provide possibility to override expression used for validation

[Sebbstar]

We use some textfields to bind to arrays or other not supported datatypes. We rely on DataAnnotations on our models and do not want to copy these rules to the razor files.

For these properties we created proxy properties which convert between string and the type of the models.

Currently MatBlazor extracts the FieldIdentifier for the validation from the valueexpression which makes it impossible to use different properties for value and validation.
To workaround this problem we had to disable the validation of the field and manually set the validation classes. What we need to solve our problem is a way to override the BaseMatInputComponent<T>.FieldIdentifier to point to the real model and not to our value-providing proxy.
Currently our code looks like this:

   <MatTextField InputClass="@DnsValidationClass" ValidationDisabled="true" @bind-Value="@PROXYDnsEntries" Label="DnsEntries" HelperText="(comma seperated list)" />
        <ValidationMessage For="(() => ProxySettings.DnsEntries)" />

        public string DnsValidationClass
        {
            get
            {
                if (EditContext.GetValidationMessages(EditContext.Field(nameof(Model.DnsEntries))).Any())
                {
                    return "invalid";
                }
                else if (EditContext.IsModified(EditContext.Field(nameof(Model.DnsEntries))))
                {
                    return "valid modified";
                }
                else
                {
                    return string.Empty;
                }
            }
        }

        public string PROXYDnsEntries
        {
            get
            {
                return Model.DnsEntries == null ? string.Empty : string.Join(", ", Model.DnsEntries);
            }
            set
            {
            {
                Model.DnsEntries = value.Split(',').Select(x => x.Trim()).ToArray();
                EditContext.NotifyFieldChanged(EditContext.Field(nameof(Model.DnsEntries)));
            }
        }

With the proposed change from this merge request we could simplify our code like this:

   <MatTextField ValidateValueExpression="@(() => Model.DnsEntries)" @bind-Value="@PROXYDnsEntries"  />
        <ValidationMessage For="(() => Model.DnsEntries)" />

        public string PROXYDnsEntries
        {
            get
            {
                return Model.DnsEntries == null ? string.Empty : string.Join(", ", Model.DnsEntries);
            }
            set
            {
            {
                Model.DnsEntries = value.Split(',').Select(x => x.Trim()).ToArray();
            }
        }

[mattgenious]

Is it correctly understood that this pull request fixes #278 #213 #184 ?

[SamProf]

I am sorry, but at this branch (https://github.com/SamProf/MatBlazor/tree/feature/forms-2.1)and this issue (#271) I working on new forms components.
So, I will check this PR after I will complete all stuff with EditContext and etc...

[Christian-Oleson]

@Sebbstar ,
There are merge conflicts here