= Server Side Events Extension

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The SSE (Server Side Events) module integrates with the xref:vw:ROOT:about.adoc[Wicket Viewer], and provides the ability to dynamically update properties of type xref:valuetypes:markdown:about.adoc[Markdown] and xref:valuetypes:asciidoc:about.adoc[AsciiDoc].
One use case could be to render a progress bar for a long-running action.



== Setup

=== Dependency Management

In your application's top level `pom.xml`, add a dependency for this module's own bill of materials (BOM):

[source,xml,subs="attributes+"]
.pom.xml
----
<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>org.apache.causeway.extensions</groupId>
            <artifactId>causeway-extensions-sse</artifactId>
            <scope>import</scope>
            <type>pom</type>
            <version>{page-causewayreleaseversion}</version>
        </dependency>
    </dependencies>
</dependencyManagement>
----

=== Dependencies / Imports

In those modules where there are domain objects to be dynamically updated using SSE, add a dependency/import to the applib module:

* add this dependency:
+
[source,xml,subs="attributes+"]
.pom.xml
----
<dependencies>
    <dependency>
        <groupId>org.apache.causeway.extensions</groupId>
        <artifactId>causeway-extensions-sse-applib</artifactId>
    </dependency>
</dependencies>
----

* and `@Import` this module:
+
[source,java]
.MyModule.java
----
@Configuration
@Import({
    CausewayModuleExtSseApplib.class,
    // ...
})
public class MyModule { ... }
----

In addition, in the webapp module of your application, add the following dependency:

[source,xml]
.pom.xml
----
<dependency>
    <groupId>org.apache.causeway.extensions</groupId>
    <artifactId>causeway-extensions-sse-wicket</artifactId>
</dependency>
----

And in your application's xref:userguide::modules.adoc#appmanifest[App Manifest], import the extension's implementation module:

[source,java]
.AppManifest.java
----
@Configuration
@Import({
        CausewayModuleExtSseWicket.class,
        ...
})
public class AppManifest {
}
----


== Usage

By way of example, let's see how to render a progress bar.
We'll start with a background task which for demo purposes wil just count slowly up to some figure.
This acts as a source of SSE events:

[source,java]
.DemoTask.java
----
@Named("demo.DemoTask")
@DomainObject(nature=Nature.VIEW_MODEL, editing=Editing.DISABLED)
@RequiredArgsConstructor(staticName="of")
public class DemoTask implements SseSource {                // <.>

    @ObjectSupport public String title() {
        return String.format("DemoTask '%s'", Integer.toHexString(hashCode()));
    }

    private final int totalSteps;
    private TaskProgress taskProgress;

    @Override
    public void run(final SseChannel eventStream) {
        taskProgress = TaskProgress.of(new LongAdder(), totalSteps);
        for(int i=0; i<totalSteps; ++i) {
            _ThreadSleep.millis(1000);
            taskProgress.getStepsProgressed().increment();
            eventStream.fire(this);                     // <.>
        }
    }

    @Override
    public String getPayload() {                        // <.>
        return taskProgress.toHtmlProgressBar();
    }
}
----
<.> Implements xref:refguide:extensions:index/sse/applib/annotations/SseSource.adoc[SseSource], meaning that it must implement `run(SseChannel)`.
<.> Periodically emits events using the provided xref:refguide:extensions:index/sse/applib/service/SseChannel.adoc[SseChannel].
<.> Also provides a payload, being a string representation of progress to date.

It's idiomatic to use HTML as the payload, so that it can be rendered in a xref:refguide:applib:index/value/Markup.adoc[Markup] property (see later).
This HTML payload is built using a helper class `TaskProgress`:

[source,java]
.TaskProgress.java
----
@Data(staticConstructor="of")
public class TaskProgress {

    private final LongAdder stepsProgressed;
    private final long totalSteps;

    public double progressedRelative() {
        final double totalReciprocal = 1. / totalSteps;
        return stepsProgressed.doubleValue() * totalReciprocal;
    }
    public double progressedPercent() {
        return Math.min(progressedRelative()*100., 100.);
    }
    public int progressedPercentAsInt() {
        return (int) Math.round(progressedPercent());
    }

    public String toHtmlProgressBar() {
        final int percent = progressedPercentAsInt();
        return stepsProgressed + "/" + totalSteps +
                "<br/>" +
                "<br/>" +
                "<div class=\"progress\">" +
                "    <div class=\"progress-bar\" " +
                         "role=\"progressbar\" " +
                         "style=\"width: " + percent + "%\" " +
                         "aria-valuenow=\""+percent+"\" " +
                         "aria-valuemin=\"0\" " +
                         "aria-valuemax=\"100\">" +
                    "</div>" +
                "</div>";
    }
}
----

As mentioned above, we will use  a xref:refguide:applib:index/value/Markup.adoc[Markup] property to display the render the progress:

[source,java]
----
@XmlElement @XmlJavaTypeAdapter(Markup.JaxbToStringAdapter.class)
@Property
@ServerSentEvents(observe=DemoTask.class)           // <.>
@Getter @Setter Markup progressView;
----
<.> observes the state of this background task.
+
IMPORTANT: If more than one instance of a task (of this class) runs at the same time, there is (currently) no way to distinguish their events.

Finally, the task is kicked off using the xref:refguide:extensions:index/sse/applib/service/SseService.adoc[].
This runs in the background using a separate thread:

[source,java]
----
@Action
public SseDemoPage startSimpleTask() {
    final DemoTask demoTask = DemoTask.of(100);
    sseService.submit(demoTask, ExecutionBehavior.REQUIRES_NEW_SESSION);  // <.>
    return this;
}

@Inject SseService sseService;
----
<.> The `ExecutionBehaviour` determines whether the background thread should run in the context of a full-blown xref:refguide:applib:index/services/iactn/Interaction.adoc[].
+
Most of the time this will be what you want, but if the background thread runs outside of the framework (eg perhaps just calls out to a web service), then instead `Execution.SIMPLE` can be used.

