Android: Support pregenerated cache in assets file engine

This provides a way for androiddeployqt to pregenerate the
entry list cache for the assets file engine, greatly improving
performance the first time a directory is read. If the file
is not present, the cache will operate as before.

Some numbers from testing on Samsung Galaxy 2, doing
QDir::entryList() on a directory inside the assets folder:

10 files
--------
Before:
     280 ms for first read,
       5 ms for subsequent reads
After:
       2 ms for reading pregenerated cache
       5 ms for first read
       5 ms for subsequent reads

2000 files
----------
Before:
    1000 ms for first read,
     150 ms for subsequent reads
After:
       5 ms for reading pregenerated cache
     150 ms for first read
     150 ms for subsequent reads

4000 files
----------
Before:
     3000 ms for first read
      300 ms for subsequent reads
After:
        8 ms for reading pregenerated cache
      300 ms for first read
      300 ms for subsequent reads

[ChangeLog][Android] Speed up first time directory listing
in assets by using pregenerated entry list.

Task-number: QTBUG-33704
Change-Id: I3973a1d823b8b38e88a2cc7843326cbe885f8bc2
Reviewed-by: Christian Stromme <christian.stromme@digia.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/plugins/platforms/android/qandroidassetsfileenginehandler.h b/src/plugins/platforms/android/qandroidassetsfileenginehandler.h
index 7bd560886c..d56367d4d8 100644
--- a/src/plugins/platforms/android/qandroidassetsfileenginehandler.h
+++ b/src/plugins/platforms/android/qandroidassetsfileenginehandler.h
@@ -58,9 +58,12 @@ public:
     QAbstractFileEngine *create(const QString &fileName) const;
 
 private:
+    void prepopulateCache();
+
     AAssetManager *m_assetManager;
     mutable QCache<QByteArray, QSharedPointer<AndroidAssetDir>> m_assetsCache;
     mutable QMutex m_assetsCacheMutext;
+    bool m_hasPrepopulatedCache;
 };
 
 #endif // QANDROIDASSETSFILEENGINEHANDLER_H