groupBy

auto sub = new SubjectObject!int;

auto group = sub.groupBy!(n => n % 10);

auto tester = new CounterObserver!(typeof(group).ElementType);
auto disposable = group.subscribe(tester);

sub.put(0);
assert(tester.putCount == 1);
assert(tester.lastValue.key == 0);

sub.put(10);
assert(tester.putCount == 1);

auto sub = new SubjectObject!string;

auto group = sub.groupBy!(text => text);

auto tester = new CounterObserver!(typeof(group).ElementType);
auto disposable = group.subscribe(tester);

sub.put("A");
assert(tester.putCount == 1);
assert(tester.lastValue.key == "A");

sub.put("B");
assert(tester.putCount == 2);
assert(tester.lastValue.key == "B");

sub.put("XXX");
assert(tester.putCount == 3);
assert(tester.lastValue.key == "XXX");