Fetch data from IPFS URI

• In smart contracts, some methods return a URI pointing to IPFS.

• To fetch data from this URI we need first to register an oracle:

oracle(ipfs, r, 'hyperdapp.mypinata.cloud/ipfs/').

• Then, we need to extract the CID from the IPFS URI. The URI root is what comes before the CID. For example, https://ipfs.io/ipfs/

// SYNTAX
extract_cid([URI, CID]) :-
  atom_chars(URI, S0),
  append(URI_ROOT, S1, S0),
  append(S2, "", S1),
  atom_chars(CID, S2).
  
// EXAMPLE
extract_cid([URI, CID]) :-
  atom_chars(URI, S0),
  append(https://ipfs.io/ipfs/, S1, S0),
  append(S2, "", S1),
  atom_chars(CID, S2).

• Finally, we can prompt the data returned from IPFS:

prompt :-
  call_fn(hyperdapp, tokenURI('55'),  [URI]),
  extract_cid([URI, CID]),
  get_http(ipfs, CID, { 
    name: Name, 
    description: Description,
    image: Image
  }),
  show [ 
    text('Name: ', Name),
    text('Description: ', Description),
    image(Image)
  ].

• We call the tokenURI method from the smart contract.

• We extract the CID from the URI.

• We use the get_http() predicate on the ipfs oracle. The output can be a big object, but we extract only what we need to display. For example, the name, description and image.

The complete dApp treated in this example can be found here - RINKEBY