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Apache's Handler Use
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Thus there is no static link between a Handler and any particular window or model object. However, in order to be useful, a Handler must be able to observe and manipulate both its corresponding window and model objects. In Example 1 through Example 6 , the created UIInfo object has an attribute object whose value is the object sam. Whenever a window event causes a Handler method to be called, TraitsUI passes the corresponding UIInfo object as one of the method arguments. This gives the Handler the information necessary to perform its tasks.
In accordance with the MVC design pattern, Handlers and Views are separate entities belonging to distinct classes.
The Handler - Wikipedia
In order for a custom Handler to provide the control logic for a window, it must be explicitly associated with the View for that window. The TraitsUI package provides three ways to accomplish this:. The result of this technique, as shown in Example 9 , is that the window created by the View object is automatically controlled by the specified handler instance.
It is also possible to associate a custom Handler with a specific window without assigning it permanently to the View. Assigning an instance of Handler to this argument gives that handler instance control only of the specific window being created by the method call. You seldom need to associate a single custom Handler with several different Views or vice versa, although you can in theory and there are cases where it is useful to be able to do so. In most real-life scenarios, a custom Handler is tailored to a particular View with which it is always used.
One way to reflect this usage in the program design is to define the View as part of the Handler. Note that for these versions of the display methods, the context keyword parameter is not optional. Both of these classes are designed to simplify the process of creating an MVC-based application.
The model attribute provides convenient access to the model object associated with either subclass. Normally, the model attribute is set in the constructor when an instance of ModelView or Controller is created. The info attribute provides convenient access to the UIInfo object associated with the active user interface view for the handler object. They also can accept metadata as keyword parameters. The difference between the ModelView and Controller classes lies in the context dictionary that each one passes to its associated user interface, as described in the following sections.
Using a Controller as the handler class assumes that the model object contains most, if not all, of the data to be viewed. If you create a custom Handler subclass, depending on the behavior you want to implement, you might override the standard methods of Handler, or you might create methods that respond to changes to specific trait attributes.
The Handler class provides methods that are automatically executed at certain points in the lifespan of the window controlled by a given Handler. By overriding these methods, you can implement a variety of custom window behaviors.
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The following sequence shows the points at which the Handler methods are called. When Handler methods are called, and when to override them. The setattr method described above is called whenever any trait value is changed in the UI. However, TraitsUI also provides a mechanism for calling methods that are automatically executed whenever the user edits a particular trait.
While you can use static notification handler methods on the HasTraits object, you might want to implement behavior that concerns only the user interface. To differentiate between code that should be executed when the window is first initialized and code that should be executed when the trait actually changes, use the initialized attribute of the UIInfo object i.
Example 9: Using a Handler that reacts to trait changes. Another use of a Handler is to define custom window actions, which can be presented as buttons, menu items, or toolbar buttons. In TraitsUI, window commands are implemented as instances of the Action class. Actions can be used in command button s, menus, and toolbars. Suppose you want to build a window with a custom Recalculate action.
Suppose further that you have defined a subclass of Handler called MyHandler to provide the logic for the window. To create the action:. Add a method to MyHandler that implements the command logic.
The Handler - Lyric Video
This method can have any name e. The simplest way to turn an Action into a window command is to add it to the buttons attribute for the View. It appears in the button area of the window, along with any standard buttons you specify.
Define the handler method and action, as described in Actions. Another way to install an Action such as recalc as a window command is to make it into a menu option.