Perl web-app testing with PageObjects

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Weasel models a hierarchy of page objects on any given page to be tested. A typical Weasel model for a login page could be:

   +-> LoginSection
   |     |
   |     +-> Username control (Weasel::Widgets::HTML::Input/regular input mode)
   |     +-> Password control (Weasel::Widgets::HTML::Input/password mode)
   |     +-> Production/Test environment selector (Weasel::Widgets::HTML::Select)
   |     \-> Login button (Weasel::Widgets::HTML::Button)
   +-> OtherSection1
   \-> OtherSectionN

where each level of the hierarchy owns a set of children. Each child itself may own a number of children. Each level encapsulates DOM internals into services for the next higher level.

The need for the abstraction of the individual widgets becomes apparent when the application switches from the use of SELECT tags to DojoToolkit’s FilteringSelect or Vuetify Selects. When making such a switch, the basic service of the widget remains the same (“select a value from a list”), but the DOM internals completely change. Weasel’s widgets handle the abstraction of internals of (small) parts of a page for the higher levels in the hierarchy to use.



Elements represent a part of a page. They have 2 roles with associated methods and properties:

  1. Elements fullfil the role of DOM tags
  2. Elements fullfil the role of PageObject

As tags, elements have methods to search for child-DOM elements, query and set tag properties and all other operations generally supported by web application testing frameworks.

In their role of PageObjects, elements (called Widgets in this role, but note that this is the same object in a different role!) encapsulate DOM internals and offer services to tests or to other PageObjects. E.g. “select a value from a list” as offered by the SELECT tag (and all re-implementations in all web frameworks thereof).

In order to increase encapsulation of DOM internals into Widgets, each Widget publishes a CSS/XPath search pattern using an alias. This keeps the details of the DOM internals inside the Widget while the search pattern can be used by other Widgets without breaking the encapsulation.


FindExpanders are functions that, given a set of arguments, return an XPath expression to be used for finding one or more PageObjects of a given type.

One of the FindExpanders that Weasel comes with (*password), is implemented as:

sub password_expander {
    my %args = @_;

    my @clauses;
    for my $clause (qw/ id name /) {
        push @clauses, "\@$clause='$args{$clause}'"
            if defined $args{$clause};
    my $clause = @clauses ? join ' and ', ('', @clauses) : '';

    return ".//input[\@type='password' $clause]";

Which defines a function which takes two named arguments (id and name) and returns an XPath expression to find an INPUT tag of type PASSWORD, with the value for either or both of the attributes id and name specified as given.

Later, this expander is registered as:

register_find_expander('*password', 'HTML', \&password_expander);

Code can then use the password expander without knowledge of the DOM internals as:

  my $pwd_widget = $self->find('*password', name => 'login_password');


The previous paragraph covered how widgets encapsulate their DOM internals by providing FindExpanders in addition to the general encapsulation that should be expected from PageObjects.

Weasel enables further abstraction: Widgets define their own mapping from DOM elements to Perl classes. With this mapping, Weasel makes sure that values returned by DOM search and modification functions, represent the correct widgets. By consequece, the returned values behave both as DOM elements and provide the page services the caller may expect from them.

The automatic mapping has an additional benefit: With Weasel, there’s no need for large-scale rewriting of tests when an application developer chooses to move SELECT implementation to another. As a switch between various UIs doesn’t influence application functionality, Widgets for the different UIs offer the same application services. Due to the fact that the widgets themselves encapsulate the DOM and the XPath to find the Widget in the page, the test developer doesn’t need to be impacted by the change at all.