Struct field reverts

I'm playing with Go a bit but found this weird situation while doing some tests.

I'm using method in a struct to send a variable to another method that should change a field, but when I check it at the end, the field goes back to the first value, which has me confused.

func (this TVManager) sendMessage(message string) {
    fmt.Println("5", this.connector)
    payload := map[string]string {
	   "id": "0",
	   "type": "request",
	   "uri": "ssap://system.notifications/createToast",
	   "payload": "{'message': 'This is a message'}"}
    this.connector.sendCommand(payload)
    fmt.Println("4", this.connector)
}

This is the method I'm testing, it calls sendCommand of connector.

func (this MockConnector) sendCommand(payload map[string]string) {
    fmt.Println("0", this)
    this.last_command = payload
    this.value = true
    fmt.Println("0", this)
}

Which in the mock object I'm using is simply changing the value of this struct fields.

	manager.sendMessage("This is a message")

fmt.Println("1", connector)
assert.Equal(t, expected, connector.last_command, "Command should be equal")

But when I check it, it goes back to internal. I set some prints to try an d track the values and they change the values as expected, but then it reverts.

 1 {false map[]}
 5 {false map[]}
 0 {false map[]}
 0 {true map[uri:ssap://system.notifications/createToast payload:{'message': 'This is a message'} id:0 type:request]}
 4 {false map[]}
 1 {false map[]}
 --- FAIL: TestTVManagerSendsNotificationDownToConnector (0.00s)

This is just a small program I'm going over to learn some Go, so I appreciate any help anybody could give me.

You are passing the structures by value. This works fine so long as you are not modifying the structure, but if you do modify it you are actually only modifying a copy. To make this work you need to use pointers to the structures you need to modify.

Instead of:

func (this MockConnector) sendCommand(payload map[string]string)

Use:

func (this *MockConnector) sendCommand(payload map[string]string)

Also, it is considered a bad idea to use this (or self) as a receiver name in Go, as a receiver is not the same thing as a this pointer/reference in other languages.

Another best practice, is if one method for a given type needs a pointer receiver, all methods for that type should have pointer receivers. This is so that the method set remains consistent no matter if the value is a pointer or not.

See method sets, and these FAQ answers for more information.