Add support for computing function 4-byte selectors from ABI

[LogvinovLeon]

It would be nice to be able to compute 4-byte selectors from ABI. I'm talking about those: https://www.4byte.directory/signatures/
Smth like:

ethers.utils.getMethodSelector('callDeposit(address,address,uint256)') // `0xac9f2b7b`

ethers.utils.getMethodSelector('CaptureTheFlag(string flagName)') // `0x95907291 `

Should be pretty simple as it just requires removing the parameter names and hashing the thing taking the first 4 bytes.

[LogvinovLeon]

I can create a PR for it if you agree it's useful and we agree on the interface.

[ricmoo]

This may make sense, but generally if you need the sighash, you often need most of the other parts of the interface as well, which would just be thrown away...

let descr = new utils.Interface([ "function transfer(address, uint)" ]);

// The signhash
console.log("sighash", descr.functions..transfer.sighash);
// "0xa9059cbb"

// But also, you would often need/want to encode parameters
console.log(descr.functions.transfer.encode(["0x0123456789012345678901234567890123456789", 1234]));
// "0xa9059cbb000000000000000000000000012345678901234567890123456789012345678900000000000000000000000000000000000000000000000000000000000004d2"

// And the ability to parse the response
console.log(descr.functions.transfer.decode(callResponse));

It may make sense to add as a static method on the Interface, but let me think about it.

[LogvinovLeon]

Ok, I agree. This is redundant. One other question. Can I also decode parameters using the Interface? I haven't found that feature. I want to decode the call data, not the return data.

[ricmoo]

const abi = [
    "function transfer(address to, uint256 amount)"
];

const iface = new ethers.utils.Interface(abi);
const transferFunc = iface.functions.transfer;

// Encoding
let address = "0x0123456789012345678901234567890123456789";
let amount = 1000;
let calldata = transferFunc.encode([address, amount]);
console.log(calldata);
// "0xa9059cbb000000000000000000000000012345678901234567890123456789012345678900000000000000000000000000000000000000000000000000000000000003e8"

// Decoding
console.log(iface.parseTransaction({ data: calldata, value: "0x0" }));
// _TransactionDescription {
//  args:
//   [ '0x0123456789012345678901234567890123456789',
//     BigNumber { _hex: '0x03e8' } ],
//  decode: [Function],
//  name: 'transfer(address,uint256)',
//  signature: 'transfer(address,uint256)',
//  sighash: '0xa9059cbb',
//  value: BigNumber { _hex: '0x0' } }

(note this example just pointed out a bug; for now, value is required; I'll remove this requirement shortly)

[ricmoo]

This should now work with value omitted. Let me know if you have any issues.

Thanks! :)

[wellitongervickas]

Get first the fragment of abi function

const fragment = contract.interface.getFunction(method)

Then, get bytes4 signature of function

const selectorHash = contract.interface.getSighash(fragment)