Introduce QImage{Reader,Writer}::imageFormatsForMimeType()

It can be used if, for example, you get an image MIME type
from the user, and you want to find the appropriate format
for loading or saving this image.

Task-number: QTBUG-49714
Change-Id: Iae5a7e9d658d6c3d1cd750a8bbc279f1812f99df
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/gui/image/qimagereader.cpp

@@ -1552,4 +1552,21 @@ QList<QByteArray> QImageReader::supportedMimeTypes()
     return QImageReaderWriterHelpers::supportedMimeTypes(QImageReaderWriterHelpers::CanRead);
 }
 
+/*!
+    \since 5.12
+
+    Returns the list of image formats corresponding to \mimeType.
+
+    Note that the QGuiApplication instance must be created before this function is
+    called.
+
+    \sa supportedImageFormats(), supportedMimeTypes()
+*/
+
+QList<QByteArray> QImageReader::imageFormatsForMimeType(const QByteArray &mimeType)
+{
+    return QImageReaderWriterHelpers::imageFormatsForMimeType(mimeType,
+                                                              QImageReaderWriterHelpers::CanRead);
+}
+
 QT_END_NAMESPACE

src/gui/image/qimagereader.h

@@ -144,6 +144,7 @@ public:
     static QByteArray imageFormat(QIODevice *device);
     static QList<QByteArray> supportedImageFormats();
     static QList<QByteArray> supportedMimeTypes();
+    static QList<QByteArray> imageFormatsForMimeType(const QByteArray &mimeType);
 
 private:
     Q_DISABLE_COPY(QImageReader)

src/gui/image/qimagereaderwriterhelpers.cpp

@@ -78,7 +78,8 @@ static void appendImagePluginFormats(QFactoryLoader *loader,
 
 static void appendImagePluginMimeTypes(QFactoryLoader *loader,
                                        QImageIOPlugin::Capability cap,
-                                       QList<QByteArray> *result)
+                                       QList<QByteArray> *result,
+                                       QList<QByteArray> *resultKeys = nullptr)
 {
     QList<QJsonObject> metaDataList = loader->metaData();
 
@@ -90,8 +91,12 @@ static void appendImagePluginMimeTypes(QFactoryLoader *loader,
         QImageIOPlugin *plugin = qobject_cast<QImageIOPlugin *>(loader->instance(i));
         const int keyCount = keys.size();
         for (int k = 0; k < keyCount; ++k) {
-            if (plugin && (plugin->capabilities(0, keys.at(k).toString().toLatin1()) & cap) != 0)
+            const QByteArray key = keys.at(k).toString().toLatin1();
+            if (plugin && (plugin->capabilities(0, key) & cap) != 0) {
                 result->append(mimeTypes.at(k).toString().toLatin1());
+                if (resultKeys)
+                    resultKeys->append(key);
+            }
         }
     }
 }
@@ -143,6 +148,33 @@ QList<QByteArray> supportedMimeTypes(Capability cap)
     return mimeTypes;
 }
 
+QList<QByteArray> imageFormatsForMimeType(const QByteArray &mimeType, Capability cap)
+{
+    QList<QByteArray> formats;
+    if (mimeType.startsWith("image/")) {
+        const QByteArray type = mimeType.mid(sizeof("image/") - 1);
+        for (const auto &fmt : _qt_BuiltInFormats) {
+            if (fmt.mimeType == type && !formats.contains(fmt.extension))
+                formats << fmt.extension;
+        }
+    }
+
+#ifndef QT_NO_IMAGEFORMATPLUGIN
+    QList<QByteArray> mimeTypes;
+    QList<QByteArray> keys;
+    appendImagePluginMimeTypes(loader(), pluginCapability(cap), &mimeTypes, &keys);
+    for (int i = 0; i < mimeTypes.size(); ++i) {
+        if (mimeTypes.at(i) == mimeType) {
+            const auto &key = keys.at(i);
+            if (!formats.contains(key))
+                formats << key;
+        }
+    }
+#endif // QT_NO_IMAGEFORMATPLUGIN
+
+    return formats;
+}
+
 } // QImageReaderWriterHelpers
 
 QT_END_NAMESPACE

src/gui/image/qimagereaderwriterhelpers_p.h

@@ -130,6 +130,7 @@ enum Capability {
 };
 QList<QByteArray> supportedImageFormats(Capability cap);
 QList<QByteArray> supportedMimeTypes(Capability cap);
+QList<QByteArray> imageFormatsForMimeType(const QByteArray &mimeType, Capability cap);
 
 }
 

src/gui/image/qimagewriter.cpp

@@ -863,4 +863,21 @@ QList<QByteArray> QImageWriter::supportedMimeTypes()
     return QImageReaderWriterHelpers::supportedMimeTypes(QImageReaderWriterHelpers::CanWrite);
 }
 
+/*!
+    \since 5.12
+
+    Returns the list of image formats corresponding to \mimeType.
+
+    Note that the QGuiApplication instance must be created before this function is
+    called.
+
+    \sa supportedImageFormats(), supportedMimeTypes()
+*/
+
+QList<QByteArray> QImageWriter::imageFormatsForMimeType(const QByteArray &mimeType)
+{
+    return QImageReaderWriterHelpers::imageFormatsForMimeType(mimeType,
+                                                              QImageReaderWriterHelpers::CanWrite);
+}
+
 QT_END_NAMESPACE

src/gui/image/qimagewriter.h

@@ -116,6 +116,7 @@ public:
 
     static QList<QByteArray> supportedImageFormats();
     static QList<QByteArray> supportedMimeTypes();
+    static QList<QByteArray> imageFormatsForMimeType(const QByteArray &mimeType);
 
 private:
     Q_DISABLE_COPY(QImageWriter)