Go Mocking & Interfaces Demo

There are two major libs for mocking:

• golang/mock (we'll focus on this one)
• testify/mock

Setup mockgen

# Go version < 1.16
GO111MODULE=on go get github.com/golang/mock/mockgen@v1.5.0

or

# Go 1.16+
go install github.com/golang/mock/mockgen@v1.5.0

Generating Mocks

mockgen -source basic/basic.go -destination basic/mock/basic.go 
mockgen -source ethclient/ethclient.go -destination ethclient/mock/ethclient.go 

Basic Example

This is a simple example of an interface being used for a database. We can generate mocks for this interface and then write tests using it. See TestStoreMulti.

Note that we explicitly specify the expected interactions with the interface, thus we can verify exactly which calls are being made. In TestStoreMultiFailing we have specified an expected call which is different from the one that is made given the inputs, thus the test fails.

Ethclient Example

Here we have a struct which uses ethclient.Client.

We have replaced the usage of ethclient.Client with an interface - very easy, much wow!

We can generate mocks for this interface and easily test the functionality of the ChainReader.

Once you start using more interfaces, you can refactor your code to be much more modular, like we have with CheckBlocks which removes the need for the ChainReader struct.

Notes

Interfaces

• By convention, one-method interfaces are named by the method name plus an -er suffix or similar modification to construct an agent noun: Reader, Writer, Formatter, CloseNotifier etc. (source)
• Smaller interfaces are better
• Be cautious which types you use as this may restrict the usefulness of an interface