[core] introduce Blob for compressed and uncompressed dataupstream/blob

- Blob is a wrapper type for a shared_ptr<const string> that has accessor functions for getting compressed and uncompressed data
- Moved util::writeFile, util::readFile, util::compress, util::uncompress, decodeImage, and encodePNG to the Blob interface
- Added Blob support to Request and file sources
- Added Blob support to VectorTile objects
- Added support for gzip decoding to util::uncompress
- We're no longer compressing WebP, PNG, and JPEG data when storing in the OfflineDatabase
- Android's HTTPRequest returns compressed Blobs by default

One caveat is that our previous decompress function didn't support gzip, so once users upgrade to this version, their offline cache may contain both zlib-compressed data and gzip-compressed data, but older versions won't be able to decompress gzip data. On the other hand, we don't support downgrading SDKs anyway, so this shouldn't be a problem. To be on the safe side, we could bump the user_version of the SQLite DB.

1 files changed, 7 insertions, 0 deletions

diff --git a/include/mbgl/util/compression.hpp b/include/mbgl/util/compression.hpp
index 93a3ddb8bc..cbce764683 100644
--- a/include/mbgl/util/compression.hpp
+++ b/include/mbgl/util/compression.hpp
@@ -5,8 +5,15 @@
 namespace mbgl {
 namespace util {
 
+// Compresses data with the deflate algorithm.
 std::string compress(const std::string& raw);
+
+// Decompresses data that is in deflate format, optionally wrapped in a gzip container.
 std::string decompress(const std::string& raw);
 
+// Returns true when there's a good chance that the string can be compressed.
+// In particular, it returns false when the data is an already compressed image format.
+bool isCompressible(const std::string& raw);
+
 } // namespace util
 } // namespace mbgl