How to get std::basic_istream from AAssetManager?

Question

I am using NDK and I need to read resource media file. So, as far as I understand in order to access to resources I need to use AAssetManager , and eventually I need to get std::basic_istream to work with it.

So, question is, how to get std::basic_istream from AAssetManager ?

Answer 1

The answer is actually very different whether you have a compressed asset (eg text) or uncompressed (by default, Jpeg images and mp3 are stored by the packer). For these uncompressed assets, you can get the file descriptor with AAsset_openFileDescriptor() , and then follow the ways of How to construct a c++ fstream from a POSIX file descriptor? . For compressed assets, you may look for a (potentially API-level-dependent) hack that will let you get the file descriptor (or file path) to the transient file that the OS opens for you when it unpacks your asset.

Answer 2

You can adapt this example to work with the "buffer" mode of AAssetManager .

This will read the entire asset into memory, though. You could make an implementation that works more like std::fstream by reading chunks into a memory buffer, but that is significantly more complex.

Note: I only test-compiled the code below.

class asset_streambuf : public std::streambuf {
    public:
        asset_streambuf(AAsset * the_asset)
            : the_asset_(the_asset) {
                char * begin = (char *)AAsset_getBuffer(the_asset);
                char * end = begin + AAsset_getLength64(the_asset);
                setg(begin, begin, end);
            }
        ~asset_streambuf() {
            AAsset_close(the_asset_);
        }
    private:
        AAsset * the_asset_;
};

Usage:

AAsset * asset = AAssetManager_open(mgr, "some_asset.bin", AASSET_MODE_BUFFER);
asset_streambuf sb(asset);
std::istream is(&sb);

EDIT: Found a shorter way based on this answer .