Vaadin utils - compose value providers

[javier-godoy]

ValueProvider extends SerializableFunction. However, one cannot use Function.compose because the return type of Function.compose is Function instead of ValueProvider

Consider the following helper method:

public static <R, T, V> ValueProvider<T, V> compose(final ValueProvider<? super T, ? extends R> this_,
			final ValueProvider<? super R, ? extends V> after) {
		Objects.requireNonNull(this_);
		Objects.requireNonNull(after);
		return (T t) -> Optional.ofNullable(this_.apply(t)).map(after).orElse(null);
}

it allows you to specifiy a ValueProvider by composing method references
grid.addColumn(compose(Person::getAddress, Address::getCity)) instead of grid.addColumn(p->Optional.ofNullable(p.getAddress()).map(Address::getCity))

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