read-cell-value

Generic Function

Package: common-graphics

Arguments: row column

This generic function may be called to retrieve a value from an application's master data to display in the grid cell defined by the row and column arguments. Typically it is called by a draw-cell method to find the value to draw in a particular cell. If the grid column uses one of the built-in grid-column classes such as editable-text-column-mixin, then the draw-cell method that is supplied with the built-in column class will call read-cell-value internally. Otherwise an application may supply custom draw-cell methods that call read-cell-value. The default read-cell-value method (shown below) assumes that the common grid paradigm is being used where each grid row represents a data object (such as an employee) and each grid column represents an attribute of those objects (such as an employee's department).

;; The default read-cell-value method
(defmethod read-cell-value ((row grid-row)(column grid-column))
  (let ((data-object (data-object row)))
    (and data-object
         (funcall (data-reader column)
                  data-object))))

The default method may be used for cells that fit this paradigm if the application has therefore supplied data-object methods for the grid rows and data-reader methods for the grid columns. For grids that do not fit the object-rows-and-attribute-columns paradigm, the application could either supply a read-cell-value method that reads grid data from the application in a custom way, or else not use read-cell-value at all in its draw-cell methods.

For example, a draw-cell method to display a pixmap representing each employee's department could be written as follows (where the application has supplied a department-pixmap function):

(defmethod draw-cell ((row employee-row) (col department-column)
                      subrow-num subcolumn-num cell-box stream)
  (let* ((department (read-cell-value row column)))
    (copy-to-stream (department-pixmap department)
      stream cell-box)))

The above call to the default read-cell-value method depends on the grid row having a data-object (which would typically be an instance of the "employee" class), and on the grid column having a data-reader that is the name of a function, such as employee-department, that will return the department of that data-object. Alternately, since the above method already knows that the column is a department-column, instead of calling read-cell-value it could instead call (employee-department (data-object row)).

See also write-cell-value and the description of the grid-widget.

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Created 2000.10.5.