Implementation of the Shadow Thing mechanism on the device side

[ilcato]

It allows the synchronization of property values between cloud and devices, at startup or after a disconnection period, based on synchronization policies configurable at the property level.
It adds a new parameter to the addProperty method that specify the synchronization mode to adopt for the property. This parameter is a callback function that will specify the synchronization logic to use. Standard synchronization logic is available by using the following callbacks:

• AUTO_SYNC: that compares the cloud timestamp of the last change with the corresponding device timestamp. The property is assigned the value with the higher timestamp.
• FORCE_CLOUD_SYNC: the property is assigned the value coming from cloud regardless of timestamps and device value.
• FORCE_DEVICE_SYNC: the device property value is kept. The cloud property value will be updated at the next update cycle.

A custom synchronization logic may be implemented by setting a custom callback function with the following signature: void (*)(ArduinoCloudProperty<T> property); use one of the specific types supported. The property object exposes several methods which will enable the custom logic to select the appropriate value.

Example: assuming a Switch boolean READ_WRITE property is defined the following statement in thingProperties.h will specify an AUTO_SYNC mode:

ArduinoCloud.addProperty(Switch, READWRITE, ON_CHANGE, onSwitchChange, AUTO_SYNC);

[aentinger]

The pull request looks generally good to me, however, some minor changes are recommended.

[mbanzi]

I gave a quick look at this, it looks good the only thing I would recommend is that we use a more human friendly way to specify the syncronisation policy i.e. AUTO_SYNC , FORCE_CLOUD_SYNC, FORCE_DEVICE_SYNC could become something lighter like CLOUD_WINS
DEVICE_WINS
MOST_RECENT_WINS. just a thought to make it more understandable

[ilcato]

I gave a quick look at this, it looks good the only thing I would recommend is that we use a more human friendly way to specify the syncronisation policy i.e. AUTO_SYNC , FORCE_CLOUD_SYNC, FORCE_DEVICE_SYNC could become something lighter like CLOUD_WINS
DEVICE_WINS
MOST_RECENT_WINS. just a thought to make it more understandable

Done

[aentinger]

Good by me ;)

[ilcato]

Mmmh, isn't this the exact opposite of what the patch was supposed to do?

Restored the initial order of the call sequence.
The previous commit was related to the call of the wrong method in the connection managers state machine.

[ubidefeo]

I think this workflow works better.
As @ilcato noticed we had things a bit wrong when it came to implementing a fallback :)

[ubidefeo]

If @lxrobotics and @facchinm are good with this I'd merge it into master without yet making a new release

[aentinger]

@ubidefeo I'm good with the changes 👍

[aentinger]

@ubidefeo Just curious - what's the state on the PR? Maybe @facchinm could take a look on it so we can merge it into master and be done with it.

[ubidefeo]

hi @lxrobotics @ilcato
I am still away but slowly getting back to work remotely.
I think we can merge this in and close the PR.
I can then add a release and get the libs updated