The framework calls this member function by the framework for the owner of an owner-draw button, combo box, list box, or menu item when the control is created.
Override this member function and fill in the MEASUREITEMSTRUCT data structure pointed to by lpMeasureItemStruct and return; this informs Windows of the dimensions of the control and allows Windows to process user interaction with the control correctly.
If a list box or combo box is created with the LBS_OWNERDRAWVARIABLE or CBS_OWNERDRAWVARIABLE style, the framework calls this function for the owner for each item in the control; otherwise this function is called once.
Windows initiates the call to OnMeasureItem for the owner of combo boxes and list boxes created with the OWNERDRAWFIXED style before sending the WM_INITDIALOG message. As a result, when the owner receives this call, Windows has not yet determined the height and width of the font used in the control; function calls and calculations that require these values should occur in the main function of the application or library.
If the item being measured is a CMenu, CListBox or CComboBox object, then the MeasureItem virtual function of the appropriate class is called. Override the MeasureItem member function of the appropriate control's class to calculate and set the size of each item.
OnMeasureItem will be called only if the control's class is created at run time, or it is created with the LBS_OWNERDRAWVARIABLE or CBS_OWNERDRAWVARIABLE style. If the control is created by the dialog editor, OnMeasureItem will not be called. This is because the WM_MEASUREITEM message is sent early in the creation process of the control. If you subclass by using DDX_Control, SubclassDlgItem, or SubclassWindow, the subclassing usually occurs after the creation process. Therefore, there is no way to handle the WM_MEASUREITEM message in the control's OnChildNotify function, which is the mechanism MFC uses to implement ON_WM_MEASUREITEM_REFLECT.
This member function is called by the framework to allow your application to handle a Windows message. The parameters passed to your function reflect the parameters received by the framework when the message was received. If you call the base-class implementation of this function, that implementation will use the parameters originally passed with the message and not the parameters you supply to the function.