Continuum is a new feature that Microsoft has introduced as part of the Windows 10 operating system. After the Build 2015 conference I have noticed that people are providing different speculations on the technology and how it was demoed. I am going to attempt to define what it is, what it isn't, and how new Universal Windows Platform (UWP) and existing Windows Phone 8.x applications can utilize it.
What is Continuum?
Continuum was previously explained by Microsoft at the Windows 10 announcement event as the ability to have a uniform experience across your devices. They described it as the ability to be using an app on one device, such as a tablet, and continue that same experience on another device, such as a phone, without any disorientation.
Microsoft may have been intentionally vague when they made that description or maybe they weren't sure where the technology would land, but it now seems like they have provided a different and only slightly more concrete definition for Continuum. They are now describing Continuum as the ability to use a device to power an experience on an external display. But how is this different than plugging in a monitor to a desktop? Well, it isn't. The key distinction with Continuum is that the focus is on using a phone or other mobile device to power an experience on an external monitor while having that experience appear fluid and native to the external monitor. This is an important distinction because having this functionality on a phone will allow people to take a full computer experience with them wherever they go.
Some specific visions that Microsoft shared during Build included hybrid laptop/tablets that are just a screen and keyboard with no internals and are powered by a phone, and watching a movie on your TV powered by your phone while continuing to answer texts and emails separately on the phone itself. In today's world where screens are everywhere and everybody has a phone, this just makes sense. For those who have financial constraints and need to choose between a phone and a computer (and the phone is usually the decision), this provides an option to have both.
One thing that I noticed during the Continuum demos was that the experience on the external monitor is not a typical desktop experience for Windows 10. Although the start button was there it had cellular signal strength and other icons at the top of the screen and I didn't see Cortana in the bottom left as it is in the current Windows 10 desktop preview builds. This makes me think that there may still be work Microsoft has to do to provide a full desktop experience from a phone, but they don't seem to be too far off. Microsoft has also mentioned the hardware constraints noting that the phones with this feature aren't available yet and will need to utilize new Qualcomm technology.
What isn't Continuum?
One distinction I want to make is that Continuum is not, to my knowledge at least, defined by Microsoft. Considering that, I want to be clear about my interpretation of Continuum. There are two other ways that Microsoft has provided to interact with separate displays on a device in Windows 10. One way is by using it to project media, such as a video or images, and the other way is by using a second monitor as a separate frame/page for an application. Although these are great features to take advantage of in certain situations, I personally would not consider these features "Continuum" since they are custom to each application and the experience is still focused on the initial device (i.e. phone).
How to Support Continuum in Windows Phone 8.x Applications
One mystery that wasn't quite clear during Build was how to provide Continuum support for existing Windows Phone 8.x applications. The main direction given was "migrate it to a Universal Windows 10 application." This is certainly an option but one that might be more difficult for people since the adoption rate of Windows 10 is still unknown and there may be some additional work needed to do a full app conversion for certain applications. Based on the demos that were shown I would expect that the apps will be able to open on the external monitors but probably will not work well. They may decide to add a restriction to not even allow older Windows Phone apps to open in the Continuum experience, but since they are still in development they aren't revealing much.
How to Support Continuum in Universal Windows Platform (UWP) 10 Applications
Support for Continuum in UWP applications is simple: use best practices to allow apps to adjust for all screen sizes (blog on this coming soon) and always consider different input methods such as a keyboard and mouse. Knowing that UWP apps can be restricted to certain device families, such as phones, some apps may decide not to follow some of these best practices. Based on the discussions from the Build conference there most likely won't be a way to prevent a user from opening an app on an external display if it works on their phone. Considering this I would recommend at least ensuring that the content scales in some way, and at that point it's most likely worth the effort to just support all of the device families.
In summary, I hope that this provides a little bit clearer picture on Continuum. Hopefully Microsoft will be able to polish this experience and provide a new way for people to think about mobile software.