Leveraging the Watch Action
As explained in the Using Resources tutorial, the watch
action will notify a reference when the value of its resource changes.
This allows developers to implement cross-domain responsiveness.
The watch action is not limited to only the Data Api, but for most development watch actions pertain to this Api.
Watch:
Watch actions vary slightly from the actions explained in the Basic Data Sharing tutorial, as we now have to introduce a callback function into the signature:
Parameters
| parameter | type | description |
|---|---|---|
| callback | Function | The callback function to be called when the given reference's resource changes. |
Returns
A promise that resolves on the acknowledgement of the request. If the resource exists, the value of the resource is passed to the promise resolution.
This promise function is not called on resource value change. The callback parameter is called on resource change. The purpose for the promise resolution is to allow a watcher to obtain immediate state of a resource when watching without an additional request.
Callback
The callback function passed to the watch action has 2 parameters:
| parameter | type | description |
|---|---|---|
| changes | Object | The changes of the resource that triggered this callback |
| changes.newValue | Primative or Array | The new value of the resource. |
| changes.oldValue | Primative or Array | The old value of the resource. |
| changes.deleted | Boolean | A boolean flag notifying if the resource was deleted. |
| done | Function | A function to call if the callback is to be unregistered. |
The watch has additional configurations that can pass additional values to the
callback's changes object. For basic use of the IWC they are not needed,
they are covered in a later advanced tutorial.
Example
Below is a simple watch example where the callback's response and the promise resolution's response are written to the screen.
Take note that the count value written out above the increment/decrement buttons is done in response to the
data acquired from the callback/promise resolution response and is never written locally except for declaring the
count to 0 if it is undefined.
Advanced Watch information
While more tutorials will be added,check out the watch action documentation in our gitbook.