Communicating between a window and a user object

Often you need to exchange information between a window and a visual user object in the window. Consider these situations:

This section discusses two techniques for handling this communication and presents a simple example.

Table 15-4: Techniques for communicating information in a window

Technique

Advantages

Disadvantages

Functions

Easy to use

Supports parameters and return types, so is not prone to errors

Supports data encapsulation and information hiding

Best for complex operations

Creates overhead, might be unnecessary for simple operations

User events

Very flexible and powerful

Uses no type checking, so is prone to error

Communication with both techniques can be either synchronous (using Send for functions and the EVENT keyword for events) or asynchronous (using Post for functions and the POST keyword for events).

Directly referencing properties

Instead of using functions or user events, it is possible to reference properties of a user object directly. If you have a user object control, uo_1, associated with a custom user object that has a SingleLineEdit, sle_1, you can use the following in a script for the window:

uo_1.sle_1.Text = "new text"

However, it is better to communicate with user objects through functions and user events, as described below, in order to maintain a clean interface between your user object and the rest of your application.

The functions technique

Exchanging information using functions is straightforward. After a user object calls a function, any return value is available to any control within that object.

For how to use this technique, see “Example 1: using functions”.

StepsTo pass information from a window to a user object:

  1. Define a public, user object-level function that takes as arguments the information needed from the window.

  2. Place the user object in the window.

  3. When appropriate, call the function from a script in the window, passing the needed information as arguments.

StepsTo pass information from a user object to a window:

  1. Define a public, window-level function that takes as parameters the information needed from the user object.

  2. Place the user object in the window.

  3. When appropriate, call the function from a script in the user object, passing the needed information as parameters.

The user events technique

You can define user-defined events, also called user events, to communicate between a window and a user object. You can declare user events for any PowerBuilder object or control.

A custom visual user object often requires a user event. After you place a custom visual user object in a window or in another custom user object, you can write scripts only for events that occur in the user object itself. You cannot write scripts for events in the controls in the user object.

You can, however, define user events for the user object, and trigger those events in scripts for the controls contained in that user object. In the Window painter, you write scripts for the user events, referencing components of the window as needed.

For more information about user events, see Chapter 9, “Working with User Events,” and Application Techniques. For instructions for using this technique, see “Example 2: using user events”.

StepsTo define and trigger a user event in a visual user object:

  1. In the User Object painter, select the user object.

    Make sure no control in the user object is selected.

  2. In the Event List view, select Add from the pop-up menu.

  3. In the Prototype window that displays, define the user event.

    For how to do so, see “Defining user events”.

  4. Use the Event keyword in scripts for a control to trigger the user event in the user object:

    userobject.Event eventname ( )
    

    For example, the following statement in the Clicked event of a CommandButton contained in a custom visual user object triggers the Max_requested event in the user object:

    Parent.Event Max_requested()
    

    This statement uses the pronoun Parent, referring to the custom visual user object itself, to trigger the Max_requested event in that user object.

  5. Implement these user events in the Window painter.

StepsTo implement the user event in the window:

  1. Open the window.

  2. In the Window painter, select Insert>Control from the menu bar and place the custom visual user object in the window.

  3. Double-click the user object and then in the Script view, write scripts for the user events you defined in the User Object painter.