Invoke node, labview

Invoke node, labview DEFAULT

Manipulating Application and VI Settings

Properties are attributes of a VI, object, or application. Methods perform actions on a VI, object, or application. You can get or set properties and methods on local or remote application instances, VIs, and objects by using the Property Node and the Invoke Node. You can get and set many application, VI, and object settings only through properties and methods.

Refer to the labview\examples\viserver directory for examples of using the Application and VI Class properties and methods.

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Property Nodes

Use the Property Node to get and set various properties on an application or VI. Select properties from the node by using the Operating tool to click the property terminal or by right-clicking the white area of the node and selecting Properties from the shortcut menu.

The following are examples of how properties can enhance ease of use in an application or VI:

  • Set the text focus to the main, most commonly used control.
  • Disable or hide controls that are not currently relevant or valid.
  • Guide the user through steps by highlighting controls.
  • Change colors to bring attention to error conditions.

You can read or write multiple properties using a single node. However, some properties are not readable and some are not writable. Use the Positioning tool to resize the Property Node to add new terminals. A small direction arrow to the right of the property indicates a property you read. A small direction arrow to the left of the property indicates a property you write. Right-click the property and select Change to Read or Change to Write from the shortcut menu to change the operation of the property.

The node executes from top to bottom. The Property Node does not execute if an error occurs before it executes, so always check for the possibility of errors. If an error occurs in a property, LabVIEW ignores the remaining properties and returns an error. If you right-click the Property Node and select Ignore Errors inside Node, LabVIEW executes the remaining properties on the Property Node. Only the first error is returned by the Property Node. The error out cluster contains information about which property caused the error.

If the Property Node opens and returns a reference to an object, use the Close Reference function to close the reference.

Implicitly Linked Property Nodes

When you create a property from a front panel object by right-clicking the object, selecting Create�Property Node, and selecting a property from the shortcut menu, LabVIEW creates a Property Node on the block diagram that is implicitly linked to the front panel object. Because such a Property Node is implicitly linked to the front panel object that you created it from, the node has no reference input and you do not need to wire the node to the terminal of the front panel object or the control reference. To remove the link to the front panel object, right-click the Property Node on the block diagram and select Disconnect From Control from the shortcut menu.

Invoke Nodes

Use the Invoke Node to perform actions, or methods, on an application or VI. Unlike the Property Node, a single Invoke Node executes only a single method on an application or VI. Select a method by using the Operating tool to click the method terminal or by right-clicking the white area of the node and selecting Methods from the shortcut menu. You also can create an implicitly linked Invoke Node by right-clicking a front panel object, selecting Create�Invoke Node, and selecting a method from the shortcut menu.

The name of the method is always the first terminal in the list of parameters in the Invoke Node. If the method returns a value, the method terminal displays the return value. Otherwise, the method terminal has no value.

The Invoke Node lists the parameters from top to bottom with the name of the method at the top and the optional parameters, which are dimmed, at the bottom.

Manipulating Application Class Properties and Methods

You can get or set properties on a local or remote application instance, perform methods on LabVIEW, or both. The following block diagram shows how to display all VIs in memory on a local computer in a string array on the front panel.

If you do not wire a refnum to the reference input, the Property Node or Invoke Node uses a reference to the current application instance. If you want to manipulate the properties or methods of another application instance, you must wire an application reference to the reference input. If you have multiple application instances open simultaneously, be sure to wire an application reference to the reference input. For example, LabVIEW opens a new application instance each time you create a LabVIEW project or a target for a LabVIEW project. LabVIEW also creates a main application instance, which contains open VIs that are not part of a project and VIs that you did not open from a project.

To find the exported VIs in memory on a remote computer, wire a string control to the machine name input of the Open Application Reference function, as shown in the following block diagram, and enter the IP address or domain name. You also must select the Exported VIs In Memory property because the All VIs In Memory property used in the previous block diagram applies only to local application instances.

You also can use the Default:Application property to return the default application reference programmatically. Use the Application property to open the target application instance programmatically.

Manipulating VI Class Properties and Methods

You can get or set properties of a VI, perform methods on a VI, or both. Use the Open VI Reference function to return a VI reference, and then use Property Nodes and Invoke Nodes as you would when manipulating Application Class properties and methods.

If you do not wire a refnum to the reference input, the property or method uses a reference to the VI containing the Property Node or Invoke Node. If you want to manipulate the properties or methods of another VI, you must wire a VI refnum to the reference input.

The Property Node operates similarly to the Invoke Node. After you wire a VI refnum to the Property Node, you can access all the VI Class properties.

Manipulating Application and VI Class Properties and Methods

In some VIs, you must access both Application and VI Class properties or methods. You must open and close the Application and VI Class refnums separately, as shown in the following block diagram.

The following block diagram shows how to determine the exported VIs in memory on a local computer and to display the path to each of the VIs on the front panel. To find the exported VIs in memory, you must access an Application Class property. To determine the paths to each of these VIs, you must access a VI Class property. The number of exported VIs in memory determines the number of times the For Loop executes. Place the Open VI Reference and Close Reference functions inside the For Loop because you need a VI refnum for each VI in memory.


Invoke Node

The Invoke Node is used to perform a method on a VI using VI Server, very similar to the Call By Reference Node. Invoke Node, however, does not need a strictly typed reference and as such is not bound to the VI's connector pane and is free to call any VI. On the other hand, as the connector pane of the callee is unknown at editing time, one must use several calls to Invoke Node in order to set control values, run the VI and retrieve its results.

When using the Invoke Node it is very common to stumble upon an error:

VI is not in appropriate state for the invoke node

This is a known behaviour, but I wouldn't call it an error. It's just how LabVIEW works. A lot of the methods for a VI reference can not be executed for VIs which are either in a reserved state or already running. When a VI is running is rather obvious but a reserved state means that the VI is either part of a running hierarchy of VIs (usually your application) or a strict typedefed VI reference is open to it. The reasons for this are not so simple but it comes down to protecting the VI from influences from different caller types. The Run method of the Invoke Node (amongst others) is asynchronous as it starts the VI but not necessarily waits for termination of the VI. An implicit reference to the VI as an icon in your block diagram or an explicit one in the form of a strict typedef to be used with the Call by Reference Node are synchronous as far as the caller is concerned.

If you are unable to resolve that error message, you may eventually try to use the Call by Reference Node instead. If you need for whatever reasons to use the Run method you MUST make sure that the VI is not an explicit part of your application by not adding its icon anywhere in your app (and not opening a strict typedefed reference to it either).

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Invoke Nodes

Invoke nodes are very similar to property nodes. Calling an invoke node runs a single method on the front panel object and sometimes requires inputs, (also known as "arguments"). In a way, calling an invoke node is similar to calling a subVI or calling a function or method in other languages.

The difference between a property node and an invoke node is that calling an invoke node, "executes something"it doesn't just change the value of a property. You can think of invoke nodes as functions that you execute; whereas a property node is a property, or state, that can be read from or written to.

To create an invoke node, pop up on either the front panel object or its terminal, and select one of the control's methods from the Create>>Invoke node>> short-cut menu. A terminal with the same name as the variable will appear on the diagram. The method name will appear as the first item in the invoke node, and all the arguments will appear as items beneath the method name. You can change the method being called by the invoke node with the Operating tool or pop up on the node and choose Methods>>. Now you have the choice of which method you wish to select. Each object has a set of base methods, and usually, an additional set of methods specific to that object. One base method that is available for all controls is Reinitialize to Default (see Figure 13.40). It doesn't have any arguments, and calling it will do just what it saysreinitialize the control to its default value.

Figure 13.40. Configuring a control's invoke node to call the Reinitialize to Default method

Activity 13-5: Using Invoke Nodes to Export an Image from a Graph

In this activity, you'll see how to use an invoke node specific to graphs, which allows you to export the graph to an image file.

We'll use the VI you created in Chapter 8, Graph Sine, so you should open that (or use the one on CD-ROM, in the folder).


Open Graph Sine


From the block diagram, right-click on Waveform Graph and select Create>>Invoke Node>> ExportImage. You should see an invoke node like the one in Figure 13.41.

Figure 13.41. Invoke node configured to call the Waveform Graph's Export Image method


The Export Image method creates an image of the graph it's associated with, and sends this image to the clipboard or to a file. The following inputs are required:

  1. FileType can be BMP, EPS, EMF, or PICT.

  2. Target can be Clipboard or File.

The following inputs are optional:

  1. Path is the path to the file to be saved.

  2. HideGrid is a Boolean, which if TRUE, hides the graph's grid on the image.

  3. AlwaysOverwrite is a Boolean, which if TRUE, overwrites the image file even if one with the same path and filename exists.


To use this node, you can pop up on each of its inputs and select the appropriate value. Set FileType to "BMP" and Target to "File."


For the path, it will be best if we allow the user to choose the path with a dialog. Here's an easy way to program a prompt for the user to enter a filename: Use the Express VI File Dialog (shown in Figure 13.42), which may be found on the Express>>Input palette.

Figure 13.42. File Dialog express VI

When you drop the File Dialog Express VI onto the block diagram, you will be asked to configure some options for this dialog. Select "Files only" and "New or existing."


Finally, use a sequence structure to make sure that things happen in the right order, and wire the output of File Dialog to the "path" input in ExportImage.

Your VI's block diagram should look like Figure 13.43.

Figure 13.43. Block diagram of the VI you will create during this activity


Save your VI as in the directory.

When you run it, you should be able to choose a filename to save the bitmap under. Open the bitmap file this VI creates and you'll see an image created from the waveform graph.



The major difference between calling by reference using an Invoke Node and the Call by Reference Node is the ease of programming versus the flexibility of the architecture. When you use a Call by Reference Node you have to wire in a strictly typed reference. A strictly typed reference includes the connector pane with the data types as well as the reference to the VI. Therefore when using the Call by Reference Node, LabVIEW knows about the data type and creates a connector pane for you. In this way you can actually take advantage of loading a VI dynamically, but at the same time be able to have the convenience of simple wiring.

Using the Call by Reference Node

Note: If you want to call asynchronously the target VI, use the Start Asynchronous Call Node instead of the Call by Reference Node. This will allow the target VI to run on a separate thread.

When you use an Invoke Node, it doesn’t matter if you use a strictly typed reference. You can therefore pass any VI to the Invoke Node to run it. This allows much more flexibility because you don’t have to type cast and only call VIs that have the same connector pane. However, if you want to pass values you will actually need to know the names of the controls and their data types, which requires much more programming. However, if all you want to do is open the front panel and run that VI, then this becomes an equally simple way to do it. It also becomes more flexible because you can pass any VI path into this setup and it will be able to open and load that VI. This allows for a plug-in architecture, which is only valid in the development environment.

Using an Invoke Node to Call by Reference

Another benefit of using the Invoke Node to call a VI by reference is it provides the ability to interact with the VI. The Invoke Node method allows interacting with the VI if it needs to be controlled or if the outputs need to be read multiple times. Using the Call by Reference Node, the connector pane will output values when the VI finishes execution, but it does not provide the capability to interact with a running VI.

Note: Using the Invoke Node method may have the target VI run in the UI thread, which can lead to some issues if you want to have the calling VI perform adequately while the called VI is running. This happens even if you have a False constant wired to the Wait Until Done terminal. For a complete asynchronous call in a separate thread, you need to use the modify the target VI’s execution by going to File»VI Properties, selecting Execution in the dialog box and selecting an execution system different than user interface or same as caller.

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Labview invoke node

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LabVIEW Property Nodes

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