I have a situation where I wanted to clear a picker field if the entry did not have a valid value in the Picker Field. This is how I did it. If anybody has a tidier solution that would also be good.
In the 'When Picker is resolving' Rule. I added the following.
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When Picker is Resolving
also populate Picker list control with data (Configure)
If Picker contains a value
then set the properties of the checkbox control ' I use this just as a visual verification check on the form.
If Picker does not contain a value
the complete the following one after another
then set the properties of checkbox control (configure) - DeSelect the checkbox visual verification check box and set a label No account found.
then set the properties of picker control - I clear the watermark otherwise it puts it under the text of the username.
then populate Picker list control with data (configure) - I set the Account value of my Picker to 'Try again.' I think any text would work.
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What it effectively does is wipe the incorrect Account Name from the Picker field and puts an insertion point for the client to reenter an appropriate account name.
I realise this might be a bit of a fundge, but it seems to work.