Use qmlRegisterModuleImport() to register styles

This patch completes the cumulative work done in previous patches.

- Uses qmlRegisterModuleImport() to register styles. This has some
  added requirements:
  - Each style must now be a QML module -- that is, it must have a
    qmldir file.
  - As a result of the above, the module must be available within the
    QML import path in order to be found.
  - The various forms of accepted style names have been reduced down to
    one ("Material", "MyStyle", etc). See below for an explanation of
    why.
  - The following API in QQuickStyle is removed:
    addStylePath(), availableStyles(), path(), stylePathList(). These
    no longer make sense now that we reuse the existing QML import
    system.
- Adds the tst_qquickstyleselector auto test back as "styleimports".

qmlRegisterModuleImport() vs resolvedUrl()

    Previously we would use QQuickStyleSelector to select individual
    QML files based on which style was set. We'd do this once when
    QtQuick.Controls was first imported.

    With Qt 6, and the requirement that each style be a proper QML
    module, qmlRegisterModuleImport() was introduced. This allows us
    to "link" one import with another. For an example of what this
    looks like in practice, suppose the style was set to "MyStyle",
    and the fallback to "Material". The "QtQuick.Controls" import
    will be linked to "MyStyle", "MyStyle" to
    "QtQuick.Controls.Material", and as a final fallback (for controls
    like Action which only the Default style implements),
    "QtQuick.Controls.Material" to "QtQuick.Controls.Default".
    This is the same behavior as in Qt 5 (see qquickstyleselector.cpp):

    // 1) requested style (e.g. "MyStyle", included in d->selectors)
    // 2) fallback style (e.g. "Material", included in d->selectors)
    // 3) default style (empty selector, not in d->selectors)

    This is a necessary step to enable compilation of QML to C++.

Reducing the set of accepted style names

    The problem

    In QtQuickControls2Plugin() we need to call
    QQuickStylePrivate::init(baseUrl()) in order to detect if the style
    is a custom style in QQuickStyleSpec::resolve() (by checking if the
    style path starts with the base URL). In Qt 5, init() is called in
    QtQuickControls2Plugin::registerTypes(), but in Qt 6 that's too
    late, because we need to call qmlRegisterModuleImport() in the
    constructor. qmlRegisterModuleImport() itself requires the style to
    have already been set in order to create the correct import URI
    ("QtQuick.Controls.X" for built-in styles, "MyCustomStyle" for
    custom styles).

    The solution

    By reducing the valid forms for style names down to one:

    ./myapp -style MyStyle

    we solve the problem of needing baseUrl() to determine if the
    style is a custom style or not, but needing to call it too early
    (since we now call qmlRegisterModuleImport() in
    QtQuickControls2Plugin(), which itself requires the style to have
    already been set). baseUrl() can't have been set before the
    constructor is finished.

    All of the various forms for _setting_ a style are still valid;
    environment variables, qtquickcontrols2.conf, etc.

[ChangeLog][Important Behavior Changes] Custom styles must now have
a qmldir that lists the files that the style implements. For example,
for a style that only implements Button:
---
module MyStyle
Button 1.0 Button.qml
---
In addition, there is now only one valid, case-sensitive form for style
names: "Material", "MyStyle", etc.
These changes are done to help enable the compilation of QML code to
C++, as well as improve tooling capabilities.

[ChangeLog][Important Behavior Changes] The following API was removed:
- QQuickStyle::addStylePath()
- QQuickStyle::availableStyles()
- QQuickStyle::path()
- QQuickStyle::stylePathList()
- QT_QUICK_CONTROLS_STYLE_PATH
This API is no longer necessary and/or able to be provided now that
styles are treated as regular QML modules.

Task-number: QTBUG-82922
Change-Id: I3b281131903c7c3c1cf0616eb7486a872dccd730
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

2 files changed, 12 insertions, 7 deletions

diff --git a/src/imports/controls/material/qmldir b/src/imports/controls/material/qmldir
index a1064229..39dae48a 100644
--- a/src/imports/controls/material/qmldir
+++ b/src/imports/controls/material/qmldir
@@ -1,7 +1,7 @@
 module QtQuick.Controls.Material
 plugin qtquickcontrols2materialstyleplugin
 classname QtQuickControls2MaterialStylePlugin
-depends QtQuick.Controls 2.5
+import QtQuick.Controls.Default auto
 
 # QtQuick.Controls 2.0 (originally introduced in Qt 5.7)
 ApplicationWindow 2.0 ApplicationWindow.qml
diff --git a/src/imports/controls/material/qtquickcontrols2materialstyleplugin.cpp b/src/imports/controls/material/qtquickcontrols2materialstyleplugin.cpp
index dc6873bb..6066d3f5 100644
--- a/src/imports/controls/material/qtquickcontrols2materialstyleplugin.cpp
+++ b/src/imports/controls/material/qtquickcontrols2materialstyleplugin.cpp
@@ -34,12 +34,12 @@
 **
 ****************************************************************************/
 
-#include <QtQuickControls2/private/qquickstyleplugin_p.h>
-
 #include "qquickmaterialstyle_p.h"
 #include "qquickmaterialtheme_p.h"
 
+#include <QtQuickControls2/private/qquickstyleplugin_p.h>
 #include <QtQuickControls2Impl/private/qquickpaddedrectangle_p.h>
+#include <QtQuickTemplates2/private/qquicktheme_p.h>
 
 QT_BEGIN_NAMESPACE
 
@@ -52,12 +52,14 @@ public:
     QtQuickControls2MaterialStylePlugin(QObject *parent = nullptr);
 
     QString name() const override;
-    void initializeTheme(QQuickTheme *theme) override;
+
+    void registerTypes(const char *uri) override;
+
+    QQuickMaterialTheme theme;
 };
 
 QtQuickControls2MaterialStylePlugin::QtQuickControls2MaterialStylePlugin(QObject *parent) : QQuickStylePlugin(parent)
 {
-    QQuickMaterialStyle::initGlobals();
 }
 
 QString QtQuickControls2MaterialStylePlugin::name() const
@@ -65,9 +67,12 @@ QString QtQuickControls2MaterialStylePlugin::name() const
     return QStringLiteral("Material");
 }
 
-void QtQuickControls2MaterialStylePlugin::initializeTheme(QQuickTheme *theme)
+void QtQuickControls2MaterialStylePlugin::registerTypes(const char *uri)
 {
-    QQuickMaterialTheme::initialize(theme);
+    QQuickStylePlugin::registerTypes(uri);
+
+    QQuickMaterialStyle::initGlobals();
+    theme.initialize(QQuickTheme::instance());
 }
 
 QT_END_NAMESPACE