Operator: ToDictionary

[benjchristensen]

http://msdn.microsoft.com/en-us/library/hh229137(v=vs.103).aspx
http://msdn.microsoft.com/en-us/library/hh212046(v=vs.103).aspx
http://msdn.microsoft.com/en-us/library/hh212075(v=vs.103).aspx
http://msdn.microsoft.com/en-us/library/hh229073(v=vs.103).aspx

[akarnokd]

I believe the equivalent to IDictionary is a Map in Java, but there are a lot of map classes (Linked, Concurrent, etc.). Which one should be the default value type? Is it okay to call the methods Observable.toMap() ?

[abersnaze]

I vote for java.util.HashMap by default with an overload that lets you pass in a map class of any type you want.

public <K, V, M extends java.util.Map<K, V>> Observable<M> toMap(Class<M> mapClass, Func1<? super T, ? extends K> keySelector, Func1<? super T, ? extends V> elementSelector) {...}

PS: not sure if I got the co/contra-variance right on that.
PPS: I Think the consensus was to ignore EqualityComparators overloads because the idiomatic java way is to implement equals/hashCode on the class.

[samuelgruetter]

Two questions:

• How many times will this operator call onNext? Just once, or every time it gets a new item?
• Are users allowed to modify the returned map?

[akarnokd]

This is how I did it somewhere else (below):

• Takes a factory which returns a map instance on subscription
• emits the map once the source finishes
• no variance on the output observable's type

public class OperationToMap {
    public static class ToMap<T, K, V> implements OnSubscribeFunc<Map<K, V>> {
        private final Observable<T> source;
        private final Func1<? super T, ? extends K> keySelector;
        private final Func1<? super T, ? extends V> valueSelector;
        private final Func0<? extends Map<K, V>> mapFactory;
        public ToMap(
                Observable<T> source,
                Func1<? super T, ? extends K> keySelector,
                Func1<? super T, ? extends V> valueSelector,
                Func0<? extends Map<K, V>> mapFactory
                ) {
            this.source = source;
            this.keySelector = keySelector;
            this.valueSelector = valueSelector;
            this.mapFactory = mapFactory;
        }
        @Override
        public Subscription onSubscribe(Observer<? super Map<K, V>> t1) {
            return source.subscribe(new ToMapObserver(t1));
        }
        public class ToMapObserver implements Observer<T> {
            Map<K, V> map;
            private final Observer<? super Map<K, V>> t1;
            public ToMapObserver(Observer<? super Map<K, V>> t1) {
                map = mapFactory.call();
                this.t1 = t1;
            }
            @Override
            public void onCompleted() {
                Map<K, V> map0 = map;
                map = null;
                t1.onNext(map0);
                t1.onCompleted();
            }
            @Override
            public void onError(Throwable e) {
                map = null;
                t1.onError(e);
            }
            @Override
            public void onNext(T args) {
                K key = keySelector.call(args);
                V value = valueSelector.call(args);
                map.put(key, value);
            }
        }
    }
}