TThreadedQueue: interesting, but incomplete
I mentioned the new generic collection TThreadedQueue<T> in my First Look at Delphi XE. I decided to play around with it a little recently. It’s useful for passing data between one thread that produces output and another that consumes it, keeping the two in step by blocking if the consumer tries to pop from the queue while it’s empty.
The first thread goes through and pushes data into the queue however it wants to. The second has it easy; all it has to do is loop endlessly until the queue is shut down. And we all know how to do that:
for value in queue do
process(value);
Except that if you try to do that, the compiler will complain at you. There’s no enumerator. For some strange reason, out of all the collections in Generics.Collections, TThreadedQueue<T> alone does not descend from TEnumerable<T>.
Oh well. It’s not all that hard to add an enumerator to a class that doesn’t have one. Just use a class helper.
TThreadedQueueEnumerator<T> = class
private
FQueue: TThreadedQueue<T>;
FCurrent: T;
function GetCurrent: T;
public
constructor Create(queue: TThreadedQueue<T>);
property Current: T read GetCurrent;
function MoveNext: Boolean;
end;
TThreadedQueueHelper<T> = class helper for TThreadedQueue<T>
public
function GetEnumerator: TThreadedQueueEnumerator<T>;
end;
implementation
{ TThreadedQueueEnumerator<T> }
constructor TThreadedQueueEnumerator<T>.Create(queue: TThreadedQueue<T>);
begin
FQueue := queue;
end;
function TThreadedQueueEnumerator<T>.GetCurrent: T;
begin
result:= FCurrent;
end;
function TThreadedQueueEnumerator<T>.MoveNext: Boolean;
begin
result := FQueue.PopItem(FCurrent) = wrSignaled;
end;
{ TThreadedQueueHelper<T> }
function TThreadedQueueHelper<T>.GetEnumerator: TThreadedQueueEnumerator<T>;
begin
result := TThreadedQueueEnumerator.Create(self);
end;
Well, that was easy. That’s probably the simplest enumerator I’ve ever written, because of the way the queue’s design makes it easy to implement MoveNext. Except… that doesn’t compile either. Apparently you can’t put generic type parameters on a class helper, which means that as far as I can tell, you can’t apply a class helper to a generic class at all.
I suppose I could subclass it and add the enumerator that way, but TEnumerableThreadedQueue<T> is a bit of a bulky name, don’t you think? I have to wonder why the enumerator was left off of this collection, though, especially since the standard enumerator pattern is basically the only reasonable way to use a class like this…