The Android framework provides an easy way to automatically convert text patterns into clickable links. By default, Android knows how to recognize web URLs, email addresses, map addresses, and phone numbers, but it also includes a flexible mechanism for recognizing and converting additional text patterns, as well.
The Android Developers Blog has an article entitled Linkify your Text! that provides a nice overview of the system. It discusses how the Linkify class can be used to enable the default link patterns and then continues with a more advanced WikiWords example that demonstrates custom links. That article is a fine introduction to the system, so the rest of this article will primarily focus on details not covered therein.
All of the examples in this article are based on the TextView widget. The
Linkify class can also be used to add links to Spannable text, but those
use cases won’t be covered here because their usage is nearly identical to the
The TextView widget features an
android:autoLink attribute that
controls the types of text patterns that are automatically recognized and
converted to clickable links. This attribute is a convenient way to enable
one or more of the default link patterns because it can be configured directly
from a layout without involving any additional code.
However, for those cases where programmatically setting this value is useful,
setAutoLinkMask() function exists.
There is one important caveat to using this “auto-linking” functionality, however. It appears that when “auto-linking” is enabled, all additional Linkify operations are ignored. It’s unclear whether this behavior is intentional or inadvertent, so it’s possible things could change in future released of the Android SDK. Consider disabling “auto-linking” before using any of the Linkify operations discussed below.
Enabling support for one of Android’s default link patterns is very easy.
Simply use the
addLinks(TextView text, int mask) function and
specify a mask that describes the desired link types.
Detecting additional types of link patterns is easy, too. The
addLinks(TextView text, Pattern pattern, String scheme)
function detects links based on a regular expression pattern.
The text is scanned for pattern matches. Matches are converted to links that are generated by appending the matched text to the provided URL scheme base.
Note that the scheme doesn’t have to be an external web-like URL. It could also be an Android Content URI that can be used in conjunction with a content provider to reference application resources, for example.
Regular expressions are a very powerful way to match text patterns, but sometimes a bit more flexibility is needed. The MatchFilter class provides this capability by giving user code a chance to evaluate the link worthiness of some matched text.
A more complex (but useful!) example would involve matching valid dates. The regular expression could be generous enough to match strings like “2010-02-30” (February 30, 2010), but a match filter could provide the logic to reject bogus calendar dates.
Up until this point, the final link was always being generated based on the
exact matched text. There are many cases where that is not desirable,
however. For example, it’s common to mention a username using the
syntax, but the resulting link should only include the
username portion of
the text. The TransformFilter class provides a solution.
This approach uses the regular expression’s capture syntax to extract just the
username portion of the pattern as a uniquely addressable match group.
Alternatively, the transform filter could just return all of the matched text
after the first character (
@), but the above approach is nice because it
keeps all of the pattern’s details within the regular expression.
Of course, transform filters can be combined with match filters for ultimate flexibility. The Android SDK uses this approach to detect wide ranges of phone number formats (many of which include various parentheses and dashes) while always generating a simplified link containing only digits.
For more information about the specific implementation details of Android’s link generation system, the best reference is actually the source code itself. In addition to being a good resource for understanding the system, it’s also the best way to track down potential bugs or misunderstandings about how the system is intended to be used.