cmake: implement default qpa plugin behavior for static builds

This is done by adding a DEFAULT_IF argument to add_qt_plugin, which accepts
if-evaluated expressions.

e.g.

    add_qt_plugin(myplugin
	DEFAULT_IF ${foo} STREQUAL ${bar}
        ...
    )

so that this mechanism can be reused later if necessary.

Change-Id: I7eba9adaaa28e55a4f0f94cf206e868b990027e6
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

3 files changed, 30 insertions, 10 deletions

diff --git a/cmake/QtBuild.cmake b/cmake/QtBuild.cmake
index b2c25b5ef6..df9e9008ce 100644
--- a/cmake/QtBuild.cmake
+++ b/cmake/QtBuild.cmake
@@ -1449,7 +1449,7 @@ function(add_qt_plugin target)
 
     qt_parse_all_arguments(arg "add_qt_plugin" "STATIC;EXCEPTIONS"
         "TYPE;CLASS_NAME;OUTPUT_DIRECTORY;INSTALL_DIRECTORY;ARCHIVE_INSTALL_DIRECTORY"
-        "${__default_private_args};${__default_public_args}" ${ARGN})
+        "${__default_private_args};${__default_public_args};DEFAULT_IF" ${ARGN})
 
     set(output_directory_default "${QT_BUILD_DIR}/${INSTALL_PLUGINSDIR}/${arg_TYPE}")
 
@@ -1501,7 +1501,15 @@ function(add_qt_plugin target)
         set_property(TARGET "${qt_module}" APPEND PROPERTY QT_PLUGINS "${target}")
     endif()
 
-    set_property(TARGET "${target}" APPEND PROPERTY EXPORT_PROPERTIES "QT_PLUGIN_CLASS_NAME;QT_MODULE")
+    set(_default_plugin 1)
+    if (DEFINED arg_DEFAULT_IF)
+      if (NOT ${arg_DEFAULT_IF})
+          set(_default_plugin 0)
+      endif()
+    endif()
+
+    set_property(TARGET "${target}" PROPERTY QT_DEFAULT_PLUGIN "${_default_plugin}")
+    set_property(TARGET "${target}" APPEND PROPERTY EXPORT_PROPERTIES "QT_PLUGIN_CLASS_NAME;QT_MODULE;QT_DEFAULT_PLUGIN")
 
     extend_target("${target}"
         SOURCES ${arg_SOURCES}
diff --git a/cmake/QtPlugins.cmake.in b/cmake/QtPlugins.cmake.in
index b51c7cec29..e6547508a7 100644
--- a/cmake/QtPlugins.cmake.in
+++ b/cmake/QtPlugins.cmake.in
@@ -3,7 +3,7 @@
 if(NOT @BUILD_SHARED_LIBS@)
     set(_module_target "@INSTALL_CMAKE_NAMESPACE@::@QT_MODULE@")
 
-    set(_default_plugins_genex "$<GENEX_EVAL:$<TARGET_PROPERTY:QT_DEFAULT_PLUGINS>>")
+    set(_default_plugins_are_enabled "$<GENEX_EVAL:$<TARGET_PROPERTY:QT_DEFAULT_PLUGINS>>")
     set(_manual_plugins_genex "$<GENEX_EVAL:$<TARGET_PROPERTY:QT_PLUGINS>>")
     set(_no_plugins_genex "$<GENEX_EVAL:$<TARGET_PROPERTY:QT_NO_PLUGINS>>")
 
@@ -15,17 +15,17 @@ if(NOT @BUILD_SHARED_LIBS@)
           continue()
         endif()
 
-        set(_user_specified_genex "$<IN_LIST:${_plugin_target},${_manual_plugins_genex}>")
+        set(_plugin_is_default "$<TARGET_PROPERTY:${_plugin_target},QT_DEFAULT_PLUGIN>")
+        set(_plugin_is_not_blacklisted "$<NOT:$<IN_LIST:${_plugin_target},${_no_plugins_genex}>>")
+        set(_plugin_is_whitelisted "$<IN_LIST:${_plugin_target},${_manual_plugins_genex}>")
 
         string(CONCAT _plugin_condition
                     "$<BOOL:$<OR:"
-                        # Add this plugin if it\'s in the list of manual plugins or plugins for the type
-                        "${_user_specified_genex},"
-                        # Add this plugin if the default plugins haven't been disabled, the module of the plug-in
-                        # is either empty or equal to the module name, and the user hasn't blacklisted it
+                        "${_plugin_is_whitelisted},"
                         "$<AND:"
-                            "${_default_plugins_genex},"
-                            "$<NOT:$<IN_LIST:${_plugin_target},${_no_plugins_genex}>>"
+                            "${_default_plugins_are_enabled},"
+                            "${_plugin_is_default},"
+                            "${_plugin_is_not_blacklisted}"
                         ">"
                     ">>"
         )
diff --git a/cmake/QtProperties.cmake b/cmake/QtProperties.cmake
index e8b1fbf4f0..0c3507b953 100644
--- a/cmake/QtProperties.cmake
+++ b/cmake/QtProperties.cmake
@@ -38,6 +38,18 @@ define_property(TARGET
         For instance, Sql for qsqlite"
 )
 
+define_property(TARGET
+    PROPERTY
+        QT_DEFAULT_PLUGIN
+    BRIEF_DOCS
+        "Indicates whether a plug-in is added by default."
+    FULL_DOCS
+        "This is a property on Qt plug-ins.
+        It is mainly used to indicate if a plug-in should be added
+        to the default set of plug-ins when building a static app -
+        for instance, which QPA should be linked."
+)
+
 define_property(GLOBAL
     PROPERTY
         QT_KNOWN_PLUGINS