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CheckboxList         class
ListCheckbox            &CML_UI_ListCheckbox
Persister               &CML_Data_ManyToManyLinksPersisterForABC
Links                   &CML_Data_ManyToManyLinks
Construct               procedure
Destruct                procedure
DisplayCheckboxData     procedure
Init                    procedure
LoadEnrollmentData      procedure
SaveEnrollmentData      procedure
                    end

And here are the methods, declared at the end of the procedure: 

CheckboxList.Construct                           procedure
    code
    self.ListCheckbox &= new CML_UI_ListCheckbox
    self.Persister    &= new CML_Data_ManyToManyLinksPersisterForABC
    self.Links        &= new CML_Data_ManyToManyLinks
    
CheckboxList.Destruct                            procedure
    code
    dispose(self.ListCheckbox)
    dispose(self.Persister)
    dispose(self.Links)
    
CheckboxList.DisplayCheckboxData                 procedure
    code
    self.ListCheckbox.LoadDisplayableCheckboxData()      
    
CheckboxList.Init                                procedure
    code
    self.Persister.Init(Access:Enrollment,ENR:kStudentIDCourseInstanceID,|
        ENR:StudentID,ENR:CourseInstanceID)
    self.Links.SetPersister(self.Persister)
    self.ListCheckbox.Initialize(Queue:Browse,Queue:Browse.InClass_Icon,|
        Queue:Browse.CLA:CourseInstanceID,?List:Enrollment,,self.Links)
    
CheckboxList.LoadEnrollmentData                  procedure
    code
    log.write('CheckboxList.LoadEnrollmentData')
    self.SaveEnrollmentData()
    StudentBrowse.UpdateBuffer()
    self.links.LeftRecordID = STU:StudentID
    self.Links.LoadAllLinkingData()
    self.ListCheckbox.LoadDisplayableCheckboxData()    
    display(?List:Enrollment)
    
CheckboxList.SaveEnrollmentData                  procedure
    code
    self.Links.SaveAllLinkingData()

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To be safe, call either MyBrowse.UpdateBuffer (which fetches the highlighted record from the queue and updated the corresponding fields in the record), or MyBrowse.UpdateViewRecord (which additionally fetches the record from the database). Avoid low level code like GET(Queue:Browse:1,CHOICE(?Browse:1)).Also 

Alternatively, I often want the user's "click" to fire some code, even if the record hasn't changed. That's why I'll often use the TakeAccepted handler for the control event, rather than (or in addition to) the browse class' TakeNewSelection method.

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I thought I was all set, but my procedure still had some serious problemsissues. I discovered that although my linking records were saved to the database, the display was more than a little funky especially when I tried scrolling through the page loaded list of courses. 

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This time the problem was with the scroll bar buttons. When you click a scroll bar button the list box doesn't fire a TakeNewSelection event until you release the mouse. Meanwhile any new list box items that scroll up don't have their list boxes drawn:

The first solution I settled on tried was to add one line of code to the end of the CheckboxList.DisplayCheckboxData method:

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That partially solved the problem. While the mouse was held down the icons were still blank, but as soon as I released the mouse the icons were drawn. I also tried a Display() on the list but that caused the entire control to flicker. I think I'll need to take control of the icon value in SetQueueRecord.Although the code still has a flaw I haven't fixed to my satisfaction, I did find it  

I fixed the problem by adding a method call to the SetQueueRecord method, which is where you normally insert code to modify the displayed values.

Image Added

Here's the code for SetCheckboxIcon:

CheckboxList.SetCheckboxIcon                    procedure
    code
    if self.links.IsLinkBetween(self.Links.LeftRecordID,self.ListCheckbox.ListQRightRecordID)
        self.ListCheckbox.ListQIconField = CML_UI_ListCheckbox_TrueValue
    else
        self.ListCheckbox.ListQIconField = CML_UI_ListCheckbox_FalseValue
    end

As all of the values specified here are already being tracked by the classes I think I can move the code down a level rather than have to write it out every time I use the classes. I'll look into that next time.

A nice side effect

The checkbox list has a benefit I hadn't really thought through beforehand. Ordinarily if you have a child list box, any time you change the parent list box selection the child list box reloads completely. But because the list of linking records is managed internally by the class, and then applied to the right hand list box in whatever state it happens to be, you can go to whatever page you want on the right hand list box, then select any record on the left hand list box and the right hand list box will still display the same records. Only the checkboxes will change based on the underlying data.

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