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Using Variables

Using Variables

Oct 31, 2023

Overview

When writing expressions and scripts in PhixFlow, you can use different types of variable.

  • Attribute names are like the column headings in a spreadsheet.

    • You can reference an attribute using its name in other attributes, expressions and file collectors.

    • In expressions, attribute names can be qualified with pipe names like this:  <pipename>.<attributename> for example in.recordID.

    • Database collectors are a special case. They generate implicit attribute names from the names of the fields returned by the database query.

  • Internal variables are predefined variables that you can use to refer to specific things; see Internal Variables.
    PhixFlow will automatically add internal variables in some situations. For example, when you load data via a file collector, the table automatically includes the attribute  _fileName to indicate the original file.

  • User-defined variables are the variables you can create to hold data required for complex calculations in attributes or other expressions. These are prefixed with a $ character, so are called $-variables.
    For information on making $-variables reusable in an application, see Creating and Using $-variables below.

  • 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 table, 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 table, you cannot use a value from the third attribute in the table. 

    • 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 table: 

    In applications, some commonly-used internal variables are

    • _grid: to refer to a data grid in a 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

    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

    Using $-variables Throughout an Application 

      

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

    • only used withn an expression or within a attribute. 

    • used more widely, in multiple expressions or for different 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 Sizing and Positioning Screen Content.

    • 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 screen.
      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 Table-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 screen:

      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 Component.

      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 screen propert, right-click to display the context menu. Select Show Screen Details or Screen 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:
         <screen>.<field>

    For example, if the screen 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