Add hide_if_empty().

src/alia/signals/containers.hpp

@@ -0,0 +1,23 @@
+#ifndef ALIA_SIGNALS_CONTAINERS_HPP
+#define ALIA_SIGNALS_CONTAINERS_HPP
+
+#include <alia/signals/adaptors.hpp>
+#include <alia/signals/application.hpp>
+
+namespace alia {
+
+// hide_if_empty(s), where :s is a signal, yields a wrapper for :s that will
+// claim to have no value if that value would be empty. (This is determined by
+// calling the empty() member function of the value, so it works on most
+// containers (including strings).)
+template<class Signal>
+auto
+hide_if_empty(Signal s)
+{
+    return mask_reads(
+        s, lazy_apply([](auto const& x) { return !x.empty(); }, s));
+}
+
+} // namespace alia
+
+#endif

unit_tests/signals/containers.cpp

@@ -0,0 +1,32 @@
+#include <alia/signals/containers.hpp>
+
+#include <testing.hpp>
+
+using namespace alia;
+
+TEST_CASE("hide_if_empty (hidden)", "[signals][adaptors]")
+{
+    std::string x;
+    auto wrapped = direct(x);
+    auto s = hide_if_empty(wrapped);
+
+    typedef decltype(s) signal_t;
+    REQUIRE(signal_is_readable<signal_t>::value);
+    REQUIRE(signal_is_writable<signal_t>::value);
+
+    REQUIRE(!signal_has_value(s));
+}
+
+TEST_CASE("hide_if_empty (not hidden)", "[signals][adaptors]")
+{
+    std::string x("foo");
+    auto wrapped = direct(x);
+    auto s = hide_if_empty(wrapped);
+
+    typedef decltype(s) signal_t;
+    REQUIRE(signal_is_readable<signal_t>::value);
+    REQUIRE(signal_is_writable<signal_t>::value);
+
+    REQUIRE(signal_has_value(s));
+    REQUIRE(read_signal(s) == "foo");
+}