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Expression Language Overview

The expression language is used to define dynamic values for component properties and expression tags. Expressions often involve one or more other values that are used to calculate a final value. Expressions don't do anything, other than return a value.

 

The classic example for an expression is to change a temperature that is stored in Celsius to Fahrenheit in order to display it. Suppose you had a tag, Tank6/Temp that was in Celsius. If you wanted to display that tag in Fahrenheit on a Label, you would use an Expression Binding on the label's text property using the following expression:

 

1.8 * {Tank6/Temp} + 32

 

Every time that the temperature tag changes, the expression will re-calculate the value and push it into the Label's text property. Now lets say that you wanted to append a "°F" to the end of the label so that the user knew the units of the temperature. You could simply use some string concatenation in your expression, like this:

 

(1.8 * {Tank6/Temp} + 32) + " °F"

 

Lets suppose that you wanted to give the user an option to display the value in Celsius or Fahrenheit, based on checking a checkbox. You could add a Check Box component to th screen called DisplayFahrenheit. Then you could use this expression to dynamically display either unit, based upon the user's selection:

 

if({Root Container.DisplayFahrenheit.selected},

 (1.8 * {Tank6/Temp} + 32) + " °F",

 {Tankf/Temp} +  " °C")

 

Where are Expressions Used?

Expressions are used in two major areas:

1.Expression Binding. The expression property binding is the most common area to use an expression. These expressions can reference tag values and property values in the same window.
2.Expression Tags. You can use an expression to dynamically calculate the value of a tag itself using a tag expression.
3.Expression Items. Expression items found in SQLBridge Transaction Groups. This uses the standard expression set with a few additions.