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Monitoring page loads: UIToolbar and UIProgressView

Now is a great time to meet two new UIView subclasses: UIToolbar and UIProgressView. UIToolbar holds and shows a collection of UIBarButtonItem objects that the user can tap on. We already saw how each view controller has a rightBarButton item, so a UIToolbar is like having a whole bar of these items. UIProgressView is a colored bar that shows how far a task is through its work, sometimes called a "progress bar."

The way we're going to use UIToolbar is quite simple: all view controllers automatically come with a toolbarItems array that automatically gets read in when the view controller is active inside a UINavigationController.

This is very similar to the way rightBarButtonItem is shown only when the view controller is active. All we need to do is set the array, then tell our navigation controller to show its toolbar, and it will do the rest of the work for us.

We're going to create two UIBarButtonItems at first, although one is special because it's a flexible space. This is a unique UIBarButtonItem type that acts like a spring, pushing other buttons to one side until all the space is used.

In viewDidLoad(), put this new code directly below where we set the rightBarButtonItem:

let spacer = UIBarButtonItem(barButtonSystemItem: .flexibleSpace, target: nil, action: nil)
let refresh = UIBarButtonItem(barButtonSystemItem: .refresh, target: webView, action: #selector(webView.reload))

toolbarItems = [spacer, refresh]
navigationController?.isToolbarHidden = false

The first line is new, or at least part of it is: we're creating a new bar button item using the special system item type .flexibleSpace, which creates a flexible space. It doesn't need a target or action because it can't be tapped. The second line you've seen before, although now it's calling the reload() method on the web view rather than using a method of our own.

The last two lines are new: the first creates an array containing the flexible space and the refresh button, then sets it to be our view controller's toolbarItems array. The second sets the navigation controller's isToolbarHidden property to be false, so the toolbar will be shown – and its items will be loaded from our current view.

That code will compile and run, and you'll see the refresh button neatly aligned to the right – that's the effect of the flexible space automatically taking up as much room as it can on the left.

The next step is going to be to add a UIProgressView to our toolbar, which will show how far the page is through loading. However, this requires two new pieces of information:

  • You can't just add random UIView subclasses to a UIToolbar, or to the rightBarButtonItem property. Instead, you need to wrap them in a special UIBarButtonItem, and use that instead.
  • Although WKWebView tells us how much of the page has loaded using its estimatedProgress property, the WKNavigationDelegate system doesn't tell us when this value has changed. So, we're going to ask iOS to tell us using a powerful technique called key-value observing, or KVO.

First, let's create the progress view and place it inside the bar button item. Begin by declaring the property at the top of the ViewController class next to the existing WKWebView property:

var progressView: UIProgressView!

Now place this code directly before the let spacer = line in viewDidLoad():

progressView = UIProgressView(progressViewStyle: .default)
let progressButton = UIBarButtonItem(customView: progressView)

All three of those lines are new, so let's go over them:

  1. The first line creates a new UIProgressView instance, giving it the default style. There is an alternative style called .bar, which doesn't draw an unfilled line to show the extent of the progress view, but the default style looks best here.
  2. The second line tells the progress view to set its layout size so that it fits its contents fully.
  3. The last line creates a new UIBarButtonItem using the customView parameter, which is where we wrap up our UIProgressView in a UIBarButtonItem so that it can go into our toolbar.

With the new progressButton item created, we can put it into our toolbar items anywhere we want it. The existing spacer will automatically make itself smaller to give space to the progress button, so I'm going to modify my toolbarItems array to this:

toolbarItems = [progressButton, spacer, refresh]

That is, progress view first, then a space in the center, then the refresh button on the right.

If you run the app now, you'll just see a thin gray line for our progress view – that's because it's default value is 0, so there's nothing colored in. Ideally we want to set this to match our webView's estimatedProgress value, which is a number from 0 to 1, but WKNavigationDelegate doesn't tell us when this value has changed.

Apple's solution to this is huge. Apple's solution is powerful. And, best of all, Apple's solution is almost everywhere in its toolkits, so once you learn how it works you can apply it elsewhere. It's called key-value observing (KVO), and it effectively lets you say, "please tell me when the property X of object Y gets changed by anyone at any time."

We're going to use KVO to watch the estimatedProgress property, and I hope you'll agree that it's useful. First, we add ourselves as an observer of the property on the web view by adding this to viewDidLoad():

webView.addObserver(self, forKeyPath: #keyPath(WKWebView.estimatedProgress), options: .new, context: nil)

The addObserver() method takes four parameters: who the observer is (we're the observer, so we use self), what property we want to observe (we want the estimatedProgress property of WKWebView), which value we want (we want the value that was just set, so we want the new one), and a context value.

forKeyPath and context bear a little more explanation. forKeyPath isn't named forProperty because it's not just about entering a property name. You can actually specify a path: one property inside another, inside another, and so on. More advanced key paths can even add functionality, such as averaging all elements in an array! Swift has a special keyword, #keyPath, which works like the #selector keyword you saw previously: it allows the compiler to check that your code is correct – that the WKWebView class actually has an estimatedProgress property.

context is easier: if you provide a unique value, that same context value gets sent back to you when you get your notification that the value has changed. This allows you to check the context to make sure it was your observer that was called. There are some corner cases where specifying (and checking) a context is required to avoid bugs, but you won't reach them during any of this series.

Warning: in more complex applications, all calls to addObserver() should be matched with a call to removeObserver() when you're finished observing – for example, when you're done with the view controller.

Once you have registered as an observer using KVO, you must implement a method called observeValue(). This tells you when an observed value has changed, so add this method now:

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if keyPath == "estimatedProgress" {
        progressView.progress = Float(webView.estimatedProgress)

As you can see it's telling us which key path was changed, and it also sends us back the context we registered earlier so you can check whether this callback is for you or not.

In this project, all we care about is whether the keyPath parameter is set to estimatedProgress – that is, if the estimatedProgress value of the web view has changed. And if it has, we set the progress property of our progress view to the new estimatedProgress value.

Minor note: estimatedProgress is a Double, which as you should remember is one way of representing decimal numbers like 0.5 or 0.55555. Unhelpfully, UIProgressView's progress property is a Float, which is another (lower-precision) way of representing decimal numbers. Swift doesn't let you put a Double into a Float, so we need to create a new Float from the Double.

If you run your project now, you'll see the progress view fills up with blue as the page loads.

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