Convert dropdown to checkbox in PowerApps

Now here customization of this control begins. Please follow below steps to implement a multi checkbox control:

1. Customize default list form with PowerApps using option described as below:
   
2. You will be able to see default multi select drop-down.
   
3. You need to create a collection to work with new and edit form (new list item and existing list item). Write below code to create collection:

   ClearCollect(collectionHobbies,AddColumns(Choices([@CustomApprovalProcess].Hobbies),"Checked",If(SharePointForm1.Mode=FormMode.New,false,Not(IsBlank(LookUp(RenameColumns(SharePointIntegration.Selected.Hobbies,"Value","SelectedValue"),SelectedValue=Value))))));Once collection is defined then we need to implement our control. Here we are going to use Gallery control with one checkbox control. For this you need to unlock dropdown card from advance setting

   

4. After defining collection, you need to unlock dropdown card from advance setting.
5. Once data card is unlocked then you need to add gallery control in data card and add checkbox to added gallery control. Also, you need to hide your default dropdown control.
   
6. Now you need to modify some properties of this control so it will look like SharePoint control.

   Gallery -> Items -> collectionHobbies 
   Gallery -> WrapCount -> 2 (As per your choice) 
   Gallery -> Height -> 40 * Round(CountRows(Gallery1.AllItems)/2,0) (here we have 40 as checkbox size not height of checkbox and dividing our total gallery count with 2 as we have set this as WrapCount) 
   Gallery -> Checkbox -> Text - > ThisItem.Value 
   Gallery -> Checkbox -> Default - > ThisItem.Checked 
   Gallery -> Checkbox -> OnSelect -> false 

7. After these settings, out control will look like:
   
8. Now you need to implement update part where collection will be updated with selected or unselected values. For this, you need to bind action on OnCheck and OnUncheck operations of checkbox control.

   1. OnCheck
   UpdateIf(collectionHobbies, Value=ThisItem.Value, {Checked: true})OnUnCheck
   2. OnUncheck
   UpdateIf(collectionHobbies, Value=ThisItem.Value, {Checked: false})

   

9. Now you need to bind collection with selected value(Checked) with hidden default dropdown. To verify data, which you have selected, you can unhide hidden dropdown, but need to be hidden before final publishing to SharePoint. To bind selected values, you need to modify below property of dropdow:

   DefaultSelectedItems -> Filter (collectionHobbies, Checked)

   

10. One point to be noted here that SharePoint loads PowerApps screen only one time when your SharePoint pages is loaded. As a result, your collection will be bind with values only one time and will not be updated with different list item. To resolve this issue, we need to add one trick with OnNew, OnEdit and OnView operations which will ensure all statements should be executed every time when user clicks on New/Edit/View.
    

    Navigate (FormScreen1)
    

    

11. To verify this customization, play the app and start selecting some value from checkbox then you will be able to see an update in dropdown. Once verification is completed then you can hide dropdown and publish changes in SharePoint. Now your customized control will be available with SharePoint modern page experience.