How to Mock Struct Methods in Go Test Cases Without Interfaces?

Mocking Struct Methods in Go Test Cases

In Go, mocking method calls of a struct can be achieved without introducing interfaces into the source code. Here's how:

Mocking a Sample Struct and Method

Consider the following struct and method:

type A struct {}

func (a *A) perfom(string){
...
...
..
}

Mocking in Test Cases

To mock the perform method for test cases:

1. Create a Mock Interface: Define an interface that represents the method to be mocked.

type Performer interface {
    perform(string)
}

2. Create Mock and Real Implementations: Implement the interface for both a mock and a real implementation of the struct.

type AMock struct {}

func (a *AMock) perform(string) {
    // Mocked behavior
}

type A struct {}

func (a *A) perform(string) {
    // Real implementation
}

3. Use Dependency Injection: Pass the mocked or real implementation as an argument to the function under test.

func invoke(url string, p Performer) {
    out := p.perfom(url)
    ...
    ...
}

4. Inject Mock in Tests: In your test cases, inject the mock implementation into the invoke function.

func TestInvokeWithMock(t *testing.T) {
    var amok = &AMock{}
    invoke("url", amok)
    // Verify mock behavior (e.g., assert it was called with the correct argument)
}

5. Inject Real Implementation in Production Code: In your production code, inject the real implementation of the struct into the invoke function.

func TestInvokeWithReal(t *testing.T) {
    var a = &A{}
    invoke("url", a)
    // No need for verification since it's the real implementation
}

Other Options

Libraries like [testify/mock](https://godoc.org/github.com/stretchr/testify/mock) provide even more robust mocking capabilities, allowing you to control mock behavior and verify method calls.

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