Sadly, the first rule of home automation still applies: “The model number matters.” So it all comes down to the details.
There are several possible scenarios.
A. A Matter Controller from one brand controls Matter devices from another brand.
Absolutely possible when the end devices are themselves individually Matter-certified (right now, WiFi or Thread).
So for example once Matter is fully supported, you will be able to add an Eve brand sensor (Thread) to your SmartThings account, something you could not do before. But you will need at least one “Thread Border Router” to collect the messages. That could be a SmartThings/Aeotec V3 hub. But not a V2 hub.
If you have a V2 hub or no hub, you can use the Thread Border Router inside some Amazon Echo or some Google Home devices. Or an Apple HomePod Mini. Or any other Thread Border Router from any other company. You only need one.
- A Matter Controller from one brand controls devices from another brand, but the end devices (probably zigbee or Bluetooth) from the other brand are not individually Matter-certified.
This is possible if and only if those end devices are connected to a “Matter Bridge.”
This will work similarly to how the SmartThings/Philips Hue LAN integration works today. You add the bridge to your account, and it brings many of its devices with it.
This will allow for several integrations which are not possible today.
For example, several Aqara hubs will be certified Matter bridges. You will be able to connect Aqara sensors to one of these Aqara hubs and then Add that Aqara hub to your SmartThings account and it will bring most of its devices along with it. Fast, easy, no custom code required and the integration will run locally. And you won’t need a SmartThings/Aeotec hub for the integration.
Unfortunately, the SmartThings/Aeotec hub will NOT be a Matter bridge. So you cannot add it to, say, the Apple HomeKit app or the Ikea Dirigera app. (This is probably because zwave is going to be way harder to use with Matter than zigbee due to differences in address protocols.)
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So…it depends what you mean by “link a controller.”
If you mean add individual devices from one brand to another brand’s Matter-supporting app, yes if those devices are individually matter-certified.
If they are not individually matter-certified, they have to be connected to a “Matter bridge.” Then you add that matter bridge device to another brand’s app, and that bridge brings a lot of its connected devices along with it.
So far Philips Hue and Aqara have said they will have at least one model hub that is a Matter bridge.
Tuya’s statements were unclear because they said they would have some Matter support, but they used nonstandard terminology.
SmartThings said they will not offer a Matter Bridge at this time.
IKEA’s new Dirigera hub will definitely be a Matter Controller, but we don’t yet know if it will be a Matter Bridge.
So…the model number matters.
YOUR SECOND QUESTION
Will there be Matter protocol scene controllers?
I’m sorry, I don’t understand what you mean by this.
If you are just asking if the Matter standard includes handheld actuators, right now it doesn’t, but some buttons will work. again, it depends on the model.
Flic has announced that its buttons will work with Matter, but only when used with their Flic mini hub. That’s the same way they work with SmartThings and HomeKit. But there’s a trick to it: these buttons look like sensors or webhooks to the other platforms. (That’s why at the time of this writing they are the only third party buttons that can trigger an Alexa routine, for example.)
https://www.reviewgeek.com/128008/flic-extends-matter-compatibility-to-all-of-its-smart-home-buttons/
I have heard that some other device manufacturers plan to bridge their buttons as switches, although the button itself cannot be turned on with an automation. Just a physical press.
So we will just have to wait and see on this one.