Property Binding

A property binding is a very simple type of binding. It simply binds one component's property to another. When that property changes, the new value is pushed into the property that the binding is set up on.

Why aren't all properties listed?

You may notice that the list of properties available to bind to is smaller than the list of all properties. While nearly all properties can be bound, only some properties can be bound to. Only properties for which a propertyChangeEvent is fired may be bound to.

How Bindings Work: Event-based vs. Polling

While there are quite a few different binding types, they all boil down into two broad categories: event-based and polling. Some complex bindings can span both categories.

Event-based bindings are evaluated when the object they are bound to changes. For example, when you bind a property to a tag, that binding listens to the tag, and every time the tag changes, it assigns the tag's new value into the property that it is on. If you bind the value of a Cylindrical Tank to the value of a Slider, every time the slider changes, it fires a propertyChangeEvent. The binding is listening for this event, and when it is fired, updates the tank's value. The following bindings are event-based:

  • Tag bindings

  • Property bindings

Polling bindings are evaluated when a window first opens, on a timer, or when they change. For example, if you bind the data property of a Table to the results of a SQL query, that query will run on a timer, updating the Table every time it executes. The following bindings are based on polling:

  • SQL query bindings

  • Some expression functions, like runScript() or now()

Many bindings can combine elements of a polling binding and event-based binding. An expression binding may combine lots of other bindings to calculate a final result. A query binding will often itself be dynamic, altering the query based on other bindings.

For example, you might have a dropdown on a window that lets the operator choose a type of product that is produced. Then you can use a query binding like the following to calculate the defect rate for the given product:

SELECT
SUM(defective) / COUNT(*) AS DefectRate
FROM
production_table
WHERE
productCode = '{Root Container.ProductPicker.selectedValue}'

The blue code is a property binding inside of the query binding. Every time this (event-based) binding fires, the query will run again. Using bindings like this, you can create highly dynamic and interactive screens with no scripting whatsoever.

Polling Options

The following are the options you can choose from for bindings that poll:

  • Polling Off
    A polling-off binding will execute once when the window is opened, and then it will only execute again if it changes. The typical example of a binding that can change is a SQL query binding where it uses the brace-notation ( {} ) to include dynamic information inside the query. When this dynamic information changes the query, it will run again.

  • Relative Rate
    The binding will execute at a regular rate, based on a delta off of the project's base polling rate, see Setting Project Polling Base Rate. This is usually a good idea so that you can speed up or slow down an entire client's polling system in one place.

  • Absolute Rate
    Using this option, you can specify an absolute rate for the binding to execute at, instead of one that is based off the relative rate.