GEOS SDK TechDocs
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4 Pen Input and Ink
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5.1 The Three Hierarchies
As previously mentioned in this chapter, the Input Manager channels input events to particular objects. The objects are chosen based on the current input grabs, the position of the pointer, and the state of the application. Sometimes, though, input events must be sent to a particular object not specified by one of these means. One example is keyboard input: While an object can grab the keyboard exclusive, it should not; instead, it should make itself the
focus
. The object that has the focus receives all keyboard input (
MSG_META_KBD_CHAR
).
Similarly, your application or other objects may need to send messages to the object which is currently active. For example, a Text Sizes menu may need to know which of several text objects currently has an active selection. Then, when the user selects a new text size, the menu can send its message to the proper text object. The object that receives these messages is called the target object.
The menu in this case, however, should not have to explicitly name the destination of its message. The menu should be able to specify that its message should go to the current target object; the UI knows which object is the target, and it redirects the message automatically.
The GEOS UI uses three special hierarchical mechanisms to fulfill these two purposes (specialized input flow and specification of destinations). These hierarchical mechanisms are built into the basic structure of GEOS objects and are therefore universal and consistent for all applications and all objects. The three hierarchies used are the focus , the target , and the model . (There is a fourth, the controller, but it is used only internally by GEOS.) These hierarchies all function in the same manner but are used for different purposes.
GEOS SDK TechDocs
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| 4 Pen Input and Ink
| 5.1 The Three Hierarchies