Decimal input

ui.DecimalInput is a number input for amounts of money: it edits a *string holding an exact decimal between two bounds, which Up and Down, and the buttons beside it, change by a step. The value is written with exactly the decimals of the step, as "1234.50"; the bounds are written the same way, "" for no limit. Inside, it counts in math/big integers of units of the step's last decimal, so no amount is ever held in binary floating point.

ui.Field(c, "Amount", func() {
	if ui.DecimalInput(c, &app.amount, "0", "", "0.01").Changed() {
		app.save()
	}
})

What is typed applies as soon as it is a number in range, with . before its decimals, , between groups of three digits, and an optional -: 1,234.5 sets "1234.50", and shows so once the input loses the focus. Text and amounts in *string convert to whatever decimal type the app uses (shopspring/decimal, apd, big.Rat), and fit a database's TEXT columns.

Nothing is rounded

At a step of 0.01, 1.500 is 1.50, but 1.005 is not a number the input takes: rather than round it, it keeps the value as it was and marks the input with Error, whose message a field around it shows below, until the text is corrected. Text that is not a number, or out of range, is marked the same way, and stays when the input loses the focus, so that a typo is never silently replaced. Emptying the input leaves the value as it was, and shows it again once the input loses the focus.

A value the app sets replaces what was typed, a mistake included. A value the app sets with more decimals than the step's, or out of range, shows as it is, marked. Changed reports a new value: one written another way, as 1.5 for 1.50, is not.

As NumberInput, it reads . and , the same way in every language the window shows: 1,50 is not one and a half, but text that is not a number, which the input marks rather than read as 150.

Accessibility

Assistive technology sees a spin button of the value, named by its Label or its field, which it increments and decrements by the step, and reads the value as the input's text, exactly. An error's message describes the input, which it reads as invalid.