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 <span id="projectnumber">3.1.0</span>
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<div class="fragment"><div class="line"><a name="l00001"></a><span class="lineno"> 1</span> # Crash Course: service locator</div><div class="line"><a name="l00002"></a><span class="lineno"> 2</span> </div><div class="line"><a name="l00003"></a><span class="lineno"> 3</span> <!--</div><div class="line"><a name="l00004"></a><span class="lineno"> 4</span> @cond TURN_OFF_DOXYGEN</div><div class="line"><a name="l00005"></a><span class="lineno"> 5</span> --></div><div class="line"><a name="l00006"></a><span class="lineno"> 6</span> # Table of Contents</div><div class="line"><a name="l00007"></a><span class="lineno"> 7</span> </div><div class="line"><a name="l00008"></a><span class="lineno"> 8</span> * [Introduction](#introduction)</div><div class="line"><a name="l00009"></a><span class="lineno"> 9</span> * [Service locator](#service-locator)</div><div class="line"><a name="l00010"></a><span class="lineno"> 10</span> <!--</div><div class="line"><a name="l00011"></a><span class="lineno"> 11</span> @endcond TURN_OFF_DOXYGEN</div><div class="line"><a name="l00012"></a><span class="lineno"> 12</span> --></div><div class="line"><a name="l00013"></a><span class="lineno"> 13</span> </div><div class="line"><a name="l00014"></a><span class="lineno"> 14</span> # Introduction</div><div class="line"><a name="l00015"></a><span class="lineno"> 15</span> </div><div class="line"><a name="l00016"></a><span class="lineno"> 16</span> Usually service locators are tightly bound to the services they expose and it's</div><div class="line"><a name="l00017"></a><span class="lineno"> 17</span> hard to define a general purpose solution. This template based implementation</div><div class="line"><a name="l00018"></a><span class="lineno"> 18</span> tries to fill the gap and to get rid of the burden of defining a different</div><div class="line"><a name="l00019"></a><span class="lineno"> 19</span> specific locator for each application.<br/></div><div class="line"><a name="l00020"></a><span class="lineno"> 20</span> This class is tiny, partially unsafe and thus risky to use. Moreover it doesn't</div><div class="line"><a name="l00021"></a><span class="lineno"> 21</span> fit probably most of the scenarios in which a service locator is required. Look</div><div class="line"><a name="l00022"></a><span class="lineno"> 22</span> at it as a small tool that can sometimes be useful if users know how to handle</div><div class="line"><a name="l00023"></a><span class="lineno"> 23</span> it.</div><div class="line"><a name="l00024"></a><span class="lineno"> 24</span> </div><div class="line"><a name="l00025"></a><span class="lineno"> 25</span> # Service locator</div><div class="line"><a name="l00026"></a><span class="lineno"> 26</span> </div><div class="line"><a name="l00027"></a><span class="lineno"> 27</span> The API is straightforward. The basic idea is that services are implemented by</div><div class="line"><a name="l00028"></a><span class="lineno"> 28</span> means of interfaces and rely on polymorphism.<br/></div><div class="line"><a name="l00029"></a><span class="lineno"> 29</span> The locator is instantiated with the base type of the service if any and a</div><div class="line"><a name="l00030"></a><span class="lineno"> 30</span> concrete implementation is provided along with all the parameters required to</div><div class="line"><a name="l00031"></a><span class="lineno"> 31</span> initialize it. As an example:</div><div class="line"><a name="l00032"></a><span class="lineno"> 32</span> </div><div class="line"><a name="l00033"></a><span class="lineno"> 33</span> ```cpp</div><div class="line"><a name="l00034"></a><span class="lineno"> 34</span> // the service has no base type, a locator is used to treat it as a kind of singleton</div><div class="line"><a name="l00035"></a><span class="lineno"> 35</span> entt::service_locator<my_service>::set(params...);</div><div class="line"><a name="l00036"></a><span class="lineno"> 36</span> </div><div class="line"><a name="l00037"></a><span class="lineno"> 37</span> // sets up an opaque service</div><div class="line"><a name="l00038"></a><span class="lineno"> 38</span> entt::service_locator<audio_interface>::set<audio_implementation>(params...);</div><div class="line"><a name="l00039"></a><span class="lineno"> 39</span> </div><div class="line"><a name="l00040"></a><span class="lineno"> 40</span> // resets (destroys) the service</div><div class="line"><a name="l00041"></a><span class="lineno"> 41</span> entt::service_locator<audio_interface>::reset();</div><div class="line"><a name="l00042"></a><span class="lineno"> 42</span> ```</div><div class="line"><a name="l00043"></a><span class="lineno"> 43</span> </div><div class="line"><a name="l00044"></a><span class="lineno"> 44</span> The locator can also be queried to know if an active service is currently set</div><div class="line"><a name="l00045"></a><span class="lineno"> 45</span> and to retrieve it if necessary (either as a pointer or as a reference):</div><div class="line"><a name="l00046"></a><span class="lineno"> 46</span> </div><div class="line"><a name="l00047"></a><span class="lineno"> 47</span> ```cpp</div><div class="line"><a name="l00048"></a><span class="lineno"> 48</span> // no service currently set</div><div class="line"><a name="l00049"></a><span class="lineno"> 49</span> auto empty = entt::service_locator<audio_interface>::empty();</div><div class="line"><a name="l00050"></a><span class="lineno"> 50</span> </div><div class="line"><a name="l00051"></a><span class="lineno"> 51</span> // gets a (possibly empty) shared pointer to the service ...</div><div class="line"><a name="l00052"></a><span class="lineno"> 52</span> std::shared_ptr<audio_interface> ptr = entt::service_locator<audio_interface>::get();</div><div class="line"><a name="l00053"></a><span class="lineno"> 53</span> </div><div class="line"><a name="l00054"></a><span class="lineno"> 54</span> // ... or a reference, but it's undefined behaviour if the service isn't set yet</div><div class="line"><a name="l00055"></a><span class="lineno"> 55</span> audio_interface &ref = entt::service_locator<audio_interface>::ref();</div><div class="line"><a name="l00056"></a><span class="lineno"> 56</span> ```</div><div class="line"><a name="l00057"></a><span class="lineno"> 57</span> </div><div class="line"><a name="l00058"></a><span class="lineno"> 58</span> A common use is to wrap the different locators in a container class, creating</div><div class="line"><a name="l00059"></a><span class="lineno"> 59</span> aliases for the various services:</div><div class="line"><a name="l00060"></a><span class="lineno"> 60</span> </div><div class="line"><a name="l00061"></a><span class="lineno"> 61</span> ```cpp</div><div class="line"><a name="l00062"></a><span class="lineno"> 62</span> struct locator {</div><div class="line"><a name="l00063"></a><span class="lineno"> 63</span>  using camera = entt::service_locator<camera_interface>;</div><div class="line"><a name="l00064"></a><span class="lineno"> 64</span>  using audio = entt::service_locator<audio_interface>;</div><div class="line"><a name="l00065"></a><span class="lineno"> 65</span>  // ...</div><div class="line"><a name="l00066"></a><span class="lineno"> 66</span> };</div><div class="line"><a name="l00067"></a><span class="lineno"> 67</span> </div><div class="line"><a name="l00068"></a><span class="lineno"> 68</span> // ...</div><div class="line"><a name="l00069"></a><span class="lineno"> 69</span> </div><div class="line"><a name="l00070"></a><span class="lineno"> 70</span> void init() {</div><div class="line"><a name="l00071"></a><span class="lineno"> 71</span>  locator::camera::set<camera_null>();</div><div class="line"><a name="l00072"></a><span class="lineno"> 72</span>  locator::audio::set<audio_implementation>(params...);</div><div class="line"><a name="l00073"></a><span class="lineno"> 73</span>  // ...</div><div class="line"><a name="l00074"></a><span class="lineno"> 74</span> }</div><div class="line"><a name="l00075"></a><span class="lineno"> 75</span> ```</div></div><!-- fragment --></div><!-- contents -->
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