Something I have always enjoyed doing is creating programs or interfaces that do things you don’t expect LabVIEW to be able to do. Consequently, I thought I would take a couple posts and consider some useful and perhaps surprising interfaces.
The first one I want to look at is actually an idea I got from another LabVIEW Champion – Ben Rayner by name. Some time ago, he posted on the user forum a small proof-of-concept VI for an interface that is so cool I though I’d polish it up, flesh it out and see what came out of it.
What it does
At first glance, the interface appears similar to the test bed application that we have built on this blog. There is a common display area and by making selections from a pop up menu, you can display screens showing different data.
The difference is that if you click a button, the GUI will “undock” the current screen and turn it into an independent floating window that is no longer accessible from the popup menu.
The software allows an operator to undock as many screen as they want at one time. The only limitations are screen space and logic that mandates leaving at least one screen docked. Likewise, if you close one of the floating windows, it will again take up its original position in the selection menu. As is our usual policy, a link to the code is a little further along in this post.
Why This Interface?
I fully plan to get into how the code works, but first we need to consider why you would want to use this interface in the first place. What is the use case that this interface addresses?
Hopefully it should be obvious to anyone who considers the matter for more than a moment or two that user interfaces are always compromises. If you do your due diligence when designing an interface you try to put together in one screen or window the various bits of information that are logically related, or at least will be used together. However, requirements can change, or a particular type of user might need to be able to correlate pieces of data that are on different screens.
You could create a separate screen for just that user, but that solution requires some additional development effort. However, if you create windows that can be undocked to be moved around the screen as desired, users that want to see their data in a particular way may be able to do so without waiting for you to create a custom screen.
So with all that in mind, let’s think about how we might accomplish this sort of GUI.
How it Works in Theory…
The first thing to you need to realize as we begin looking at how to create this interface, is that if you have been following this blog you already know everything required to make this happen. The only thing missing is an understanding of how to make all the pieces fit together in a slightly different way.
For example, we have talked many times about how to create a basic multiscreen interface using subpanels. Likewise, we know how to make windows float on top of one another by defining their behavior as floating. Now if you think about it, these are the two states that our screens can be in: as a subpanel (when docked) and as a floating window (when undocked).
So our problem is really as simple as deciding how to manage the transition from one state to the other. If you are following the recommendations I have been making, the screens are already written as separate processes that don’t know (or care) how they are being displayed – or even if they are being displayed.
So how do we want this to behave? Well for the undocking there are a variety of options. LabVIEW supports drag-and-drop so we might be able to do something with that. Alternatively, we could create a pull down menu with an “Undock Current Window” on it, or even a custom shortcut menu when the user right-clicks on the on the interface’s front panel. But those are really just different ways of triggering the same logic, so for this demo I’m just going to create a button below the subpanel that is labeled something obscure like, “Click to Undock the Current Display”.
For docking one of the floating windows back in the interface, our options are more limited. But when you have a really good technique available, you only need one. How about this: when you click the window’s close button it gets docked back into the interface. After all, it’s what most users would try anyway.
…and in Practice
Now that we basically understand where we are going, lets start looking at some code.
The block diagram of the top-level VI is pretty much what a regular reader of this blog would expect. It starts by running a subVI that loads into memory, and starts executing, all the VIs that are going to be available through the subpanel. References to these VIs are stored in a DVR so they can be used later to populate the subpanel itself. This subVI also outputs an array of strings that are the names of the VIs it loaded. The code uses this array to initialize the strings in a ring control, and then programmatically fires the value change event associated with the ring.
With this work done, the VI next registers to receive a UDE that we will discuss in a moment, and enters the program’s main event loop. This loop includes a value change event for the ring control that changes the VI that is visible in the subpanel, and two additional events that manage the undocking and docking processes.
The first of these new events handles the undocking of windows and, understandably, is a value change event on the undocking button. The event logic goes about its work in two steps that are contained in separate subVIs. The first one is called
Float the VI.vi.
Its job is to remove the VI that is being undocked from the subpanel using the
Remove VI subpanel control method, and then (after looking up its VI reference in the DVR) open its front panel using the
FP:Open VI method. The other half of the operation is performed in
This VI’s purpose is to update the user interface to the VI’s change of state – which in this case means simply disabling its selection in the ring control, and updating the DVR. This subVI is also responsible for getting a reference to the next available docked window and inserting it into the subpanel. As a by-product of this operation, the subVI also generates a flag that indicates when there is only one docked window remaining. The event handler uses this flag to disable the
undock button when all but one of the displays have been undocked.
Docking a Window
Like the previous one, this event handles its duties as a two-step process. But unlike the previous one, it is driven by a UDE from the VI that is being docked back into the main display. As stated before, this action should be triggered when the user closes the VI’s front panel. The plugins accomplish this task by intercepting the
Panel Close? event and instead of closing its front panel, simply fires a UDE (
Redock Screen) that tells the GUI to reincorporate it back into the subpanel. The first subVI in the event handler is
Unfloat the VI.vi.
After looking up the VI’s reference in the DVR, it closes the VI’s front panel and inserts it back into the subpanel. Note that it is not necessary to remove the VI that is already there. The other subVI that the event handler calls is
Basically reversing the operation that were performed when the window was undocked, this VI removes the window’s name in the ring control from the list of disabled items and updates the status stored in the DVR.
The Big Tease
So there’s a basic implementation of a pretty neat capability, but what now? What sort of enhancements might we reasonably want to make? I have an idea. You notice that when the window’s undock, they always open in the middle of the main screen. This is certainly a reasonable approach, but an enhancement that would be sure to make your users happy is to provide some sort of mechanism whereby each window would remember its last position. With that capability in place a windows would reopen in the same place as it was when it was last closed.
So when we next get together, let’s do that. It will be a good thing to know how to do in general.
Until Next Time…