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109 lines
3.5 KiB
Markdown
109 lines
3.5 KiB
Markdown
# Overview of heterogeneous classes
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<!--begintoc-->
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## Table Of Contents
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* [Description](#a2_1)
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* [Warning](#a2_2)
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* [Usage](#a2_3)
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* [Headers](#a3_1)
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* [Template parameters](#a3_2)
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* [Differences between heterogeneous classes vs homogeneous classes](#a2_4)
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<!--endtoc-->
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<a id="a2_1"></a>
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## Description
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'CallbackList', 'EventDispatcher', and 'EventQueue' are homogeneous. All listeners must have the same prototype. For example,
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```c++
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eventpp::EventDispatcher<int, void ()> dispatcher;
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dispatcher.appendListener(3, []() {}); // OK
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dispatcher.appendListener(3, [](std::string) {}); // wrong, can't listen for void(std::string)
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```
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There are heterogeneous counterparts, 'HeterCallbackList', 'HeterEventDispatcher', and 'HeterEventQueue'. These classes allow listeners having different prototypes. For example,
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```c++
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eventpp::HeterEventDispatcher<int, eventpp::HeterTuple<void (), void (std::string)> > dispatcher;
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dispatcher.appendListener(3, []() {}); // OK
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dispatcher.appendListener(3, [](std::string) {}); // OK
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```
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<a id="a2_2"></a>
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## Warning
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The heterogeneous classes are mostly for proof of concept purpose. Misusing them most likely means your application design has flaws.
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You should stick with the homogeneous classes, even though sometimes the heterogeneous classes look convenient (but with overhead).
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The heterogeneous classes may be not well maintained or supported in the future. You use them on your own risk.
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<a id="a2_3"></a>
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## Usage
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<a id="a3_1"></a>
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### Headers
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eventpp/hetercallbacklist.h
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eventpp/hetereventdispatcher.h
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eventpp/hetereventqueue.h
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<a id="a3_2"></a>
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### Template parameters
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```c++
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template <
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typename PrototypeList,
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typename Policies = DefaultPolicies
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>
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class HeterCallbackList;
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template <
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typename Event,
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typename PrototypeList,
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typename Policies = DefaultPolicies
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>
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class HeterEventDispatcher;
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template <
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typename Event,
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typename PrototypeList,
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typename Policies = DefaultPolicies
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>
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class HeterEventQueue;
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```
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For comparison, below are the template parameters for the homogeneous counterparts
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```c++
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template <
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typename Prototype,
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typename Policies = DefaultPolicies
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>
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class CallbackList;
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template <
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typename Event,
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typename Prototype,
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typename Policies = DefaultPolicies
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>
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class EventDispatcher;
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template <
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typename Event,
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typename Prototype,
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typename Policies = DefaultPolicies
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>
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class EventQueue;
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```
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The only difference is the `Prototype` in homo-classes becomes `PrototypeList` in heter-classes.
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In the homo-classes, `Prototype` is a single function type such as `void ()`.
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In the heter-classes, `PrototypeList` is a list of function types in `eventpp::HeterTuple`, such as `eventpp::HeterTuple<void (), void (std::string), void (int, int)>`.
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Note: Ideally it would be better to use `std::tuple` instead of `eventpp::HeterTuple`, but the problem is that the tuple is instantiated in HeterEventDispatcher which cause compile error that function type can't be instantiated.
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<a id="a2_4"></a>
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## Differences between heterogeneous classes vs homogeneous classes
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1. Heterogeneous classes has both overhead on performance and memory usage. Usually event system is the core component in an application, the performance is critical.
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2. Heterogeneous classes can't have the same API interface as homogeneous classes, because some APIs are impossible or very difficult to implement in heterogeneous classes.
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3. Heterogeneous classes doesn't support eventpp::ArgumentPassingAutoDetect. That means the event in the argument can't be detected automatically.
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