Angular differentiates between a Service (like Singleton) and Factory (new Instance). Those classes should not be a singleton. If we really need / want a singleton the DI container should handle this.

Otherwise I don't see what the problem is with creating an instance of those classes. It is I think currently only done in core and they usually do not have any state anyway.

Didn't get an email for this comment either...

This issue is about comparing component objects. For example, to see if two dimensions are equal (needed by PivotByDimension filter) you have to write:

!empty($lhs) && !empty($rhs) && $lhs->getId() == $rhs->getId()

This is more code than should be necessary. And the nature of these objects demands that there should only be one of them. A Dimension instance holds a description of an analytics dimension. It does not store state that can be altered, it is not manipulated or transformed by any algorithm and it does not provide logic that can be re-used by different types of software. There should not be more than one instance of a Dimension.

To illustrate better, consider scala, which has first class support for singletons (but different than Piwik's Singleton implementation). A dimension in Scala might look like this:

object Keyword extends VisitDimension {
    // dimension setup goes here (this is the constructor)
}

A report for the dimension would look like this:

object GetKeywords extends Report {
    this.module = "Referrers";
    this.action = "getKeywords";
    this.dimension = Keywords;
}

So the PivotByDimension check for a subtable dimension might look like:

val report = GetKeywords,
      pivotByDimension = City; // UserCountry.City
return report.subtableReport.dimension == pivotByDimension;

Which I think is a much better way of expressing the logic.

Of course using Piwik's Singleton base is not the same as a scala singleton and thus using it would be a mistake. DI is what I had in mind when I created the ticket.

This will be done as part of the UI & Client side improvements project, where we slowly move logic from server side controllers towards client side controllers (AngularJS), and improving our HTTP APIs so that clients (eg. browser) can do everything with such APIs. Some work is already done in Piwik 3.0 branch, for more info see this issue #7131