#Mocking Examples

The basic flow for making a test involving mock_gcs_client involves creating a mock, and calling On() on it to mock out the function calls.

var testBytes []byte = ...

func TestFetchFromGCS(t *testing.T) {
	testutils.SmallTest(t)
	m := test_gcsclient.NewMockClient()
	defer m.AssertExpectations(t)

	o := MyObjStruct{client:m}

	// AnythingOfType is handy when you don't really care what gets passed in, as long
	// as it matches the type specified.
	ctx := mock.AnythingOfType("*context.emptyCtx")

	m.On("GetFileContents", ctx, "my-file").Return(testBytes, nil)

	o.DoAThing()

	// Make sure we don't spam GCS with requests
	m.AssertNumberOfCalls(t, "GetFileContents", 1)
}

type MyObjStruct {
	client gcs.GCSClient
}


func (o *MyObj) DoAThing() {
	...
	content, err := o.client.GetFileContents(context.Background(), path)
	...
}

If you don‘t mock out a call, or the parameters don’t match exactly, the test will fail and give you a (hopefully) helpful message about how close the params were.

If you mock out a call that doesn't get used at all, AssertExpectations(t) will catch this and make the test fail.

mock.Anything() and mock.AnythingOfType(string) are useful for keeping the specifications too crazy.

In the above example, every call to m.GetFileContents("my-file") will return the same set of testBytes. If your test wants to change this (i.e. simulating the file goes missing), you'll need to do something like:

m.On("GetFileContents", ctx, "my-file").Return(testBytes, nil).Times(6)
m.On("GetFileContents", ctx, "my-file").Return(nil, errors.New("Not found"))

Where it will return the test bytes for the first 6 calls, and then return an error.

It is very tempting to do something like

	m.On("GetFileContents", ctx, "my-file").Return(testBytes, nil)

	o.DoAThing()

	m.On("GetFileContents", ctx, "my-file").Return(differentTestBytes, nil)

	// One might expect GetFileContents to start returning differentTestBytes, for "my-file",
	// but that is not the case.
	o.DoAThing()

But this does not work. As of this writing, unless .Times() is called, only the first m.On() is used.

##More Information

The Mock API is at https://godoc.org/github.com/stretchr/testify/mock