Getting Started
Event-to-action connections cause an action to be performed when a certain
event takes place. The default color of this connection type is
green.
For event-to-action connections, the event is always the source and the
action is always the target. If you try to connect an action to an
event, VisualAge automatically reverses the source and target for you, so the
connection is an event-to-action. A simple example of an
event-to-action connection follows:
- Place a Push Button part within the default Window part in the Composition
Editor.
- Place a second Window part on the free-form surface of the Composition
Editor.
- Connect the clicked event of the Push Button to the
openWidget action of the new window part you added.
This causes a second window to display when the push button is
pressed.
Attributes can also be used to trigger an action and events can be used to
change the value of an attribute. This behavior is made possible
because an attribute typically signals when it is changed.
As a result, you can make the following connections with attributes:
- Connecting an attribute to an action
- When an attribute value has changed, it can be used to cause an action to
be performed. A simple example of an attribute-to-action connection
follows:
- Place a Text part and a Push Button part within the default
Window part in the Composition Editor.
- Double click on the Push Button part to get the properties view.
Set the enabled attribute to false. This means
its default visual appearance is disabled.
- Connect the object attribute of the Text part to the
enable action of Push Button part.
The push button will become enabled as soon as a user types text into the
Text part.
- Connecting an event to an attribute
- An event can be used to set an attribute value. In this case, a
parameter must be used with the connection to supply the attribute
value. A simple example of an event-to-attribute connection
follows:
- Place a Label part within the default Window part in the Composition
Editor and change its object attribute in the property view to the
string This is a window title.
- Connect the openedWidget event of the default window part to
the title attribute of the default window part.
Note: | Yes, connecting an event to an attribute for the same part does make sense in
the right situation.
|
- Now, to provide the parameter for the event-to-attribute connection you
just created, connect the object attribute of the Label part to the
value attribute from the connection pop-up menu.
When the window is opened its title text will be set to This is a
window title. This example is rather contrived, but it conveys
the idea. For more information, see Connection parameters.
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