QQuickImageBase: allow derived classes to affect devicePixelRatio usage

QQuickImageBase currently sets its d->devicePixelRatio member in three
scenarios:

- The URL uses the image: scheme (an image provider)
- The URL is an svg/svgz
- The URL contains the "@2x" syntax

In all other cases, it sets d->devicePixelRatio to 1.0. QQuickIconImage
derives from QQuickImageBase, and can display theme icons in addition
to regular images. Theme icons have their own directory structure that
determines what is and is not a high DPI image:

https://standards.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html

In particular, they use e.g. a 22x22@2 directory to indicate that
the icons contained within it are high DPI images, as opposed to
storing that information in the file name.

To account for these images, QQuickIconImage must be able to affect
the decision making progress that determines whether or not an image
is loaded at a higher DPI. This patch adds a virtual function to
QQuickImageBasePrivate that allows subclasses to have some control over
this.

Change-Id: Id45e0580c8e2ea9a96a41c54ef5fa765ce5922cf
Reviewed-by: J-P Nurmi <jpnurmi@qt.io>
Reviewed-by: Robin Burchell <robin.burchell@crimson.no>

1 files changed, 28 insertions, 19 deletions

diff --git a/src/quick/items/qquickimagebase.cpp b/src/quick/items/qquickimagebase.cpp
index 22d631e917..33d69f5032 100644
--- a/src/quick/items/qquickimagebase.cpp
+++ b/src/quick/items/qquickimagebase.cpp
@@ -49,6 +49,30 @@
 
 QT_BEGIN_NAMESPACE
 
+// This function gives derived classes the chance set the devicePixelRatio
+// if they're not happy with our implementation of it.
+bool QQuickImageBasePrivate::updateDevicePixelRatio(qreal targetDevicePixelRatio)
+{
+    // QQuickImageProvider and SVG can generate a high resolution image when
+    // sourceSize is set (this function is only called if it's set).
+    // If sourceSize is not set then the provider default size will be used, as usual.
+    bool setDevicePixelRatio = false;
+    if (url.scheme() == QLatin1String("image")) {
+        setDevicePixelRatio = true;
+    } else {
+        QString stringUrl = url.path(QUrl::PrettyDecoded);
+        if (stringUrl.endsWith(QLatin1String("svg")) ||
+            stringUrl.endsWith(QLatin1String("svgz"))) {
+            setDevicePixelRatio = true;
+        }
+    }
+
+    if (setDevicePixelRatio)
+        devicePixelRatio = targetDevicePixelRatio;
+
+    return setDevicePixelRatio;
+}
+
 QQuickImageBase::QQuickImageBase(QQuickItem *parent)
 : QQuickImplicitSizeItem(*(new QQuickImageBasePrivate), parent)
 {
@@ -221,26 +245,11 @@ void QQuickImageBase::load()
 
         QUrl loadUrl = d->url;
 
-        // QQuickImageProvider and SVG can generate a high resolution image when
-        // sourceSize is set. If sourceSize is not set then the provider default size
-        // will be used, as usual.
-        bool setDevicePixelRatio = false;
-        if (d->sourcesize.isValid()) {
-            if (loadUrl.scheme() == QLatin1String("image")) {
-                setDevicePixelRatio = true;
-            } else {
-                QString stringUrl = loadUrl.path(QUrl::PrettyDecoded);
-                if (stringUrl.endsWith(QLatin1String("svg")) ||
-                    stringUrl.endsWith(QLatin1String("svgz"))) {
-                    setDevicePixelRatio = true;
-                }
-            }
-
-            if (setDevicePixelRatio)
-                d->devicePixelRatio = targetDevicePixelRatio;
-        }
+        bool updatedDevicePixelRatio = false;
+        if (d->sourcesize.isValid())
+            updatedDevicePixelRatio = d->updateDevicePixelRatio(targetDevicePixelRatio);
 
-        if (!setDevicePixelRatio) {
+        if (!updatedDevicePixelRatio) {
             // (possible) local file: loadUrl and d->devicePixelRatio will be modified if
             // an "@2x" file is found.
             resolve2xLocalFile(d->url, targetDevicePixelRatio, &loadUrl, &d->devicePixelRatio);