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POImplementing Observer pattern when observers wish to observe different items
 

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  1. POImplementing Observer pattern when observers wish to observe different items
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    <p>Below I have attempted to write a sudo code for the Observer pattern when observers wish to observe different items.</p> <p>Ignore the syntax errors. I wish to know if this is the correct way to implement this. If not, please suggest better ways.</p> <pre><code>// Used by the subject for keeping a track of what items the observer wants to observe typedef struct observerListStruct { bool getTemperatureUpdate; bool getHumidityUpdate; bool getPressureUpdate; observer's-function pointer's address; }; // Subject's class class weatherData { public: // Observers will call this function to register themselves. The function pointer will point to the function which will get called when updates are available. void registerObservers (observer obj, observer's-FunctionPointer) { // This observer's function returns which items to observe. char* f = obj.returnItemsToObserve (); if f[0] = `1` observerListStruct.getTemperatureUpdate = true; } void unregisterObservers (observer obj) {} private: vector &lt;observerListStruct&gt; observerList; float temperature; float humidity; float pressure; void notifyObservers () {} float getTemperature () {} float getHumidity () {} float getPressure () {} } weatherDataObject; // Base class for observers containing common functions class observers { char ItemsToObserve [3] = {1, 2, 3}; // This observer's function returns which items to observe. Default - return all items virtual char* returnItemsToObserve () { return ItemsToObserve; } }; class observerDisplayElementCurrentConditions : public observers { char ItemsToObserve [3] = {1, 2}; char* returnItemsToObserve () { return ItemsToObserve; } // this function will be used as a function pointer for getting updates void getUpdatesAndDisplayWeatherData (float, float) {} }; </code></pre>
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    1. USAquarius_Girl
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    1. CO`Subject` should not know which observer is interested in which `Subject`'s property. Each `ConcreteObserver` knows what `ConcreteSubject`'s properties it is interested in. `ConcreteSubject` should have public getters so `ConcreteObserver` can fetch the latest values of those properties (when `ConcreteSubject` triggers event or calls `Subject::Notify()` method - Google for Gang Of Four Observer implementation). `registerObservers` should just be adding a new `ConcreteObserver` to the list of Observers. You can register each `ConcreteObserver` with multiple `ConcreteSubject`s.
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      1. USBojan Komazec
    2. CO@BojanKomazec What did you mean by "public getters"? Please explain. When subject calls "notify" function, then what should happen?
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    3. COI mean public accessor methods. See AquilaRapax' answer below and look for `WheatherData::getTemperature()`, `WheatherData::getHumidity()` etc. `Notify()` goes through the list of all registered Observers and calls `Update()` on them. Each `ConcreteObserver` implements `Update()` and inside this method it obtains the latest values of `ConcreteSubject`'s properties through those getters.
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