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| | #REDIRECT [[ist:Observer (padrão de desenho)]] |
| <!--Material correspondente à aula 28.-->
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| O padrão ''observer'' permite observar o estado de um objecto. Os observadores registam o seu interesse no estado junto do objecto; quando o estado do objecto muda, os observadores são notificados.
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| ==Estrutura==
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| ===Diagrama de classes===
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| O padrão ''observer'' tem a seguinte estrutura de classes:
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| [[Image:observer-dpcd.png|600px]] | |
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| ===Diagrama de sequência===
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| As colaborações entre os intervenientes são as que figuram no seguinte diagrama de sequência:
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| [[Image:observer-dpsd.png|500px]]
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| ==Exemplo==
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| ===Observadores e Observados===
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| <B>public</B> <B>interface</B> Subject {
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| <B>public</B> <FONT COLOR="#800000">void</FONT> <FONT COLOR="#000080">registerObserver</FONT>(Observer o);
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| <B>public</B> <FONT COLOR="#800000">void</FONT> <FONT COLOR="#000080">removeObserver</FONT>(Observer o);
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| <B>public</B> <FONT COLOR="#800000">void</FONT> <FONT COLOR="#000080">notifyObservers</FONT>();
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| }
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| <B>public</B> <B>interface</B> Observer {
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| <B>public</B> <FONT COLOR="#800000">void</FONT> <FONT COLOR="#000080">update</FONT>(<FONT COLOR="#800000">float</FONT> temp, <FONT COLOR="#800000">float</FONT> humidity, <FONT COLOR="#800000">float</FONT> pressure);
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| }
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| ===Apresentação===
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| Esta interface define o método básico para apresentação de dados.
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| <B>public</B> <B>interface</B> DisplayElement {
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| <B>public</B> <FONT COLOR="#800000">void</FONT> <FONT COLOR="#000080">display</FONT>();
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| }
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| As implementações da interface de apresentação, que implementam também a de observação, definem várias formas de exibição de dados.
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| <B>public</B> <B>class</B> CurrentConditionsDisplay <B>implements</B> Observer, DisplayElement {
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| <B>private</B> <FONT COLOR="#800000">float</FONT> temperature;
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| <B>private</B> <FONT COLOR="#800000">float</FONT> humidity;
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| <B>private</B> Subject weatherData;
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|
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| <B>public</B> <FONT COLOR="#000080">CurrentConditionsDisplay</FONT>(Subject weatherData) {
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| <B>this</B>.<FONT COLOR="#000080">weatherData</FONT> = weatherData;
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| weatherData.<FONT COLOR="#000080">registerObserver</FONT>(<B>this</B>);
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| }
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|
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| <B>public</B> <FONT COLOR="#800000">void</FONT> <FONT COLOR="#000080">update</FONT>(<FONT COLOR="#800000">float</FONT> temperature, <FONT COLOR="#800000">float</FONT> humidity, <FONT COLOR="#800000">float</FONT> pressure) {
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| <B>this</B>.<FONT COLOR="#000080">temperature</FONT> = temperature;
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| <B>this</B>.<FONT COLOR="#000080">humidity</FONT> = humidity;
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| <FONT COLOR="#000080">display</FONT>();
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| }
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|
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| <B>public</B> <FONT COLOR="#800000">void</FONT> <FONT COLOR="#000080">display</FONT>() {
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| <B><FONT COLOR="#0095ff">System</FONT></B>.<FONT COLOR="#000080">out</FONT>.<FONT COLOR="#000080">println</FONT>(<FONT COLOR="#dd0000">"Current conditions: "</FONT> + temperature
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| + <FONT COLOR="#dd0000">"F degrees and "</FONT> + humidity + <FONT COLOR="#dd0000">"% humidity"</FONT>);
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| }
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| }
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| <B>public</B> <B>class</B> ForecastDisplay <B>implements</B> Observer, DisplayElement {
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|
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| <B>private</B> <FONT COLOR="#800000">float</FONT> currentPressure = <FONT COLOR="#800080">29.92f</FONT>;
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| <B>private</B> <FONT COLOR="#800000">float</FONT> lastPressure;
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| <B>private</B> WeatherData weatherData;
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|
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| <B>public</B> <FONT COLOR="#000080">ForecastDisplay</FONT>(WeatherData weatherData) {
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| <B>this</B>.<FONT COLOR="#000080">weatherData</FONT> = weatherData;
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| weatherData.<FONT COLOR="#000080">registerObserver</FONT>(<B>this</B>);
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| }
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|
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| <B>public</B> <FONT COLOR="#800000">void</FONT> <FONT COLOR="#000080">update</FONT>(<FONT COLOR="#800000">float</FONT> temp, <FONT COLOR="#800000">float</FONT> humidity, <FONT COLOR="#800000">float</FONT> pressure) {
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| lastPressure = currentPressure;
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| currentPressure = pressure;
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| <FONT COLOR="#000080">display</FONT>();
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| }
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|
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| <B>public</B> <FONT COLOR="#800000">void</FONT> <FONT COLOR="#000080">display</FONT>() {
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| <B><FONT COLOR="#0095ff">System</FONT></B>.<FONT COLOR="#000080">out</FONT>.<FONT COLOR="#000080">print</FONT>(<FONT COLOR="#dd0000">"Forecast: "</FONT>);
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| <B>if</B> (currentPressure > lastPressure) {
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| <B><FONT COLOR="#0095ff">System</FONT></B>.<FONT COLOR="#000080">out</FONT>.<FONT COLOR="#000080">println</FONT>(<FONT COLOR="#dd0000">"Improving weather on the way!"</FONT>);
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| } <B>else</B> <B>if</B> (currentPressure == lastPressure) {
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| <B><FONT COLOR="#0095ff">System</FONT></B>.<FONT COLOR="#000080">out</FONT>.<FONT COLOR="#000080">println</FONT>(<FONT COLOR="#dd0000">"More of the same"</FONT>);
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| } <B>else</B> <B>if</B> (currentPressure < lastPressure) {
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| <B><FONT COLOR="#0095ff">System</FONT></B>.<FONT COLOR="#000080">out</FONT>.<FONT COLOR="#000080">println</FONT>(<FONT COLOR="#dd0000">"Watch out for cooler, rainy weather"</FONT>);
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| }
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| }
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| }
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| ===Representação de dados===
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| Os dados de meteorologia correspondem ao objecto observado.
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| <cpp>
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| public class WeatherData implements Subject {
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| private ArrayList<Observer> _observers = new ArrayList<Observer>();
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| private float _temperature;
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| private float _humidity;
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| private float _pressure;
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|
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| public WeatherData() { }
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|
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| public void registerObserver(Observer o) { _observers.add(o); }
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|
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| public void removeObserver(Observer o) {
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| int i = _observers.indexOf(o);
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| if (i >= 0) { _observers.remove(i); }
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| }
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|
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| public void notifyObservers() {
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| for (int i = 0; i < _observers.size(); i++) {
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| Observer observer = _observers.get(i);
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| observer.update(temperature, humidity, pressure);
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| }
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| }
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|
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| public void measurementsChanged() { notifyObservers(); }
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|
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| public void setMeasurements(float temperature, float humidity, float pressure) {
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| _temperature = temperature;
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| _humidity = humidity;
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| _pressure = pressure;
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| measurementsChanged();
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| }
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| }
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| </cpp>
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| ===Dois contextos===
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| Os dois contextos de utilização correspondem a duas estações meteorológicas: em cada uma são utilizados observadores diferentes sobre os mesmos dados.
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| <B>public</B> <B>class</B> WeatherStation {
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| <B>public</B> <FONT COLOR="#800000">static</FONT> <FONT COLOR="#800000">void</FONT> <FONT COLOR="#000080">main</FONT>(<B><FONT COLOR="#0095ff">String</FONT></B>[] args) {
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| WeatherData weatherData = <B>new</B> <FONT COLOR="#000080">WeatherData</FONT>();
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|
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| CurrentConditionsDisplay currentDisplay = <B>new</B> <FONT COLOR="#000080">CurrentConditionsDisplay</FONT>(weatherData);
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| StatisticsDisplay statisticsDisplay = <B>new</B> <FONT COLOR="#000080">StatisticsDisplay</FONT>(weatherData);
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| ForecastDisplay forecastDisplay = <B>new</B> <FONT COLOR="#000080">ForecastDisplay</FONT>(weatherData);
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|
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| weatherData.<FONT COLOR="#000080">setMeasurements</FONT>(<FONT COLOR="#0000ff">80</FONT>, <FONT COLOR="#0000ff">65</FONT>, <FONT COLOR="#800080">30.4f</FONT>);
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| weatherData.<FONT COLOR="#000080">setMeasurements</FONT>(<FONT COLOR="#0000ff">82</FONT>, <FONT COLOR="#0000ff">70</FONT>, <FONT COLOR="#800080">29.2f</FONT>);
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| weatherData.<FONT COLOR="#000080">setMeasurements</FONT>(<FONT COLOR="#0000ff">78</FONT>, <FONT COLOR="#0000ff">90</FONT>, <FONT COLOR="#800080">29.2f</FONT>);
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| }
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| }
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| <B>public</B> <B>class</B> WeatherStationHeatIndex {
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| <B>public</B> <FONT COLOR="#800000">static</FONT> <FONT COLOR="#800000">void</FONT> <FONT COLOR="#000080">main</FONT>(<B><FONT COLOR="#0095ff">String</FONT></B>[] args) {
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| WeatherData weatherData = <B>new</B> <FONT COLOR="#000080">WeatherData</FONT>();
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|
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| CurrentConditionsDisplay currentDisplay = <B>new</B> <FONT COLOR="#000080">CurrentConditionsDisplay</FONT>(weatherData);
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| StatisticsDisplay statisticsDisplay = <B>new</B> <FONT COLOR="#000080">StatisticsDisplay</FONT>(weatherData);
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| ForecastDisplay forecastDisplay = <B>new</B> <FONT COLOR="#000080">ForecastDisplay</FONT>(weatherData);
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| HeatIndexDisplay heatIndexDisplay = <B>new</B> <FONT COLOR="#000080">HeatIndexDisplay</FONT>(weatherData);
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|
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| weatherData.<FONT COLOR="#000080">setMeasurements</FONT>(<FONT COLOR="#0000ff">80</FONT>, <FONT COLOR="#0000ff">65</FONT>, <FONT COLOR="#800080">30.4f</FONT>);
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| weatherData.<FONT COLOR="#000080">setMeasurements</FONT>(<FONT COLOR="#0000ff">82</FONT>, <FONT COLOR="#0000ff">70</FONT>, <FONT COLOR="#800080">29.2f</FONT>);
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| weatherData.<FONT COLOR="#000080">setMeasurements</FONT>(<FONT COLOR="#0000ff">78</FONT>, <FONT COLOR="#0000ff">90</FONT>, <FONT COLOR="#800080">29.2f</FONT>);
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| }
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| }
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| [[category:OOP]]
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| [[category:Teaching]]
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