What are the pros and cons in using fluent interfaces in Delphi?
Fluent interfaces are supposed to increase the readability, but I’m a bit skeptical to have one long LOC that contains a lot of chained methods.
Are there any compiler issues?
Are there any debugging issues?
Are there any runtime/error handling issues?
Fluent interfaces are used in e.g. TStringBuilder, THTMLWriter and TGpFluentXMLBuilder.
Updated:
David Heffernan asked which issues I was concerned about. I’ve been given this some thought, and the overall issue is the difference between “explicitly specifying how it’s done” versus “letting the compiler decide how it’s done”.
AFAICS, there is no documentation on how chained methods actually is handled by the compiler, nor any specification on how the compiler should treat chained methods.
In this article we can read about how the compiler adds two additional var-parameters to methods declared as functions, and that the standard calling convention puts three params in the register and the next ones on the stack. A “fluent function method” with 2 params will therefor use the stack, whereas an “ordinary procedure method” with 2 params only uses the register.
We also know that the compiler does some magic to optimize the binary (e.g. string as function result, evaluation order, ref to local proc), but sometimes with surprising side effects for the programmer.
So the fact that the memory/stack/register-management is more complex and the fact that compiler could do some magic with unintentional side effects, is pretty smelly to me. Hence the question.
After I’ve read the answers (very good ones), my concern is strongly reduced but my preference is still the same 🙂
Compiler issues:
If you’re using interfaces (rather than objects), each call in the chain will result in a reference count overhead, even if the exact same interface is returned all the time, the compiler has no way of knowing it. You’ll thus generate a larger code, with a more complex stack.
Debugging issues:
The call chain being seen as a single instruction, you can’t step or breakpoint on the intermediate steps. You also can’t evaluate state at intermediate steps.
The only way to debug intermediate steps is to trace in the asm view.
The call stack in the debugger will also not be clear, if the same methods happens multiple times in the fluent chain.
Runtime issues:
When using interfaces for the chain (rather than objects), you have to pay for the reference counting overhead, as well as a more complex exception frame.
You can’t have try..finally constructs in a chain, so no guarantee of closing what was opened in a fluent chain f.i.
Debug/Error logging issues:
Exceptions and their stack trace will see the chain as a single instruction, so if you crashed in .DoSomething, and the call chain has several .DoSomething calls, you won’t know which caused the issue.
Code Formatting issues:
AFAICT none of the existing code formatters will properly layout a fluent call chain, so it’s only manual formatting that can keep a fluent call chain readable. If an automated formatter is run, it’ll typically turn a chain into a readability mess.