Opening hyperlinks in UILabel on iOS
UILabel supports displaying attributed strings with link attributes but what it does not support is allowing to tap on hyperlinks and opening those in Safari. An Alternative way is using an UITextView which does support opening hyperlinks but on the other hand it is a more heavy view component and therefore might not be the best choice when we just need to display some text with hyperlinks. This time lets create a simple UILabel subclass which adds support for opening hyperlinks and custom hyperlink styles.
Creating NSAttributedStrings with hyperlinks
Before we jump into implementing an UILabel subclass HyperlinkLabel, let’s first take a look on how to create a NSAttributedString with hyperlinks. In the example app we will have several labels with different configurations: default and custom link styles and left, centre, right text alignments. UITextView has a linkTextAttributes property but in our UILabel subclass we’ll need to implement custom link styling ourselves. The approach we are going to take is creating a new NSAttributedString.Key value named hyperlink and adding text attributes when the attribute is present.
Let’s now create a convenience method which creates NSAttributedString and sets it to the HyperlinkLabel.
The final NSAttributedString is created by first initializing a mutable version with a format string. The format string follows the familiar format specifiers like we used with NSString and String APIs when dealing with re-orderable arguments. Format specifiers are replaced with new instances of NSAttributedStrings where the string value equals to a hyperlink name and the URL value is stored on the NSAttributedString.Key.link or NSAttributedString.Key.hyperlink attribute. The former gives us the default link style defined by Apple and the latter our custom link style.
Creating HyperlinkLabel which supports tapping on hyperlinks
UILabel does not provide access to its text layout and therefore it is not possible to know which hyperlink was tapped. For finding the tapped hyperlink we’ll need to use our own NSLayoutManager, NSTextStorage, and NSTextContainer. If we configure those properly we can figure out which character was tapped and therefore if it was part of the hyperlink. If it is, then we can let UIApplication to open the tapped URL. Let’s contain this in a function which gives us a configured NSTextStorage.
Now we can use this function when handling UITouch events. We’ll add another private function which will take current UITouches and figure out which hyperlink was tapped. The general flow consists of creating a NSTextStorage with the function we just defined, then asking from the NSLayoutManager which character index was tapped. NSLayoutManager returns the closest character index of the touch. Therefore, we’ll need to go one step deeper and ask for the actual bounding rectangle of the glyphs representing the character and then verifying if the touch location was inside the glyph’s bounding rectangle. This is important when dealing with different text alignments and when tapping on the free space around characters. After figuring out which character was tapped we’ll need to check for the hyperlink attributes. If the tapped character has either attribute set, then we can conclude that we tapped on a hyperlink. This function can then be called in touchesEnded and if we tapped on a hyperlink, then we can open it. One thing to note is that userInteractionEnabled needs to be set to true before UILabel can handle touch events.
Next thing we need to do is overriding attributedText property and handling the custom style of our custom hyperlink attribute. If the attributed text has this attribute set, then we will apply custom hyperlink text attributes. The same way as Apple’s link attribute works, when the attributed string gets displayed then custom styling is used. Secondly, we’ll set the UILabel’s font to the attributed string’s ranges which do not have a font attribute set. UILabel internally use UILabel’s font when font attributes are not set, so we want to force the same behaviour when the stored attributed string is set to our own NSTextStorage. If we do not do this, then NSAttributedString just uses its default font and the displayed string is not going to be equal to the string set to NSTextStorage. This in turn will lead to invalid character index calculations because fonts are different.
We created a UILabel subclass which has a capability of opening tapped links. Useful, if we need just a label for displaying some text along with hyperlinks. Please take a look on the full implementation available here: HyperlinkLabel.
UILabelHyperlinks (Xcode 12.3)