[FEATURE][API] Add a content cache for raster images

This new class QgsImageCache is the equivalent of QgsSvgCache
but for raster images.

QgsImageCache stores pre-rendered resampled versions of raster
image files, allowing efficient reuse without incurring the
cost of resampling on every render.

Additionally, it offers the other benefits QgsSvgCache has,
such as thread safety, ability to transparently download remote
images, and support for base64 encoded strings.

python/core/auto_generated/qgsabstractcontentcache.sip.in

@@ -33,7 +33,6 @@ Constructor for QgsAbstractContentCacheEntry for an entry relating to the specif
     virtual ~QgsAbstractContentCacheEntry();
 
 
-
     QString path;
 
     QDateTime fileModified;

python/core/auto_generated/qgsapplication.sip.in

@@ -649,7 +649,18 @@ providers that may add items to the browser tree.
 Returns the application's SVG cache, used for caching SVG images and handling parameter replacement
 within SVG files.
 
+.. seealso:: :py:func:`imageCache`
+
 .. versionadded:: 3.0
+%End
+
+    static QgsImageCache *imageCache();
+%Docstring
+Returns the application's image cache, used for caching resampled versions of raster images.
+
+.. seealso:: :py:func:`svgCache`
+
+.. versionadded:: 3.6
 %End
 
     static QgsNetworkContentFetcherRegistry *networkContentFetcherRegistry();

python/core/auto_generated/qgsimagecache.sip.in

@@ -0,0 +1,71 @@
+/************************************************************************
+ * This file has been generated automatically from                      *
+ *                                                                      *
+ * src/core/qgsimagecache.h                                             *
+ *                                                                      *
+ * Do not edit manually ! Edit header and run scripts/sipify.pl again   *
+ ************************************************************************/
+
+
+
+
+
+
+class QgsImageCache : QgsAbstractContentCacheBase
+{
+%Docstring
+A cache for images derived from raster files.
+
+QgsImageCache stores pre-rendered resampled versions of raster image files, allowing efficient
+reuse without incurring the cost of resampling on every render.
+
+QgsImageCache is not usually directly created, but rather accessed through
+:py:func:`QgsApplication.imageCache()`
+
+.. versionadded:: 3.6
+%End
+
+%TypeHeaderCode
+#include "qgsimagecache.h"
+%End
+  public:
+
+    QgsImageCache( QObject *parent /TransferThis/ = 0 );
+%Docstring
+Constructor for QgsImageCache, with the specified ``parent`` object.
+%End
+
+    QImage pathAsImage( const QString &path, QSize size, bool keepAspectRatio, bool &fitsInCache /Out/ );
+%Docstring
+Returns the specified ``path`` rendered as an image. If possible, a pre-existing cached
+version of the image will be used. If not, the image is fetched and resampled to the desired
+size, and then the result cached for subsequent lookups.
+
+``path`` may be a local file, remote (HTTP) url, or a base 64 encoded string (with a "base64:" prefix).
+
+The ``size`` parameter dictates the target size of the image. An invalid size indicates the
+original raster image size (with no resampling).
+
+If ``keepAspectRatio`` is true, then the original raster aspect ratio will be maintained during
+any resampling operations.
+
+If the resultant raster was of a sufficiently small size to store in the cache, then ``fitsInCache``
+will be set to true.
+%End
+
+  signals:
+
+    void remoteImageFetched( const QString &url );
+%Docstring
+Emitted when the cache has finished retrieving an image file from a remote ``url``.
+%End
+
+};
+
+/************************************************************************
+ * This file has been generated automatically from                      *
+ *                                                                      *
+ * src/core/qgsimagecache.h                                             *
+ *                                                                      *
+ * Do not edit manually ! Edit header and run scripts/sipify.pl again   *
+ ************************************************************************/

python/core/core_auto.sip

@@ -332,6 +332,7 @@
 %Include auto_generated/qgsgeometryvalidator.sip
 %Include auto_generated/qgsgml.sip
 %Include auto_generated/qgsgmlschema.sip
+%Include auto_generated/qgsimagecache.sip
 %Include auto_generated/qgsmaplayer.sip
 %Include auto_generated/qgsmaplayerlegend.sip
 %Include auto_generated/qgsmaplayermodel.sip

scripts/spell_check/spelling.dat

@@ -6554,7 +6554,7 @@ stength:strength
 steriods:steroids
 sterotypes:stereotypes
 stilus:stylus
-stingent:stringent
+stingent:stringent:*
 stiring:stirring
 stirng:string
 stirngs:strings

src/core/CMakeLists.txt

@@ -214,6 +214,7 @@ SET(QGIS_CORE_SRCS
   qgshistogram.cpp
   qgshstoreutils.cpp
   qgshtmlutils.cpp
+  qgsimagecache.cpp
   qgsinterval.cpp
   qgsjsonutils.cpp
   qgslabelfeature.cpp
@@ -616,6 +617,7 @@ SET(QGIS_CORE_MOC_HDRS
   qgsgeometryvalidator.h
   qgsgml.h
   qgsgmlschema.h
+  qgsimagecache.h
   qgsmaplayer.h
   qgsmaplayerlegend.h
   qgsmaplayermodel.h

src/core/qgsabstractcontentcache.h

@@ -52,8 +52,6 @@ class CORE_EXPORT QgsAbstractContentCacheEntry
 
     virtual ~QgsAbstractContentCacheEntry() = default;
 
-    QgsAbstractContentCacheEntry() = delete;
-
     //! QgsAbstractContentCacheEntry cannot be copied.
     QgsAbstractContentCacheEntry( const QgsAbstractContentCacheEntry &rh ) = delete;
     //! QgsAbstractContentCacheEntry cannot be copied.
@@ -176,7 +174,7 @@ class CORE_EXPORT QgsAbstractContentCacheBase: public QObject
 #ifndef SIP_RUN
 
 /**
- * \class QgsAbstractContentCacheBase
+ * \class QgsAbstractContentCache
  * \ingroup core
  *
  * Abstract base class for file content caches, such as SVG or raster image caches.
@@ -560,6 +558,7 @@ class CORE_EXPORT QgsAbstractContentCache : public QgsAbstractContentCacheBase
     QString mTypeString;
 
     friend class TestQgsSvgCache;
+    friend class TestQgsImageCache;
 };
 
 #endif

src/core/qgsapplication.cpp

@@ -30,6 +30,7 @@
 #include "qgstaskmanager.h"
 #include "qgsfieldformatterregistry.h"
 #include "qgssvgcache.h"
+#include "qgsimagecache.h"
 #include "qgscolorschemeregistry.h"
 #include "qgspainteffectregistry.h"
 #include "qgsprojectstorageregistry.h"
@@ -1772,6 +1773,11 @@ QgsSvgCache *QgsApplication::svgCache()
   return members()->mSvgCache;
 }
 
+QgsImageCache *QgsApplication::imageCache()
+{
+  return members()->mImageCache;
+}
+
 QgsNetworkContentFetcherRegistry *QgsApplication::networkContentFetcherRegistry()
 {
   return members()->mNetworkContentFetcherRegistry;
@@ -1842,6 +1848,7 @@ QgsApplication::ApplicationMembers::ApplicationMembers()
   mActionScopeRegistry = new QgsActionScopeRegistry();
   mFieldFormatterRegistry = new QgsFieldFormatterRegistry();
   mSvgCache = new QgsSvgCache();
+  mImageCache = new QgsImageCache();
   mColorSchemeRegistry = new QgsColorSchemeRegistry();
   mPaintEffectRegistry = new QgsPaintEffectRegistry();
   mSymbolLayerRegistry = new QgsSymbolLayerRegistry();
@@ -1878,6 +1885,7 @@ QgsApplication::ApplicationMembers::~ApplicationMembers()
   delete mRasterRendererRegistry;
   delete mRendererRegistry;
   delete mSvgCache;
+  delete mImageCache;
   delete mSymbolLayerRegistry;
   delete mTaskManager;
   delete mNetworkContentFetcherRegistry;

src/core/qgsapplication.h

@@ -34,6 +34,7 @@ class QgsPaintEffectRegistry;
 class QgsProjectStorageRegistry;
 class QgsRendererRegistry;
 class QgsSvgCache;
+class QgsImageCache;
 class QgsSymbolLayerRegistry;
 class QgsRasterRendererRegistry;
 class QgsGpsConnectionRegistry;
@@ -595,10 +596,20 @@ class CORE_EXPORT QgsApplication : public QApplication
     /**
      * Returns the application's SVG cache, used for caching SVG images and handling parameter replacement
      * within SVG files.
+     *
+     * \see imageCache()
      * \since QGIS 3.0
      */
     static QgsSvgCache *svgCache();
 
+    /**
+     * Returns the application's image cache, used for caching resampled versions of raster images.
+     *
+     * \see svgCache()
+     * \since QGIS 3.6
+     */
+    static QgsImageCache *imageCache();
+
     /**
      * Returns the application's network content registry used for fetching temporary files during QGIS session
      * \since QGIS 3.2
@@ -867,6 +878,7 @@ class CORE_EXPORT QgsApplication : public QApplication
       QgsRendererRegistry *mRendererRegistry = nullptr;
       QgsRuntimeProfiler *mProfiler = nullptr;
       QgsSvgCache *mSvgCache = nullptr;
+      QgsImageCache *mImageCache = nullptr;
       QgsSymbolLayerRegistry *mSymbolLayerRegistry = nullptr;
       QgsTaskManager *mTaskManager = nullptr;
       QgsLayoutItemRegistry *mLayoutItemRegistry = nullptr;