Let the committed model be amounts and an exchange rate , with invariant . Editing pounds keeps the rate and computes euros; editing euros keeps the rate and computes pounds; editing the rate keeps pounds and computes euros. A usable rate must be strictly positive, and every accepted floating-point number and derived result must be finite. Negative amounts are allowed. Rejecting invalid text preserves the last committed model.
Insert the following members inside the supplied The invariant is interpreted with the usual floating-point rounding in multiplication and division. All candidate values are calculated before committing any of them, so invalid text, zero/negative rates, NaN, infinity and floating-point overflow leave the old model intact. The committed
MainFrame, after the three fields have been initialized. Its existing layout can remain as written:private var p = 10.0
private var e = 11.5
private var r = 1.15
private var refreshing = false
private def finite(x: Double): Boolean = !x.isNaN && !x.isInfinity
private def showModel(): Unit = {
refreshing = true
try {
pounds.text = java.lang.String.format(java.util.Locale.ROOT, "%.2f", Double.box(p))
euros.text = java.lang.String.format(java.util.Locale.ROOT, "%.2f", Double.box(e))
rate.text = r.toString
} finally {
refreshing = false
}
}
private def edit(source: TextField): Unit = {
try {
val v = source.text.trim.toDouble
if (finite(v)) {
var nextP = p
var nextE = e
var nextR = r
if (source eq pounds) {
nextP = v
nextE = v * r
} else if (source eq euros) {
nextE = v
nextP = v / r
} else if (source eq rate) {
nextR = v
nextE = p * v
}
if (nextR > 0.0 && finite(nextP) && finite(nextE) && finite(nextR)) {
p = nextP
e = nextE
r = nextR
}
}
} catch {
case _: NumberFormatException => ()
}
showModel()
}
listenTo(pounds, euros, rate)
reactions += {
case EditDone(source) if !refreshing => edit(source)
}r remains positive, so euro-to-pound division is defined. Using a fixed formatting locale keeps the decimal separator compatible with toDouble. The displayed amounts are rounded to two decimal places, while the model retains their full available precision. The rate is not forcibly rounded to two places, which could otherwise turn a valid small positive rate into displayed zero.The
refreshing guard prevents synchronous derived-field notifications from recursively committing another edit. EditDone is an edit-completion event, not a request to react to every partial keystroke; pressing the field's completion action is enough to trigger the conversion. If a particular UI additionally needs live updates, it can subscribe to the appropriate text-change events with the same validation and authoritative-source policy. The supplied API and archived Scala Swing documentation suffice for the requested completion-event implementation; no unidentified practical library is required.[ref]Scala Swing event-system documentation. Publisher/Reactor subscription design; archived Swing event API: EditDone carries its source TextField
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