The last time we were together, we discussed the first of two common use cases for tree controls: displaying tabular data. This time out, we are going to look at the other major use case: using tree controls as a sort of menu system to control an application’s operation – or at least its GUI.

The Problem We’re Solving

If you look at the testbed application that we have been working on for almost a year, it’s pretty clear that much of the work has been going on “behind the scenes” and not in the GUI. Oh it is nicely modularized thanks to a structure built around a subpanel interface, but the actual controls are really pretty bare bones. A good example of the utilitarian, but unsophisticated structure is the usage of a simple pop-up menu to select the screen to view. Right now it works pretty well because there is only a handful of plugin screens from which we can choose. However, it doesn’t take much imagination to visualize the mess that would result if there were a dozen, or even hundreds of screens available. We need better organization.

Fixing the Data

The biggest conceptual difference between our current goal, and the one we worked on last time is that in our earlier discussion we were displaying data that already existed outside the application. In other words, my disk has directories that contain files and other directories whether or not I chose to create a program that can read and display the directory’s contents. By contrast, the data we are going to be displaying now only exists within the context of our program, or perhaps within the context of our test environment as a whole. One big consequence of this fact is that we a lot more freedom to define the data’s structure and presentation.

For instance, when our testbed runs right now, there are two “acquisition” processes and three “temperature controllers”. Let’s say, for the sake of argument, that the controller functions are dispersed geographically, and what we see on the local interface is status data from three remote processes. In such a situation, we can observe that there is no “correct” way of viewing that overall structure. Depending upon who the user is and what they need to do there are (at least) two ways that these systems could be organized.

One user, might want to see a top-level breakdown that groups systems based on the function they perform. With this approach to organization, you would have sections for “Data Sources” and “Temperature Controllers”. The individual screen would then be grouped under one or the other of those headings:

Alternatively, a different user might want to see the network resources grouped primarily by each system’s geographical location, with the functions for each site then grouped together like so:

However, as I said before, neither view is any more “correct” than the other. Therefore. we need to be able to support either one – and any other structure that our customers request, as well. Although this level of flexibility might seem to be a tall order, the truth of the matter is that the tree control’s basic operation is very simple, so all we are really talking about is a matter of data management. Moreover, we already have in our hands the tools we need to accomplish the job. I am talking, of course about our database.

Creating the Data Management Structures

So in defining our data structures, we can start with what we already know: The user needs to be able to select basic menu structures by changing a single value. From this requirement it’s obvious that we’re going to need a table to identify the menu’s basic context. We will then use the values stored in that table to qualify the menu item groupings. Here is the definition for this table, and the three records we are going to insert into it:

CREATE TABLE menu_context (
  id     AUTOINCREMENT PRIMARY KEY,
  label  TEXT(100),
  CONSTRAINT contextlabel_uc UNIQUE(label)
  )
;

INSERT INTO menu_context (id, label) VALUES (0,'NULL');
INSERT INTO menu_context (label) VALUES ('By Function');
INSERT INTO menu_context (label) VALUES ('By Location');

The second table we need to define, will hold the records that describe the actual menu entries. Each record defines one line of the tree control’s contents.

CREATE TABLE menu_group (
  id           AUTOINCREMENT PRIMARY KEY,
  context_id   INTEGER NOT NULL,
  item_name    TEXT(100),
  parent_id    INTEGER NOT NULL,
  sort_order   INTEGER,
  CONSTRAINT context_group_fk FOREIGN KEY (context_id) REFERENCES menu_context(id),
  CONSTRAINT self_ref_fk FOREIGN KEY (parent_id) REFERENCES menu_group(id)
  )
;

As is typical, the data for each record incorporates a primary key that uniquely identifies it. Next, comes a foreign key value that relates each record to one of the menu context values defined in the menu_context table. The last three fields store the data that controls the entry’s appearance in the tree control. The item_name field contains the text that will appear for the item’s entry in the tree control. The parent_id is the ID key for the item’s parent. A key value of 0 indicates a top-level item. Note that this values relates to the id field value in the same table. This sort of self-referential relationship is common when creating tables that are, in essence, linked lists. Finally, the sort_order field defines the order in which the menu entries will be added to the tree control. This last field is necessary because we are storing the configuration data in a database – and as you will recall DBMS make no promises about the order of data in queries unless you explicitly include an ORDER BY clause in the query.

Now that we have a table defining the overall tree control menu structure, we need to be able to insert into that structure the entries that will represent the plugin screens. In order to accomplish that task we need a table that relates data we already have in the database (the contents of the launch_item table) to the specific tree control entries that will be their parents in the tree control. The following table fulfills that task:

CREATE TABLE subpanel_group_xref (
  id               AUTOINCREMENT PRIMARY KEY,
  launch_item_id   INTEGER NOT NULL,
  menu_group_id    INTEGER NOT NULL,
  menu_context_id  INTEGER NOT NULL,
  CONSTRAINT launchid_subpanel_FK FOREIGN KEY (launch_item_id) REFERENCES launch_item(id),
  CONSTRAINT groupid_subpanel_FK FOREIGN KEY (menu_group_id) REFERENCES menu_group(id),
  CONSTRAINT contextid_subpanel_FK FOREIGN KEY (menu_context_id) REFERENCES menu_context(id)
  )
;

This table might seem a strange candidate for implementing this crucial bit of functionality because it doesn’t appear to actually store any data. The table only has 4 fields and they are all seem to be holding integers. The distinction here is that while most of the tables we have considered serve to store data, this table stores relationships – specifically the 3-way relationship that defines where each plugin will appear in the menu for each menu context. To see how these bits fit together we need to start considering the LabVIEW code that will read these structures and build the tree control based menus.

Creating the LabVIEW

The basic approach that we will take in creating the entries for the tree control is going to incorporate two distinct phases.

1. Draw the menu structure
2. Fill in the entries associated with the plugins

Reading the Data

The VI that is responsible for reading the menu data from the database (Config Data_DB_ADO:Read Tree Menu Structure.vi) has an enumerated input that selects the menu context the code will display. This value drives a subVI (Get Menu Context ID.vi) that reads and buffers the id value associated with the desired menu context.

In one sense, this subVI really isn’t necessary because you could theoretically perform this look-up operation using a so-called “subquery”, but this approach is far less efficient because it forces the DBMS to repeat the look-up with each query. In addition, these values are not going to change, so better to let LabVIEW remember them. To my way of thinking, however, the biggest issue with this approach is that it complicates the query itself. Given that this is the logic that maintainers (who may not be knowledgeable in SQL) are going to see, it’s a good idea to keep the SQL logic as simple as possible. The other thing to notice about the query is that it puts the entries in the correct order for display by incorporating the clause ORDER BY parent_id, sort_order ASC. Finally, you can see that I built this logic inside the generic ADO database subclass of the existing Config Data object structure.

For reading the tree entries associated with the plugins we use this VI, which is similar to the one for reading the main menu structure, but with some important differences.

The first obvious thing is that the query is much more complex because the primary table being queried is a cross-reference table. Consequently, we have to de-reference the id numbers to derive the data we need to build the menu entries. In learning how this de-referencing works, it’s important to remember that SQL is a language created by a mathematician – specifically a mathematician who specialized in a branch of mathematics called “Set Theory”. His (incredibly optimistic) idea was that if he could create a language based on mathematic principles, he would be able to prove, in the mathematical sense, that the program was correct (read: bug free).

While his grand hope evaporated in the face of the harsh reality that most programming has surprising little to do with mathematics (i.e. computing an answer), the set orientation of SQL has survived. For example, when you perform a query, what you are really doing is SELECTing a data subset FROM a larger set of data – which is typically a table. However, sometimes you need to gather data from a still larger set of data that is spread across multiple tables. To do that, you need to temporarily JOIN those tables together into one large virtual dataset based ON some criteria, like matching id numbers. Get the idea?

A not-so-obvious thing about this LabVIEW code is where it is located in the Config Data object structure. Unlike the routine for reading the basic menu structure, this VI is not located in the generic ADO database subclass. Instead it can be found in the JET database subclass, and the reason for this placement lies in the query. Unlike the other query operation which was implemented in generic SQL, there are aspects of this query that utilize JET-specific syntax (specifically, all the parentheses).

Generating the Menu Tags

With the data in hand that defines the tree-control menus, we now need to turn that data into menu entries. The first step in that transformation is to process the raw data we have acquired from the database to generate the tags that are needed to properly organize the tree entries. I won’t take up the room to show the code for this VI (Parse Tree Management Data.vi) because it’s easy to explain what it does – but feel free to check it out in the code. The VI’s primary program structure is a while loop that iterates through the raw tree-definition data generating the tags and formatting the data to generate the tree items. The loop has on it two shift registers: one holds an array of ID numbers that the loop has already processed, the other holds an array of clusters. Each element contains the four items that we will need to define a menu item (Parent Tag, Child Name, Child Tag and Child Only?).

With each iteration, the loop extracts the top element from the array and tests the Parent ID. If it is zero, the item is a top-level entry so the code builds its entry and continues with the next iteration. If Parent ID is anything other than 0, it searches the array of processed IDs to see if its parent has already been processed. When the comparison finds the new entry’s Parent ID it uses its tag value to synthesize the new entry’s child tag. When the new entry’s Parent ID is not found, the code adds the entry’s element back onto the bottom of the array of entries to be processed so it can be retried later. Normally, this search should never fail because the ordering in the queries should put the elements in the correct order, but this is just in case. This operation continues until there are no more entries left to process.

Finally, in terms of tree control infrastructure, the only thing we have left is to actually insert the entries that we have defined into the tree control. By the time we get to this point in the code we have gotten the definitions in the correct order so all we have left to do is disable front panel updates, clear the tree control, add the new tree entries and re-enable front panel updates. Again, this code is very simple so to save space I will refer you to the last post (https://www.notatamelion.com/2015/09/14/a-tree-grows-in-brooklyn-labview/) for details on the call and how it works.

Integration with the Testbed

To integrate this code with the existing testbed application requires very little work. First off on the front panel, we remove the existing ring control that we were using to select screens and add the tree control (I’m using he one from the System-themed palette), and a System-themed enumeration that will allow the operator to switch between the two menu context values. Note that this control could also be defined as a ring with the String[] control populated at run time to show the available options. This implementation would be useful if you want to provide the ability in your program to either allows the users to dynamically configure the basic structure of the tree control menus, or provide different options depending on who is using the system.

On the block diagram, the front panel changes impact the program logic in two places. First, we need to create a new Value Changeevent to handle the Menu Context control. This event (which is also fired when the GUI initializes itself) is responsible for rebuilding the tree-control menu display.

The event handler starts by calling a subVI (Get Tree Menu Data.vi) that accepts as an input a Menu Context value and internally calls the two database query VIs we discussed above. After concatenating the arrays that it gets from the two routines, it passes the raw data to the VI (Parse Tree Management Data.vi) I described that converts the raw data into tree control entries. Finally it returns the array of tree control entries to the event handler, which passes it, and two references to the subVI (Draw Menu.vi) which does exactly what it name says. The first of the references is, obviously, a control reference to the tree control. The other is a VI reference to the GUI itself so the subVI can defer and then re-enable front panel updates.

The other block diagram change is to purpose an existing value change event. The event n question used to handle the ring control that changed screens and while it will still be a value change event, it will be a value change event on the aptly labeled tree control, Tree.

The original logic that occupied this space took the string value of the selected ring item and used it to look up the name of the associated screen in an array of strings. The string array consisted of screen labels that were generated when the GUI loaded the subpanel VIs into memory and started them running. The resulting index was then used to index the screen’s VI reference from an array of plugin screen VI references. This VI reference would, in turn, drive the subpanel’s Insert VI method to make that screen visible in the subpanel.

The modified form works basically the same, but with a couple minor differences. Although the tree control’s value is a string, the string is the tag associated with the entry. Since the part we need to perform our search is the last item in the colon-delimited list, the first thing we need to do is strip off everything up to, and including, the lastcolon in the string. Moreover because we want this operation to be efficient as possible – so no looping. A very efficient solution is to use the built-in Match Pattern function with the rather curious-looking pattern shown. To see how it works, consider that a dot (“.”) is a special character matches any character. Next, the asterisk (“*”) is a special character that matches the longest sequence of the token that came just before it. Hence, it will match the longest sequence of any characters. Finally, the colon is not a special character so it will match just a colon. The end result is that the complete pattern will match the longest sequence of characters that are followed by a colon, and it works the same whether there is one colon in the string or a dozen. The string I want will be what is left after the match.

The other change that was needed for the tree control, is the case structure, which is there to work around a bug in the way LabVIEW handles value change events with tree controls. I configured the tree control such that only entries that are marked as Child Only are selectable. The bug is that when you click on one of the parent items, LabVIEW still fires the value change event even though the value of the tree control isn’t changing. To work around this issue, the event handler bypasses all further event processing of the “selected” item when it isn’t found in the list of screens.

Testing the Interface

As always, the “Proof of the pudding is in the eating” so let’s try running the application with its new GUI feature. One minor difference in behavior is that the subpanel now remains empty at startup until the user makes a selection. However, from the time the application starts, the user has visible a complete list of all the display screens that are available. In addition, the tree will automatically reconfigure itself when a different context is selected.

Testbed Application – Release 17
Toolbox – Release 14

The Big Tease

At one time when I started doing this work, test systems were surprisingly homogeneous. While it was true that instruments came from many different vendors, the software environment was pretty monolithic. Today, however, things have really changed. Every day it is becoming more common and accepted to have multiple applications running in parallel that were developed using a variety of development tools ranging from C++ to Java to C# and F.

In the past we have talked about creating a LabVIEW-based backend application with the main GUI built using standard web tools such as HTML, CSS and JavaScript (https://www.notatamelion.com/2015/06/08/building-a-web-backend-in-labview/). Over the next few posts I want to consider some of the other ways that your LabVIEW application can work with external applications.

Until Next Time…
Mike…