Hook Control Host Container Messages

I'm busy with a bar code reading retail app, and I would like to encapsulate
the code that responds to bar code inputs in a Bar Code control / component
/ library.  I use a special character string to prefix bar codes, so the UI
knows to give the bar code field focus before the barcode is sent, and a tab
to leave the barcode field after the whole barcode is received.



If I were to wrap the barcode field in a separate control, how could I
delegate the responsibly of detecting the start-of-code string to this
control, when it doesn't have focus and doesn't receive keystrokes.  I'm
thinking along the lines of it installing some sort of message hook in its
parent control, and focussing itself when it sees its parent receive the
start-of-code string, but I have no idea where to start.


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