Merge pull request #65 from QMatrixClient/kitsune-simplify-events

Events refactoring

1 files changed, 147 insertions, 13 deletions

diff --git a/util.h b/util.h
index 79f76860..c9e2d1c9 100644
--- a/util.h
+++ b/util.h
@@ -18,6 +18,11 @@
 
 #pragma once
 
+#include <QtCore/QMetaEnum>
+#include <QtCore/QDebug>
+
+#include <functional>
+
 namespace QMatrixClient
 {
     /**
@@ -71,14 +76,19 @@ namespace QMatrixClient
      * @brief Lookup a value by a key in a varargs list
      *
      * This function template takes the value of its first argument (selector)
-     * as a key and searches for it in the key-value map passed in a varargs list
-     * (every next pair of arguments forms a key-value pair). If a match is found,
-     * the respective value is returned; if no pairs matched, the last value
-     * (fallback) is returned.
+     * as a key and searches for it in the key-value map passed in
+     * a parameter pack (every next pair of arguments forms a key-value pair).
+     * If a match is found, the respective value is returned; if no pairs
+     * matched, the last value (fallback) is returned.
      *
      * All options should be of the same type or implicitly castable to the
-     * type of the first option. Note that pointers to methods of different
-     * classes are of different object types, in particular.
+     * type of the first option. If you need some specific type to cast to
+     * you can explicitly provide it as the ValueT template parameter
+     * (e.g. <code>lookup<void*>(parameters...)</code>). Note that pointers
+     * to methods of different classes and even to functions with different
+     * signatures are of different types. If their return types are castable
+     * to some common one, @see dispatch that deals with this by swallowing
+     * the method invocation.
      *
      * Below is an example of usage to select a parser depending on contents of
      * a JSON object:
@@ -91,7 +101,7 @@ namespace QMatrixClient
      * }
      *
      * The implementation is based on tail recursion; every recursion step
-     * removes 2 arguments (match and option). There's no selector value for the
+     * removes 2 arguments (match and value). There's no selector value for the
      * fallback option (the last one); therefore, the total number of lookup()
      * arguments should be even: selector + n key-value pairs + fallback
      *
@@ -99,20 +109,144 @@ namespace QMatrixClient
      * (the first parameter) - most likely it won't do what you expect because
      * of shallow comparison.
      */
+    template <typename ValueT, typename SelectorT>
+    ValueT lookup(SelectorT/*unused*/, ValueT&& fallback)
+    {
+        return std::forward<ValueT>(fallback);
+    }
+
     template <typename ValueT, typename SelectorT, typename KeyT, typename... Ts>
-    ValueT lookup(SelectorT selector, KeyT key, ValueT value, Ts... remainingMapping)
+    ValueT lookup(SelectorT&& selector, KeyT&& key, ValueT&& value, Ts&&... remainder)
     {
         if( selector == key )
-            return value;
+            return std::forward<ValueT>(value);
 
         // Drop the failed key-value pair and recurse with 2 arguments less.
-        return lookup(selector, remainingMapping...);
+        return lookup<ValueT>(std::forward<SelectorT>(selector),
+                              std::forward<Ts>(remainder)...);
     }
 
-    template <typename SelectorT, typename ValueT>
-    ValueT lookup(SelectorT/*unused*/, ValueT fallback)
+    /**
+     * A wrapper around lookup() for functions of different types castable
+     * to a common std::function<> form
+     *
+     * This class uses std::function<> magic to first capture arguments of
+     * a yet-unknown function or function object, and then to coerce types of
+     * all functions/function objects passed for lookup to the type
+     * std::function<ResultT(ArgTs...). Without Dispatch<>, you would have
+     * to pass the specific function type to lookup, since your functions have
+     * different signatures. The type is not always obvious, and the resulting
+     * construct in client code would almost always be rather cumbersome.
+     * Dispatch<> deduces the necessary function type (well, almost - you still
+     * have to specify the result type) and hides the clumsiness. For more
+     * information on what std::function<> can wrap around, see
+     * https://cpptruths.blogspot.jp/2015/11/covariance-and-contravariance-in-c.html
+     *
+     * The function arguments are captured by value (i.e. copied) to avoid
+     * hard-to-find issues with dangling references in cases when a Dispatch<>
+     * object is passed across different contexts (e.g. returned from another
+     * function).
+     *
+     * \tparam ResultT - the desired type of a picked function invocation (mandatory)
+     * \tparam ArgTs - function argument types (deduced)
+     */
+#if __GNUC__ < 5 && __GNUC_MINOR__ < 9
+    // GCC 4.8 cannot cope with parameter packs inside lambdas; so provide a single
+    // argument version of Dispatch<> that we only need so far.
+    template <typename ResultT, typename ArgT>
+#else
+    template <typename ResultT, typename... ArgTs>
+#endif
+    class Dispatch
+    {
+            // The implementation takes a chapter from functional programming:
+            // Dispatch<> uses a function that in turn accepts a function as its
+            // argument. The sole purpose of the outer function (initialized by
+            // a lambda-expression in the constructor) is to store the arguments
+            // to any of the functions later looked up. The inner function (its
+            // type is defined by fn_t alias) is the one returned by lookup()
+            // invocation inside to().
+            //
+            // It's a bit counterintuitive to specify function parameters before
+            // the list of functions but otherwise it would take several overloads
+            // here to match all the ways a function-like behaviour can be done:
+            // reference-to-function, pointer-to-function, function object. This
+            // probably could be done as well but I preferred a more compact
+            // solution: you show what you have and if it's possible to bring all
+            // your functions to the same std::function<> based on what you have
+            // as parameters, the code will compile. If it's not possible, modern
+            // compilers are already good enough at pinpointing a specific place
+            // where types don't match.
+        public:
+#if __GNUC__ < 5 && __GNUC_MINOR__ < 9
+            using fn_t = std::function<ResultT(ArgT)>;
+            explicit Dispatch(ArgT&& arg)
+                : boundArgs([=](fn_t &&f) { return f(std::move(arg)); })
+            { }
+#else
+            using fn_t = std::function<ResultT(ArgTs...)>;
+            explicit Dispatch(ArgTs&&... args)
+                : boundArgs([=](fn_t &&f) { return f(std::move(args)...); })
+            { }
+#endif
+
+            template <typename... LookupParamTs>
+            ResultT to(LookupParamTs&&... lookupParams)
+            {
+                // Here's the magic, two pieces of it:
+                // 1. Specifying fn_t in lookup() wraps all functions in
+                // \p lookupParams into the same std::function<> type. This
+                // includes conversion of return types from more specific to more
+                // generic (because std::function is covariant by return types and
+                // contravariant by argument types (see the link in the Doxygen
+                // part of the comments).
+                auto fn = lookup<fn_t>(std::forward<LookupParamTs>(lookupParams)...);
+                // 2. Passing the result of lookup() to boundArgs() invokes the
+                // lambda-expression mentioned in the constructor, which simply
+                // invokes this passed function with a set of arguments captured
+                // by lambda.
+                if (fn)
+                    return boundArgs(std::move(fn));
+
+                // A shortcut to allow passing nullptr for a function;
+                // a default-constructed ResultT will be returned
+                // (for pointers, it will be nullptr)
+                return {};
+            }
+
+        private:
+            std::function<ResultT(fn_t&&)> boundArgs;
+    };
+
+    /**
+     * Dispatch a set of parameters to one of a set of functions, depending on
+     * a selector value
+     *
+     * Use <code>dispatch<CommonType>(parameters).to(lookup parameters)</code>
+     * instead of lookup() if you need to pick one of several functions returning
+     * types castable to the same CommonType. See event.cpp for a typical use case.
+     *
+     * \see Dispatch
+     */
+    template <typename ResultT, typename... ArgTs>
+    Dispatch<ResultT, ArgTs...> dispatch(ArgTs&& ... args)
     {
-        return fallback;
+        return Dispatch<ResultT, ArgTs...>(std::forward<ArgTs...>(args)...);
+    };
+
+    // The below enables pretty-printing of enums in logs
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 5, 0))
+#define REGISTER_ENUM(EnumName) Q_ENUM(EnumName)
+#else
+    // Thanks to Olivier for spelling it and for making Q_ENUM to replace it:
+    // https://woboq.com/blog/q_enum.html
+#define REGISTER_ENUM(EnumName) \
+    Q_ENUMS(EnumName) \
+    friend QDebug operator<<(QDebug dbg, EnumName val) \
+    { \
+        static int enumIdx = staticMetaObject.indexOfEnumerator(#EnumName); \
+        return dbg << Event::staticMetaObject.enumerator(enumIdx).valueToKey(int(val)); \
     }
+#endif
 }  // namespace QMatrixClient