Implement preview support for GTK file dialog

This adds preview support in GTK file dialog implementation,
this is helpful for a lot of applications
like image viewers, messengers and etc.

[ChangeLog][Platform Specific Changes][Linux] A native GTK
file dialog (created via QFileDialog or QtQuick.Dialogs)
now shows an image preview pane.

Task-number: QTBUG-3796
Task-number: QTBUG-53167
Change-Id: Ib80108c09b84d774440a0445adcccab4b64652ef
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

2 files changed, 43 insertions, 0 deletions

diff --git a/src/plugins/platformthemes/gtk3/qgtk3dialoghelpers.cpp b/src/plugins/platformthemes/gtk3/qgtk3dialoghelpers.cpp
index 16d5eb3ab4..4f417d77d8 100644
--- a/src/plugins/platformthemes/gtk3/qgtk3dialoghelpers.cpp
+++ b/src/plugins/platformthemes/gtk3/qgtk3dialoghelpers.cpp
@@ -45,6 +45,7 @@
 #include <qcolor.h>
 #include <qdebug.h>
 #include <qfont.h>
+#include <qfileinfo.h>
 
 #include <private/qguiapplication_p.h>
 #include <qpa/qplatformfontdatabase.h>
@@ -55,6 +56,14 @@
 #include <gdk/gdkx.h>
 #include <pango/pango.h>
 
+// The size of the preview we display for selected image files. We set height
+// larger than width because generally there is more free space vertically
+// than horiztonally (setting the preview image will alway expand the width of
+// the dialog, but usually not the height). The image's aspect ratio will always
+// be preserved.
+#define PREVIEW_WIDTH 256
+#define PREVIEW_HEIGHT 512
+
 QT_BEGIN_NAMESPACE
 
 class QGtk3Dialog : public QWindow
@@ -250,6 +259,10 @@ QGtk3FileDialogHelper::QGtk3FileDialogHelper()
     g_signal_connect(GTK_FILE_CHOOSER(d->gtkDialog()), "selection-changed", G_CALLBACK(onSelectionChanged), this);
     g_signal_connect_swapped(GTK_FILE_CHOOSER(d->gtkDialog()), "current-folder-changed", G_CALLBACK(onCurrentFolderChanged), this);
     g_signal_connect_swapped(GTK_FILE_CHOOSER(d->gtkDialog()), "notify::filter", G_CALLBACK(onFilterChanged), this);
+
+    previewWidget = gtk_image_new();
+    g_signal_connect(G_OBJECT(d->gtkDialog()), "update-preview", G_CALLBACK(onUpdatePreview), this);
+    gtk_file_chooser_set_preview_widget(GTK_FILE_CHOOSER(d->gtkDialog()), previewWidget);
 }
 
 QGtk3FileDialogHelper::~QGtk3FileDialogHelper()
@@ -390,6 +403,33 @@ void QGtk3FileDialogHelper::onFilterChanged(QGtk3FileDialogHelper *dialog)
     emit dialog->filterSelected(dialog->selectedNameFilter());
 }
 
+void QGtk3FileDialogHelper::onUpdatePreview(GtkDialog *gtkDialog, QGtk3FileDialogHelper *helper)
+{
+    gchar *filename = gtk_file_chooser_get_preview_filename(GTK_FILE_CHOOSER(gtkDialog));
+    if (!filename) {
+        gtk_file_chooser_set_preview_widget_active(GTK_FILE_CHOOSER(gtkDialog), false);
+        return;
+    }
+
+    // Don't attempt to open anything which isn't a regular file. If a named pipe,
+    // this may hang.
+    QFileInfo fileinfo(filename);
+    if (!fileinfo.exists() || !fileinfo.isFile()) {
+        g_free(filename);
+        gtk_file_chooser_set_preview_widget_active(GTK_FILE_CHOOSER(gtkDialog), false);
+        return;
+    }
+
+    // This will preserve the image's aspect ratio.
+    GdkPixbuf *pixbuf = gdk_pixbuf_new_from_file_at_size(filename, PREVIEW_WIDTH, PREVIEW_HEIGHT, 0);
+    g_free(filename);
+    if (pixbuf) {
+        gtk_image_set_from_pixbuf(GTK_IMAGE(helper->previewWidget), pixbuf);
+        g_object_unref(pixbuf);
+    }
+    gtk_file_chooser_set_preview_widget_active(GTK_FILE_CHOOSER(gtkDialog), pixbuf ? true : false);
+}
+
 static GtkFileChooserAction gtkFileChooserAction(const QSharedPointer<QFileDialogOptions> &options)
 {
     switch (options->fileMode()) {
diff --git a/src/plugins/platformthemes/gtk3/qgtk3dialoghelpers.h b/src/plugins/platformthemes/gtk3/qgtk3dialoghelpers.h
index e78a7fc6d1..1a055ac055 100644
--- a/src/plugins/platformthemes/gtk3/qgtk3dialoghelpers.h
+++ b/src/plugins/platformthemes/gtk3/qgtk3dialoghelpers.h
@@ -47,6 +47,7 @@
 #include <QtCore/qstring.h>
 #include <qpa/qplatformdialoghelper.h>
 
+typedef struct _GtkWidget GtkWidget;
 typedef struct _GtkDialog GtkDialog;
 typedef struct _GtkFileFilter GtkFileFilter;
 
@@ -108,6 +109,7 @@ private:
     static void onSelectionChanged(GtkDialog *dialog, QGtk3FileDialogHelper *helper);
     static void onCurrentFolderChanged(QGtk3FileDialogHelper *helper);
     static void onFilterChanged(QGtk3FileDialogHelper *helper);
+    static void onUpdatePreview(GtkDialog *dialog, QGtk3FileDialogHelper *helper);
     void applyOptions();
     void setNameFilters(const QStringList &filters);
     void selectFileInternal(const QUrl &filename);
@@ -118,6 +120,7 @@ private:
     QHash<QString, GtkFileFilter*> _filters;
     QHash<GtkFileFilter*, QString> _filterNames;
     QScopedPointer<QGtk3Dialog> d;
+    GtkWidget *previewWidget;
 };
 
 class QGtk3FontDialogHelper : public QPlatformFontDialogHelper