The Per Rewrite Diaries: Day 35

This post is part of a series about rewriting my iOS app, Per. Per is a price per unit comparison app with a bunch of neat convenience figures, but it hasn’t been updated in years, so I’m rewriting it from scratch to eliminate a bunch of technical debt. Just because it’s not an open-source app doesn’t mean I can’t share what I learn as I go!

See the rest of the series here.

Fun With textDidChangeNotification

The first papercut I’m tackling since revisiting the list yesterday is a FIXME that affects the Add button in the product detail form view. We want the button to be disabled unless the form has “valid input”, which is to say, it’s got at least a price and quantity set.

The UITextFieldDelegate lets you hook into various events related to a text view, and the one we’re most interested here is when the text was changed. I kicked off this work by running a callback on textFieldDidEndEditing(_:) method that would call a method in the form view’s delegate, updateVolatileFormData() — this struct provides a temporary datastore for the price, quantity, and (optionally) units as Strings from each text field.

Instead, on each change of the text in the form, I can call that same update method — and do a little bit of additional checking. If the form data is complete, enable the Add button; otherwise, disable it.

So how do we hook in to this event? At the end of my form view’s setupView() method, I subscribe to textDidChangeNotification:

func setupView() {
    /* Set up the controls and other views here */

        selector: #selector(textDidChange(_:)),
        name: UITextField.textDidChangeNotification,
        object: nil

And then I create that selector:

@objc func textDidChange(_ notification: Notification) {
    if let textField = notification.object as! UITextField? {
        switch textField.tag {
        case 100:
            datasource?.quantity = textField.text ?? ""
        case 101:
            datasource?.units = textField.text ?? ""
        case 102:
            datasource?.price = textField.text ?? ""
            print("Unknown tag")

Notifications include an object property that you can pass to subscribers, and in this case, that object payload is the UITextField that triggered the notification (which is why I force-cast notification.object as a UITextField? here — remember that I’m subscribing to UITextField.textDidChangeNotification, so I’m pretty sure that cast is guaranteed to succeed).

Now, here’s a fun fact.

For reasons that I’m not clear on, these notifications are not fired when you change the content of a text field like so:

someTextField.text = "blah blah blah"

Instead, if you want to set the value of your text field programmatically, you’ll have to do it this way:

someTextField.text = ""
someTextField.insertText("blah blah blah")

That will fire the text-changed notification. Because I’m using a picker view whose didSelectRow: delegate method sets the value of the units text field, I had to change it from the former to the latter approach.

I guess this makes sense because inserting/removing text counts as changing the text that’s already there, whereas the straight assignment in the first approach doesn’t because that String object may not even exist yet? I’m not entirely sure, but it’s been a source of confusion in iOS for a while.

Anyways, with this work done, you can’t crash the app if you hit the Add button before you’ve got appropriate input values in your form. The last bit of work to get this to feature parity with the shipping version of Per is adding an inputAccessoryView for navigating fields and enabling the simple calculator feature, so I’ll kick off work on that tomorrow, then sort out all of the remaining papercuts before I move on to new v2.0 features (beyond being able to add multiple products, that is).

Angelo Stavrow

Montreal, Canada
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Mobile/full-stack developer. Montrealer. Internet gadabout. Your biggest fan.