从Californium CoAP客户端检索观察值

Question

I am trying to implement a CoAP client based on Californium. I make this client observing to a resource:

public static class CoapCl{

    double val = 0;
    CoapClient client = new CoapClient("coap://localhost/Ultrasonic");
    CoapObserveRelation relation = client.observe(new CoapHandler() {
    @Override public void onLoad(CoapResponse response)
    {
        val = Double.parseDouble(response.getResponseText());
    }

    @Override
    public void onError() {
        System.out.println("Failed");

    }
});
}

I want to access the value "val" from another class. How can I do it ? I tried to call a reference from the CoapCl class like this and print the value out:

CoapCl client = new CoapCl();
    while(true)
    {
    System.out.println("Testing: " + client.val);
    }   

This will print all the value I get from the CoAP client, both changed and unchanged value. What should I do if I only want to get the changed value ?

Answer 1

Well, the issue itself isn't related to Californium and CoAP. Except that CoapHandler is async but this is rather a strench. Nevertheless, I'd recommend to end up with some kind of callback:

public class CoapCl {
    private final Consumer<Double> valueChangedAction;
    private final CoapClient client = new CoapClient("coap://localhost/Ultrasonic");

    public CoapCl(Consumer<Double> valueChangedAction) {
        this.valueChangedAction = valueChangedAction;
    }

    public void run() {
        client.observe(new CoapHandler() {
            @Override
            public void onLoad(CoapResponse response) {
                valueChangedAction.accept(
                    Double.parseDouble(
                        response.getResponseText()
                    )
                );
            }

            @Override
            public void onError() {
                System.out.println("Failed");
            }
        });
    }
}

new CoapCl(val -> System.out.println("Testing: " + val)).run();

Please keep in mind you have to block the main thread someway to keep the program from immediate exit. Before, you had blocked it with your infinite loop. Now you'll have to use System.in.read() or Thread.sleep or something else if you have no such stuff yet in your program.