Apple WebObjects 3.5 Manual de usuario descargar pdf (Pagina 84)

Chapter 3 Creating a WebObjects Database Application

1. Inspect the middle image button.

Its

action attribute is bound to the action method saveChanges.

2. Look in the

Main.java class to see how saveChanges is implemented.

The method (shown below with comments omitted) simply saves any

changes that have been made to

movieDisplayGroup’s objects to the database.

public void saveChanges() throws Exception {

try {

this.session().defaultEditingContext().saveChanges();

}

catch (Exception exception) {

System.err.println("Cannot save changes ");

throw exception;

}

this.session() returns a Session object that represents a connection to the

application by a single user. A Session object provides access to an

EditingContext object. The expression

this.session().defaultEditingContext().saveChanges();

sends a saveChanges message to the Session’s defaultEditingContext. This default

EditingContext object manages graphs of objects fetched from the

database, and all changes to the database are saved through it. For more

information, see the EditingContext class speciﬁcation in the Enterprise

Objects Framework Reference.

An EditingContext’s

saveChanges method uses other Enterprise Objects

Framework objects to analyze its network of enterprise objects (Movie

objects referenced by the application) for changes and then to perform a set

of corresponding operations in the database. If an error occurs during this

process,

saveChanges throws an exception. Main.java’s saveChanges method simply

raises the exception, having the effect of returning a diagnostic page. You

could return an error page that explains the reason for the save failure

instead, but the application in this tutorial uses the default behavior.

3. Inspect the ﬁrst and third image buttons to see what their

action attributes are

bound to.

They are bound to

movieDisplayGroup.insert and movieDisplayGroup.delete,

respectively. The DisplayGroup

insert method creates a new enterprise

object, then inserts it into the display group’s list of objects just past the

current selection. The DisplayGroup

delete method deletes the display

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