Imagine: enable caching for image lookupsv5.10.0-beta3

File system lookups are somewhat expensive, so cache the results to
avoid repetitive lookups. The default cache size of 500 is a rough
estimate based on that the default assets have 300+ images per scale
factor.

Caching image lookups gives an average of 20% performance improvement
in qmlbench:

    control         before  after improvement
    ----------------------------------------------
    busyindicator   29    32      1.10344827586207
    button          53    60      1.13207547169811
    checkbox        51    60      1.17647058823529
    combobox        39    44      1.12820512820513
    dial           110   147      1.33636363636364
    itemdelegate    54    61      1.12962962962963
    label          143   152      1.06293706293706
    menuitem        26    34      1.30769230769231
    progressbar     13    15      1.15384615384615
    radiobutton     52    59      1.13461538461538
    scrollbar       37    62      1.67567567567568
    scrollview      78   121      1.55128205128205
    slider          31    40      1.29032258064516
    spinbox         75    85      1.13333333333333
    switch          35    43      1.22857142857143
    textarea        80    88      1.1
    textfield       56    63      1.125
    tumbler        126   128      1.01587301587302
    ----------------------------------------------
    average                       1.2102967624703

Caching can be disabled by setting an environment variable:

    QT_QUICK_CONTROLS_IMAGINE_CACHE=0

but this is not advertised in the docs for the time being. It is
there to help debugging, should anything go wrong with caching.

Change-Id: I1119f3d8186bc9a51cc174b06fed02ed9e1fb70c
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

1 files changed, 17 insertions, 4 deletions

diff --git a/src/imports/controls/imagine/qquickimageselector.cpp b/src/imports/controls/imagine/qquickimageselector.cpp
index 2c3b0cd3..1b3e80a9 100644
--- a/src/imports/controls/imagine/qquickimageselector.cpp
+++ b/src/imports/controls/imagine/qquickimageselector.cpp
@@ -47,6 +47,15 @@
 
 QT_BEGIN_NAMESPACE
 
+static const int DEFAULT_CACHE = 500;
+
+static inline int cacheSize()
+{
+    static bool ok = false;
+    static const int size = qEnvironmentVariableIntValue("QT_QUICK_CONTROLS_IMAGINE_CACHE", &ok);
+    return ok ? size : DEFAULT_CACHE;
+}
+
 Q_DECLARE_LOGGING_CATEGORY(lcQtQuickControlsImagine)
 
 // input: [focused, pressed]
@@ -90,12 +99,14 @@ static QString findFile(const QDir &dir, const QString &baseName, const QStringL
         if (QFile::exists(filePath))
             return QFileSelector().select(filePath);
     }
-    return QString();
+    // return an empty string to indicate that the lookup has been done
+    // even if no matching asset was found
+    return QLatin1String("");
 }
 
 QQuickImageSelector::QQuickImageSelector(QObject *parent)
     : QObject(parent),
-      m_cache(false),
+      m_cache(cacheSize() > 0),
       m_complete(false),
       m_separator(QLatin1String("-"))
 {
@@ -224,18 +235,20 @@ QString QQuickImageSelector::cacheKey() const
 
 void QQuickImageSelector::updateSource()
 {
-    static QCache<QString, QString> cache(200); // TODO: cost
+    static QCache<QString, QString> cache(cacheSize());
 
     const QString key = cacheKey();
 
     QString bestFilePath;
+
     if (m_cache) {
         QString *cachedPath = cache.object(key);
         if (cachedPath)
             bestFilePath = *cachedPath;
     }
 
-    if (bestFilePath.isEmpty()) {
+    // note: a cached file path may be empty
+    if (bestFilePath.isNull()) {
         QDir dir(m_path);
         int bestScore = -1;