SetSourceStream should not require a pointer to an interface

[bindermuehle]

I don't understand the reason for SetSourceStream to require a pointer to an interface.
the method signature should be
rs io.ReadSeeker
instead of
rs *io.ReadSeeker

a conversion to the interface would then not be necessary anymore as NewReader already implements the ReaderSeeker interface.

rs := bytes.NewReader(pdfBytes) tpl1 := gofpdi.ImportPageFromStream(pdf, rs, 1, "/TrimBox")
Let me know if you want me to create a pull request for this. I understand that this is a rather tedious issue since it actually breaks the interface and would mean this library and the gofpdf and gopdf library would have to release a new major version I suppose.

[mrtsbt]

@bindermuehle The pointer is used for internal caching. Using the interface directly would currently break multi page templates.

[bindermuehle]

I'm sorry I can't quite follow.
Importing a page from a multipage template seems to work regardless
Importing into a multipage pdf works as well.
importing multiple pages after each other from a multipage template into a multipage pdf works as well.
You can't import multiple pages at once anyways as you tell the command which page you want to import correct?
can you describe the case which breaks when using an interface instead of pointer to an interface in more detail please? I am trying to reproduce it.

[mrtsbt]

Sorry, I should have been more precise. If you take a look at SetSourceStream in importer.go, you'll see that the first thing it does is to construct a string from the pointer. That string is then used as an identifier for internal caching, analogous to SetSourceFile. Basically, the address of the stream is used as identifier for internal caching.

Using an interface with no other changes would work in a fashion, I guess, but printing the stream with fmt.Sprintf("%v") is ... less than ideal. Taking the address of the interface, on the other hand, will not produce the address of the stream. It would produce a different identifier at each call, which will break the caching mechanisms, leading to problems with multipage templates (e.g. producing the same page over again).

So, to replace the pointer-to-an-interface with an interface only you'd need some way to obtain a unique identifier for a given stream from the interface.

[awesomeunleashed]

For what it's worth, using fmt.Sprintf("%p") would work for any io.ReadSeekers that are already pointers, which is most likely the case in all real use cases. It wouldn't work well if you did happen to have an io.ReadSeeker that wasn't a pointer, though.