Updating external metadata of an ECL item

Question

I can successfully set metadata properties for an ECL item and display it in External Metadata Schema as shown below:

I am not able to figure out how to read the updated Metadata fields in the ECL code.

The idea is that properties like Alt Text can be updated in CME, read in ECL and saved back to external system. But, I am unable to read the updated field values in the following functions:

public string MetadataXml
{
    get
    {
        return metadata.ToString();
    }
    set
    {
        // set metadata values
    }
}

--OR--

public IContentLibraryItem Save(bool readback)
{
    return readback ? this : null;
}

Answer 1

I ended up with adding Metdata Schema to the newly created ECL Stub Schema. I am using the Metadata Schema to relate an alternate text information to ECL item. Its getting stored in Tridion and not in ECL.