c# - How implement an infinity list in a combobox with int values -

i want implement dynamic combobox contain int values , dynamically load new elements before user reaching end of list.

so example load list initial value of 15 , list contains +- 5 element , when user reach +- 3. element end of list add 5 more element respectively end or beginning.

how can archive this?

to that, need handler scrollchanged event of scrollviwer control inside combobox's controltemplate.

first, edit combobox template: visual studio designer selecte > edit template > edit copy,

in generated style search dropdownscrollviewer , add scrollchanged event handler

xaml

//..  <border x:name="dropdownborder" borderbrush="{dynamicresource {x:static systemcolors.windowframebrushkey}}" borderthickness="1" background="{dynamicresource {x:static systemcolors.windowbrushkey}}">                         <scrollviewer x:name="dropdownscrollviewer" scrollchanged="onscrollchanged">                             <grid x:name="grid" renderoptions.cleartypehint="enabled">                                 <canvas x:name="canvas" horizontalalignment="left" height="0" verticalalignment="top" width="0">                                     <rectangle x:name="opaquerect" fill="{binding background, elementname=dropdownborder}" height="{binding actualheight, elementname=dropdownborder}" width="{binding actualwidth, elementname=dropdownborder}"/>                                 </canvas>                                 <itemspresenter x:name="itemspresenter" keyboardnavigation.directionalnavigation="contained" snapstodevicepixels="{templatebinding snapstodevicepixels}"/>                             </grid>                         </scrollviewer>                     </border>  //.. 

now, let combobox bound observablecollection :

<combobox x:name="cbbox" itemssource="{binding cboxitems}" verticalalignment="center" horizontalalignment="center" style="{dynamicresource comboboxstyle1}"/> 

viewmodel

 private observablecollection<string> _cboxitemscollection = new observablecollection<string>()     {         "0",         "1",         "2",         "3",         "4",         "5",     };             public observablecollection<string> cboxitems     {                 {             return _cboxitemscollection;         }          set         {             if (_cboxitemscollection == value)             {                 return;             }              _cboxitemscollection = value;             onpropertychanged();         }     } 

handler

the handler should that:

private void onscrollchanged(object sender, scrollchangedeventargs e)  {      var scrollviewer = (scrollviewer)sender;      if (scrollviewer.verticaloffset == scrollviewer.scrollableheight)       {          (int = 0; < 5; i++)           {              cboxitems.add((cboxitems.count+i).tostring());           }        }   } 

edit

i forgot mention scroll top, following same principle when verticaloffset of scrollviewer equal 0, insert negative items @ top of collection:

//..         if (scrollviewer.verticaloffset == 0)          {             (int = 0; < 5; i++)             {                 cboxitems.insert(0,"negaive item");             }             scrollviewer.scrolltoverticaloffset(10);          }         //.. 

the scrollviewer.scrolltoverticaloffset(10); prevent infinite loop since default scrollviewer scroll top.