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Sol-cov uses transaction traces in order to figure out which lines of Solidity source code have been covered by your tests. In order for it to gather these traces, you must add the CoverageSubprovider to the ProviderEngine instance you use when running your Solidity tests. If you're unfamiliar with ProviderEngine, please read the Web3 Provider explained wiki article.
The CoverageSubprovider eavesdrops on the eth_sendTransaction and eth_call RPC calls and collects traces after each call using debug_traceTransaction. eth_call's' don't generate traces - so we take a snapshot, re-submit it as a transaction, get the trace and then revert the snapshot.
Coverage subprovider needs some info about your contracts (srcMap, bytecode). It gets that info from your project's artifacts. Some frameworks have their own artifact format. Some artifact formats don't actually contain all the neccessary data.
In order to use CoverageSubprovider with your favorite framework you need to pass an artifactsAdapter to it.
Sol-compiler
If you are generating your artifacts with @0xproject/sol-compiler you can use the SolCompilerArtifactsAdapter we've implemented for you.
import { SolCompilerArtifactsAdapter } from '@0xproject/sol-cov';
const artifactsPath = 'src/artifacts';
const contractsPath = 'src/contracts';
const artifactsAdapter = new SolCompilerArtifactsAdapter(artifactsPath, contractsPath);
Truffle
If your project is using Truffle, we've written a TruffleArtifactsAdapterfor you.
import { TruffleArtifactAdapter } from '@0xproject/sol-cov';
const contractsPath = 'src/contracts';
const artifactAdapter = new TruffleArtifactAdapter(contractsDir);
Because truffle artifacts don't have all the data we need - we actually will recompile your contracts under the hood. That's why you don't need to pass an artifactsPath.
Other framework/toolset
You'll need to write your own artifacts adapter. It should extend AbstractArtifactsAdapter.
Look at the code of the two adapters above for examples.
Usage
import { CoverageSubprovider } from '@0xproject/sol-cov';
import ProviderEngine = require('web3-provider-engine');
const provider = new ProviderEngine();
const artifactsPath = 'src/artifacts';
const contractsPath = 'src/contracts';
const networkId = 50;
// Some calls might not have `from` address specified. Nevertheless - transactions need to be submitted from an address with at least some funds. defaultFromAddress is the address that will be used to submit those calls as transactions from.
const defaultFromAddress = '0x5409ed021d9299bf6814279a6a1411a7e866a631';
const isVerbose = true;
const coverageSubprovider = new CoverageSubprovider(artifactsAdapter, defaultFromAddress, isVerbose);
provider.addProvider(coverageSubprovider);
After your test suite is complete (e.g in the Mocha global after hook), you'll need to call:
await coverageSubprovider.writeCoverageAsync();
This will create a coverage.json file in a coverage directory. This file has an Istanbul format - so you can use it with any of the existing Istanbul reporters.