Conditional Date Validation in Flutter

[dmarotta77]

I need to specify start and end dates for an event with the following constraints:

• Start date field is mandatory.
• End date field is optional.
• If the end date field is populated, it must be greater than or equal to the start date.

Currently, using Validators.compare, I'm getting an error notification even if the end date is not specified.

Should I implement a custom validator?

[omj-denis]

I'm in the same situation right now: min and max price filters, both optional. But if both are specified, a validation is needed (min <= max). So, I use CompareValidator for this, but it reports an error if one of the 2 fields is empty.

I see the following options:

• CompareValidator should ignore nulls. If somebody needs to ensure non-null values, they can add Validators.required. That could be the simplest solution, but, unfortunately, it's a breaking change now.
• custom validator, as you suggest. A duplication of CompareValidator. Duplication is not a good practice. Obviously, that's the reason why you want to avoid it and why you created this issue.
• combine CompareValidator inside Validators.composeOR with a custom AllowNullValidator which only purpose is to report VALID for null values (and thus suppress the error coming from CompareValidator). This solution is acceptable but AllowNullValidator doesn't make much sense on its own.
• add an optional parameter allowNull to CompareValidator to suppress this error (similar to PR allowNull flag for number validator #451)

Here's the implementation of the last option: PR #476