How can I apply the dependency inversion principle when using third party libraries?

Question

I was reading about the dependency inversion principle and as far as I understand the relation is inverted because package A (high level) defines the interface and package B (low level) implements the interface, instead of directly invoking the class in package B.

But how can I apply the dependency inversion principle when I do not own package B? I'm using PHP and through the Composer package manager I import some third party libraries. Since I'm not in control of that code, I cannot make the classes in that library implement my high level interface.

I’ve searched on Google and Stackoverflow but i can’t seem to find questions or articles that mention this use-case.

Answer 1

The standard solution is to write an Adapter over the third-party library. In C#, it would look like the following:

public interface IFoo
{
    void Bar(int baz);
    string Qux(bool quux);
}

This interface would be defined in your A pakcage.

In another package, you'd reference the third-party library and write the Adapter:

public class ThirdPartyFoo : IFoo
{
    private ThirdPartyObject thirdPartyObject;

    // Class initialisation goes here...

    public void Bar(int baz)
    {
        var corge = // perhaps translate or modify baz first...
        thirdPartyObject.SomeMethod(corge);
    }

    public string Qux(bool quux)
    {
        var grault = // perhaps translate or modify quux first...
        var res = thirdPartyObject.SomeOtherMethod(grault);
        var garply = // perhaps translate res
        return garply;
    }
}