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Figure 1-2 The main event loop, with event sourceĪ Cocoa application is event driven: It fetches an event from the queue, dispatches it to an appropriate object, and, after the event is handled, fetches the next event. The event source consists of a port for receiving events from the window server and a FIFO queue-the event queue-for holding those events until the application can process them, as shown in Figure 1-2. What especially distinguishes the main event loop is an input source called the event source, which is constructed when the global NSApplication object ( NSApp) is initialized. By default, every thread in OS X has its own run loop, and the run loop of the main thread of a Cocoa application is called the main event loop. A run loop, which in Cocoa is an NSRunLoop object, enables a process to receive input from various sources. For a Cocoa application, that mechanism is called the main event loop. These events are generated by reserved keys or key combinations, such as the power and media-eject keys.Įvery application has a mechanism specific to its environment for receiving events from the window server. These events are never routed to a Cocoa application. Note: Lower levels of the system trap and handle some events early in the event stream. As it passes through each subsystem, an event may change structure but it still identifies a specific user action. This event stream proceeds “upward” (in an architectural sense) through the various levels of the system-the hardware to the window server to the Event Manager-until each event reaches its final destination: an application. In OS X, events are delivered as an asynchronous stream.
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ACTION INITIATER WORD CODE
The window server has a translation facility that converts the virtual key code into a Unicode character.
ACTION INITIATER WORD DRIVER
The device driver translates the raw scan code into a virtual key code which it then passes off (along with other information about the key-press) to the window server in an event record. As an example, consider what happens when a user presses a key. Although applications running in the background do not normally receive key and mouse events, low-level mechanisms called event taps make it possible for a background application to receive events and act upon them.īefore it dispatches an event to an application, the window server processes it in various ways it time-stamps it, annotates it with the associated window and process port, and possibly performs other tasks as well. Note: Applications normally receive events from the keyboard and mouse only when they are in the foreground (that is, active). Figure 1-1 depicts this event-delivery system. From there the event is forwarded to the event-handling mechanism appropriate to the application environment. The window server dispatches the event to the appropriate run-loop port of the target process. Through the I/O Kit, the device driver creates a low-level event, puts it in the window server's event queue, and notifies the window server. When the user presses a key or clicks a button or moves a stylus, the device detects the action and initiates a transfer of data to the device driver associated with it.
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A typical event in OS X originates when the user manipulates an input device attached to a computer system, such as a keyboard, mouse, or tablet stylus. How an Event Enters a Cocoa ApplicationĪn event is a low-level record of a user action that is usually routed to the application in which the action occurred.
ACTION INITIATER WORD MAC
This chapter traces the possible paths of events of various types and describes the mechanisms and architectural designs for handling events in the Application Kit.įor further background, About OS X App Design in Mac App Programming Guide is recommended reading. The path taken by an event to the object in a Cocoa application that finally handles it can be a complicated one.