This page summarises which variables are available in different contexts, and explains how to create and use $-variables. See also Expression Basics.

The following tables gives some guidelines for the availability of variables, based on the PhixFlow Timing Cycle for evaluating analysis models.

Variable Scoping

When writing expressions that reference variables it is important to understand whether PhixFlow has "seen" the variable yet. This is the scope of the variable. 

In a stream, output multiplier or output filter, the attributes have a specified order. When you run analysis on a model, PhixFlow processes each attribute in order, evaluating their expressions. This means you cannot reference:

• an attribute name before PhixFlow has processed it. So in the first attribute in a stream, you cannot use a value from the third attribute in the stream. 
• a $-variable before PhixFlow has processed the expression where it is set.

You can only reference:

• an attribute name in a subsequent attribute
• a $-variable after it has been set or evaluated.

Internal Variables

PhixFlow has a list of Internal Variables. For example, some common ways to reference attributes in a stream: 

In applications, some commonly-used internal variables are

• _grid: to refer to a data grid in a stream view or layout.
• _form: to refer to a form on a layout.
• _context: to refer to a Context Parameter

Creating and Using $-variables

Defining a $-variable

• All user-defined variables must start with the $ character. 
• For multi-word variable names, use no spaces and camel case.
• It can be useful to know whether a variable is used in other expressions. As there is no way to check this, we recommend using the following naming convention:
• To distinguish between $-variables that are:
  • only used in the current expression or attribute, use a lowercase first letter, for example $percent. 
    These are sometimes called local.
  • used in other expressions or attributes, use an uppercase first letter, for example $Percent.
These are sometimes called global. 

PhixFlow assumes that you are declaring a variable and setting its value the first time it occurs in the expression.

$localVar = 'Smith' $GlobalVar = 'St.John-Smith'

Using $-Variables in Expressions

In expressions that have complex calculations, you may need a variable to hold the result of a calculation. You can then reference the variable later in the expression, or in other attribute expressions for the same stream.

The following expression uses do() function and $-variables for a simple calculation.

When to Use $-variables

$-variables are not really needed in simple calculations. They are useful:

• in forEach loops
• calculating attibute names from pipes; see Assigning Values From a Pipe.
• to store a value from a look-up pipe, when you need the value in multiple attributes
  1. In the first attribute, look-up the value.
  2. Use a $-variable to store the value that is returned by the look-up.
  3. When you need the value in another attribute, reference the $-variable instead of repeating the look-up.
• in complex expressions, to break up multiple consecutive functions into separate lines. In this case, use a $-variable to store the intermediate values; see Multiline Statements and $-variables.

Using $-variables Throughout an Application    

$-variables can have different "scope". They can be:

• only used withn an expression or within a stream attribute. 
• used more widely, in multiple expressions or for different stream attributes.

Although the variables are the same, to distinguish between these two usages we use the terms local and global respectively.

There are some situations where you want to set a $-variable to use throughout your application. For example:

• Users can access your application on a desktop or mobile platform. The form-factor affects which styles you want want to use to display the application.
  Also see Using Size and Position for Responsive Design.
• You application is used by people in different countries. This could affect different aspects of the application, such as showing the relevant flag on a dashboard.
  Also see Translating an Application Interface.
  

In both these examples, you want to reference a $-variable that identifies the device or country. You then use the $-variable to select an appropriate formatting rule to customise the application's appearance.

To make a $-variable available throughout the application, you need to set it on something that always happens and is then always available. You can acheive this by:

1. Setting a $-variable as a context parameter on the application's open action. 
2. Linking the context parameter it to something on the first screen that opens, such as the application home screen.

The steps in more detail are:

1. In the repository, find the action that opens the application. In its properties, add a Context Parameter; see Action.
2. In the context parameter's Expression field, set the $-variable; see Context Parameter.
3. In the repository, find the application's first screen. Right-click and select Display. On the dashboard:
   1. From the palette drag in a simple form field onto an area.
      For example, use the palette Basic Components →StringField. A field is a type of layout component;  see Layout.
   2. Give the field exactly the same as the context parameter.
   3. In the field properties tab, at the end of the Style Settings section, tick Hidden.
4. To display the dashboard property tab, right-click to display the context menu. Select Show Dashboard Details or Dashboard Options →  Show Details.
5. In the properties tab Basic Settings section, tick Keep Data.

You have now created a $-variable that PhixFlow always knows about, but which is not visible to users. To reference this variable in any expression or script, use:
     <dashboard>.<field>

For example, if the dashboard is called HomePage, and the field is called Mobile, you would reference HomePage.Mobile. The field name effectively redirects PhixFlow to the context parameter where the $-variable is set.

Variable Typing

When a variable is first used, PhixFlow makes a best guess about what type of data is stored in that variable; see Parameter Types. For example:


When PhixFlow has decided what type of data is stored in a variable, you usually cannot change the type. PhixFlow will reject an expression that equates different types, such as:

$thisNumber = $thisAddress

The cases where you may be able to change a data type are

• integer to a floating point value: PhixFlow may be able to convert between these.
• by using type conversion function, such as toDate. This forces a value of one type into a variable of another type.

See Also

• Expressions and Scripts
• Expression Basics
• Expression Style
• Internal Variables