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Chapter 6

Chapter 6. Event-Driven Programming and Access Events. Chapter Objectives. Learn the advantages and disadvantages of event-driven programming Trigger macros, functions, and event procedures through an Access event Develop a macro that highlights the control that has focus. Chapter Objectives.

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Chapter 6

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  1. Chapter 6 Event-Driven Programming and Access Events

  2. Chapter Objectives • Learn the advantages and disadvantages of event-driven programming • Trigger macros, functions, and event procedures through an Access event • Develop a macro that highlights the control that has focus

  3. Chapter Objectives • Distinguish between the various user actions, data retrieval, and data update events • Determine the most appropriate event to use in a particular situation

  4. Event-Driven Programming and Access Events • Events • Actions recognized by an object • Include button clicks, keystrokes in text boxes, the opening of forms, and record deletions • Event-driven programming • Type of programming where code associated with some action or change in the system’s environment executes when the event occurs

  5. Event-Driven Programming • Event-driven programming • Supports the creation of systems that can react to a wide range of user actions • Does not require the development of management components that anticipate the variety of user actions that can occur • Where system developer identifies important events and associates macros and procedures with them

  6. Event-Driven Programming • Some sources of frustration from event-driven programming include: • Events occurring outside an anticipated sequence • Anticipated events that do not occur • Events occurring at an inappropriate time • Events occurring more than once • Timing problems between events • Difficulty in locating relevant code • Difficulty in understanding how the system executes

  7. The World According to Events • Event property • An object property that associates a procedure or macro with an event that is recognized by the object • If the event occurs, the macro or procedure identified by the event property executes

  8. Overview of Events in Access • An event is associated with a form, report, or control object through an event property Figure 6-1 Using an event property to trigger a macro that runs frmProspects

  9. Overview of Events in Access • VBA procedures are identified by: • A key phrase [Event Procedure] or • An equals sign followed by a function name • Event procedure • A subroutine contained within a form or report class module that is closely associated with an event

  10. Overview of Events in Access Figure 6-2 Form_Current event procedure for frmProspects

  11. Exploring Access Events in Detail • Access recognizes three types of events: • User action events • Occur primarily in the application’s user interface • Data retrieval events • Occur when data are placed in a form or report • Data update events • Occur when updates in a form are propagated to the form’s record source

  12. Events Related to User Actions • Exit event • Occurs for the control that the cursor is leaving • LostFocus event • Occurs for the control in the old record that no longer has the cursor • BeforeUpdate and AfterUpdate events • If changes were made in the previous record, the cursor movement to a new record causes changes to be saved • Events occur for the record being saved

  13. Events Related to User Actions • Current event • Occurs for the form when the new record becomes the one (currently) being manipulated • Enter event • Occurs for the the control to which the cursor is moving • GotFocus event • Occurs for the control that contains the cursor in the new record

  14. Events Related to User Actions Table 6-1 User action events

  15. Events Related to User Actions Table 6-1 User action events (continued)

  16. Events Related to User Actions Table 6-1 User action events (continued)

  17. Events Related to User Actions Table 6-1 User action events (continued)

  18. Focus • Generally speaking, when an Access object becomes the object of attention, it has “received focus” • As the attention moves from one object to another, the object that had focus “loses focus” to the next object that is receiving the focus • Activate, Deactivate, Enter, Exit, GotFocus, and LostFocus events • Related to the process associated with moving from one object to another

  19. Event Timing • Events related to focus demonstrate the importance of understanding event timing Figure 6-3 Order of control focus events

  20. Event Timing • When the cursor is in Control X and the user clicks Control Y or tabs to Control Y, the following events occur: • The Exit event for Control X • The LostFocus event for Control X • The Enter event for Control Y • The GotFocus event for Control Y

  21. Event Timing • Other user action events are mouse clicks and keypresses • DblClick event • Access activates when the user clicks the mouse twice • Rarely used with a command button • Users expect to double-click over a text box, list box, or combo box

  22. Events Related to Data Retrieval • Unlike the user action events, most data retrieval events are not defined by the different ways in which a user interacts with the system • Data retrieval events are identified by their role in placing data from forms and reports • Opening and closing of forms and reports instigate most of the data retrieval events

  23. Events Related to Data Retrieval Table 6-2 Data retrieval events

  24. Events Related to Data Retrieval Table 6-2 Data retrieval events (continued)

  25. Events Related to Data Retrieval • Commonly used data retrieval events: • Open • Occurs when a form is opened but before the first record displays • Load • Occurs after a form is opened, after the first record is read from the database but just prior to the first record being displayed

  26. Events Related to Data Retrieval • Commonly used data retrieval events (continued): • Current • Occurs when focus moves to a new record (making it the current record) and when the form opens

  27. Events Related to Data Retrieval Figure 6-4 Sequence of events caused by opening and closing a form

  28. Data Update Events Table 6-3 Data update events

  29. Data Update Events Table 6-3 Data update events (continued)

  30. Data Update Events Figure 6-5 Sequence of events for inserting and updating records through a bound form

  31. Data Update Events • Current control buffer • Where Access writes user-entered changes to a control • Storage area that contains the displayed value of the control with focus • If code in the BeforeUpdate event procedure does not cancel the event, Access writes the data to thecurrent record bufferand Access triggers an AfterUpdate event

  32. Data Update Events Figure 6-6 Focus changes before and after update events and buffers

  33. Data Update Events • BeforeUpdate event for a control • Occurs when the user finishes adding or changing data in the control • OddValue property of a control • Allows the system to compare the user-typed value with the previous value of the control • AfterUpdate event for a control • Occurs after Access writes changed data in a control to the record buffer

  34. Data Update Events • NonList event for a control • Occurs when a user enters a value in the text box portion of a combo box, but that value is not in the combo box list • BeforeInsert event for a form • Occurs when the user types the first character in a new record, but before the record is actually created

  35. Data Update Events • AfterInsert event for a form • Occurs after changes to the record are completed and written to the database • BeforeUpdate event for a form • Occurs before changed data are written from the current record buffer to the database

  36. Data Update Events • Delete event • Occurs when a user performs any action that causes a record to be deleted • BeforeDelConfirm • Occurs after the user deletes records but before Access displays a dialog box requesting delete confirmation • AfterDelConfirm • Occurs after a user confirms a deletion and the records are actually deleted or the deletions are canceled

  37. Data Update Events • Message box • A window that displays a brief message and then waits for the user to click a button • MsgBox action • Displays a message box • MsgBox function can also be used in VBA code

  38. Time and Error Events • Timer event • Occurs in forms at regular intervals when a specified amount of time has passed • Used to create a splashscreen that appears when you open a database • TimerInterval property • Where the amount of time needed to pass before Timer event will occur, is specified

  39. Time and Error Events • Use the timer event to create a splashscreen that appears when you open a database Figure 6-7 Event properties of frmSplashScreen

  40. Time and Error Events • Error events • An event procedure attached to the OnError event property of a form specifies VBA code to execute when an error is detected on a form • Examples of problems caught by OnError event property include: • Duplicate primary keys • Violations of validation constraints • Violation of foreign key constraints

  41. Chapter Summary • Event-driven programming • An event is triggered by a change in an Access object, a change in the environment surrounding an object, or a user action • Each event is associated with an event property of an object • The value of an event property identifies the macro, function, or event procedure that should execute when the event occurs

  42. Chapter Summary • Access events can be classified into several categories: • Events related to user actions center on interactions with the user interface • Focus events are a type of user action events that occur when the cursor moves from one control to another • Data retrieval events are associated with the display of data in a form or report • Data update events respond to a user-caused changes to the values of some control

  43. Chapter Summary • Event timing and event interaction sometimes make program debugging and maintenance difficult • Macros and VBA procedures should be associated with the objects and events that are most closely aligned with the code’s results and conditions for execution

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