Polaroider : Creating formatters

Using formatters to mock objects

If a object being snapshotted has some properties with data that changes with each snapshot, it is possible to create a formatter that returns a constant value that represents the data of the type.
This can be the case with DateTimes or Guids that are timebased or generated with each object creation.

var options = SnapshotOptions.Create(o => o.AddFormatter<DateTime>(obj => "0000-00-00T00:00:00.0000"));
sn.MatchSnapshot(options);

For more complex structures create a formatter that derives IValueFormatter and add this to the SnapshotOptions.

public class MockDateTimeFormatter : IValueFormatter
{
	public string Format(object value)
	{
		if (value is DateTime)
		{
            // return a constant value instead of the changing datetime
            return "0000-00-00T00:00:00.0000";
		}
		return value?.ToString();
	}
}

Directive

The directives help find a part of the replace string that would match the changing data and convert this to a constant value

// change all occurrences of ISO 8601 DateTimes to a constant value
var options = SnapshotOptions.Create(o => 
  o.AddDirective(line => 
    line.ReplaceRegex("[0-9]{1,4}-[0-9]{1,2}-[0-9]{1,2}T[0-9]{1,2}:[0-9]{1,2}:[0-9]{1,2}.[0-9]{1,7}\\+[0-9]{1,2}:[0-9]{1,2}", "0000-00-00T00:00:00.0000")));
sn.MatchSnapshot(options);

Extensions

There are some builtin formatters for mocking objects.

Method	Description	Result
MockDateTimes	Format all DateTimes to a value that always matches	0000-00-00T00:00:00.0000
MockGuids	Format all Guids to a value that always matches	00000000-0000-0000-0000-000000000000

Mocking objects is configured in the SnapshotOptions.

// mock all occurrences of DateTimes to a constant value
var options = SnapshotOptions.Create(o => o.MockDateTimes());