TAILIEUCHUNG - Javascript bible_ Chapter 33

Tham khảo sách 'javascript bible_ chapter 33', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The Event Object Prior to Navigator 4 user and system actions events were captured predominantly by event handlers defined as attributes inside HTML tags. For instance when a user clicked on a button the click event triggered the onClick event handler in the tag. That handler might invoke a separate function or perform some inline JavaScript script. Even so the events themselves were rather dumb Either an event occurred or it didn t. Where it occurred that is the screen coordinates of the pointer when the mouse button was clicked and other pertinent event tidbits for example whether a keyboard modifier key was pressed at the same time were not part of the equation. Until Navigator 4 that is. While remaining fully backward compatible with the event handler mechanism of old the Navigator 4 event model turns events into first-class objects whose properties automatically carry a lot of relevant information about the event when it occurs. These properties are fully exposed to scripts allowing pages to respond more intelligently about what the user does with the page and its elements. Another new aspect of Navigator 4 s event model is that an event is no longer confined to only the intended object of the event. It is possible to capture events on a more global basis perform some preprocessing if necessary and then dispatch the event either to its original target or some other object under your script control. Why Events Graphical user interfaces are more difficult to program than the old-fashioned command-line interface. With a command-line or menu-driven system users were intentionally restricted in the types of actions they could take at any given moment. The world was very modal primarily as a convenience to programmers who led users through rigid program structures. That all changed in a graphical user interface such as Windows MacOS Xwindows and all others derived from the pioneering work of the Xerox Star system. The challenge for programmers is that a good user .

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