Racket Graphical Interface Toolkit – 5


Un altro post descrittivo, continuando da qui; oggi sono qui: [doc]/gui/windowing-overview.html#(part._new-containers).

Definire nuovi tipi di containers

Although nested horizontal and vertical containers can express most layout patterns, a programmer can define a new type of container with an explicit layout procedure. A programmer defines a new type of container by deriving a class from panel% or pane% and overriding the container-size and place-children methods. The container-size method takes a list of size specifications for each child and returns two values: the minimum width and height of the container. The place-children method takes the container’s size and a list of size specifications for each child, and returns a list of sizes and placements (in parallel to the original list).

An input size specification is a list of four values:

  • the child’s minimum width;
  • the child’s minimum height;
  • the child’s horizontal stretchability (#t means stretchable, #f means not stretchable); and
  • the child’s vertical stretchability.

For place-children, an output position and size specification is a list of four values:

  • the child’s new horizontal position (relative to the parent);
  • the child’s new vertical position;
  • the child’s new actual width;
  • the child’s new actual height.

The widths and heights for both the input and output include the children’s margins. The returned position for each child is automatically incremented to account for the child’s margin in placing the control.

Pronto a passare al prossimo argomento: [doc]/gui/windowing-overview.html#(part._mouseandkey).

Eventi da mouse e tastiera

Whenever the user moves the mouse, clicks or releases a mouse button, or presses a key on the keyboard, an event is generated for some window. The window that receives the event depends on the current state of the graphic display:

  • The receiving window of a mouse event is usually the window under the cursor when the mouse is moved or clicked. If the mouse is over a child window, the child window receives the event rather than its parent.
    When the user clicks in a window, the window “grabs” the mouse, so that all mouse events go to that window until the mouse button is released (regardless of the location of the cursor). As a result, a user can click on a scrollbar thumb and drag it without keeping the cursor strictly inside the scrollbar control.
    A mouse button-release event is normally generated for each mouse button-down event, but a button-release event might get dropped. For example, a modal dialog might appear and take over the mouse. More generally, any kind of mouse event can get dropped in principle, so avoid algorithms that depend on precise mouse-event sequences. For example, a mouse tracking handler should reset the tracking state when it receives an event other than a dragging event.
  • The receiving window of a keyboard event is the window that owns the keyboard focus at the time of the event. Only one window owns the focus at any time, and focus ownership is typically displayed by a window in some manner. For example, a text field control shows focus ownership by displaying a blinking caret.
    Within a top-level window, only certain kinds of subwindows can have the focus, depending on the conventions of the platform. Furthermore, the subwindow that initially owns the focus is platform-specific. A user can moves the focus in various ways, usually by clicking the target window. A program can use the focus method to move the focus to a subwindow or to set the initial focus.
    A 'wheel-up or 'wheel-down event may be sent to a window other than the one with the keyboard focus, depending on how the operating system handles wheel events.
    A key-press event may correspond to either an actual key press or an auto-key repeat. Multiple key-press events without intervening key-release events normally indicate an auto-key. Like any input event, however, key-release events sometimes get dropped (e.g., due to the appearance of a modal dialog).

Controls, such as buttons and list boxes, handle keyboard and mouse events automatically, eventually invoking the callback procedure that was provided when the control was created. A canvas propagates mouse and keyboard events to its on-event and on-char methods, respectively.

A mouse and keyboard event is delivered in a special way to its window. Each ancestor of the receiving window gets a chance to intercept the event through the on-subwindow-event and on-subwindow-char methods. See the method descriptions for more information.

The default on-subwindow-char method for a top-level window intercepts keyboard events to detect menu-shortcut events and focus-navigation events. See on-subwindow-char in frame% and on-subwindow-char in dialog% for details. Certain OS-specific key combinations are captured at a low level, and cannot be overridden. For example, on Windows and Unix, pressing and releasing Alt always moves the keyboard focus to the menu bar. Similarly, Alt-Tab switches to a different application on Windows. (Alt-Space invokes the system menu on Windows, but this shortcut is implemented by on-system-menu-char, which is called by on-subwindow-char in frame% and on-subwindow-char in dialog%.)

Yep, un altro post descrittivo, tutto copiato. E mi sa che continua ancora un po’ 🙄


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