Fix handler callbacks WebIDL, update text and examples.

[zolkis]

Fixes #102.

[danielpeintner]

Do not understand what this.hasOwnProperty(propertyName) is for in

  thing.setPropertyReadHandler(propertyName => {
    console.log("Handling read request for " + propertyName);
    return new Promise((resolve, reject) => {
        if (this.hasOwnProperty(propertyName))
          // application can dispatch fetching property values
          // if (propertyName === 'prop1') { ... }
          // but in general, just read the property value
          resolve(this[propertyName]);
        else
          reject('Property not found');
      },
      e => {
        console.log("Error");
      });
  });

I think we cannot put dispatcher code there. According do my understanding the snippet above should be simply

  thing.setPropertyReadHandler(propertyName => {
    console.log("Handling read request for " + propertyName);
    return new Promise((resolve, reject) => {
          // application can dispatch fetching property values
          // if (propertyName === 'prop1') { ... }
          // but in general, just read the property value
          resolve(...);
    });
  });

[zolkis]

@danielpeintner @knimura @mkovatsc could any of you review please?

[danielpeintner]

I believe the changes are fine.

The two comments are just to clarify what happens if no read or write handler is provided. The current text is not 100% clear to me.

Apart from that the PR is fine!

[danielpeintner]

Minor comment: in node-wot we deal with any property without a readHandler by simply returning the value as is (last set value). Hence I am a bit unsure about the 2nd sentence. Does it make sense to simplify the 2nd sentence above to something like this?

"If no handler is provided for a given Property, implementations are required to report the property value as is (e.g., set by writeProperty() or ThingProperty.value)"

[danielpeintner]

Similar to the previous comment the 2nd sentence might look like

"If no write handler is initialized for a given Property, implementations SHOULD set the property value with the provided new data and notify observers."

Maybe "default property update" means the same to you. IF so fine also.

[zolkis]

My thinking was that the handler actually implements writing the property in a way known only to it (e.g. using local call such as GPIO or network etc). Or it may only want to implement some side-effects (hooks) to setting the property value. Of course the WoT runtime can always update the "proxy" property value it maintains (i.e. in your case you only have the "proxy" property value, basically a memory location that is not bound to HW).

That is why I used the term "default property update", to leave open what that actually means (until we define the algorithms), since it may be more than just updating a local memory location.

[danielpeintner]

Agree. Makes sense. Let's leave it as is for now.

[zolkis]

Fixes #102.