You can receive callbacks when the active test run, or individual tests, starts and finishes. You can register callbacks by invoking `RegisterCallbacks` on the [TestRunnerApi](./reference-test-runner-api.md) with an instance of a class that implements [ICallbacks](./reference-icallbacks.md). There are four `ICallbacks` methods for the start and finish of both the whole run and each level of the test tree.
## Example
An example of how listeners can be set up:
> **Note**: Listeners receive callbacks from all test runs, regardless of the registered `TestRunnerApi` for that instance.
``` C#
public void SetupListeners()
{
var api = ScriptableObject.CreateInstance<TestRunnerApi>();
api.RegisterCallbacks(new MyCallbacks());
}
private class MyCallbacks : ICallbacks
{
public void RunStarted(ITestAdaptor testsToRun)
{
}
public void RunFinished(ITestResultAdaptor result)
{
}
public void TestStarted(ITestAdaptor test)
{
}
public void TestFinished(ITestResultAdaptor result)
> **Note**: The registered callbacks are not persisted on domain reloads. So it is necessary to re-register the callback after a domain reloads, usually with [InitializeOnLoad](https://docs.unity3d.com/Manual/RunningEditorCodeOnLaunch.html).
It is possible to provide a `priority` as an integer as the second argument when registering a callback. This influences the invocation order of different callbacks. The default value is zero. It is also possible to provide `RegisterCallbacks` with a class instance that implements [IErrorCallbacks](./reference-ierror-callbacks.md)that is an extended version of `ICallbacks`. `IErrorCallbacks`also has a callback method for `OnError` that invokes if the run fails to start, for example, due to compilation errors or if an [IPrebuildSetup](./reference-setup-and-cleanup.md) throws an exception.
