Validation

This example demonstrates how to add a formatter to a textbox in order to ensure that the textbox contains valid data.

The example is reasonably complex, for a number of reasons: firstly there are currently no built-in implementations of the Formatter trait included in Druid, which means we have to write versions for the example, and additionally the mechanism for reporting errors that occur during formatting are difficult to handle in Druid's current architecture, and involve some fairly ugly use of Command in order to function.