   // cause initialization of wxWidgets' global logger target
+   (void)AudacityLogger::Get();
 
 #if defined(__WXMAC__)
     // Disable window animation

src/widgets/wxWidgetsBasicUI.cpp

@@ -0,0 +1,12 @@
+/*!********************************************************************
+
+Audacity: A Digital Audio Editor
+
+@file wxWidgetsBasicUI.cpp
+
+Paul Licameli
+
+**********************************************************************/
+#include "wxWidgetsBasicUI.h"
+
+wxWidgetsBasicUI::~wxWidgetsBasicUI() = default;

src/widgets/wxWidgetsBasicUI.h

@@ -0,0 +1,23 @@
+/*!********************************************************************
+
+Audacity: A Digital Audio Editor
+
+@file wxWidgetsBasicUI.h
+@brief Implementation of BasicUI using wxWidgets
+
+Paul Licameli
+
+**********************************************************************/
+#ifndef __WXWIDGETS_BASIC_UI__
+#define __WXWIDGETS_BASIC_UI__
+
+#include "BasicUI.h"
+
+//! An implementation of BasicUI::Services in terms of the wxWidgets toolkit
+/*! This is a singleton that doesn't need AUDACITY_DLL_API visibility */
+class wxWidgetsBasicUI final : public BasicUI::Services {
+public:
+   ~wxWidgetsBasicUI() override;
+};
+
+#endif