move more view model logic into the files view model

pkg/gui/context/traits/list_cursor.go

@@ -0,0 +1,43 @@
+package traits
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/gui/types"
+	"github.com/jesseduffield/lazygit/pkg/utils"
+)
+
+type HasLength interface {
+	GetItemsLength() int
+}
+
+type ListCursor struct {
+	selectedIdx int
+	list        HasLength
+}
+
+func NewListCursor(list HasLength) *ListCursor {
+	return &ListCursor{selectedIdx: 0, list: list}
+}
+
+var _ types.IListCursor = (*ListCursor)(nil)
+
+func (self *ListCursor) GetSelectedLineIdx() int {
+	return self.selectedIdx
+}
+
+func (self *ListCursor) SetSelectedLineIdx(value int) {
+	self.selectedIdx = utils.Clamp(value, 0, self.list.GetItemsLength()-1)
+}
+
+// moves the cursor up or down by the given amount
+func (self *ListCursor) MoveSelectedLine(delta int) {
+	self.SetSelectedLineIdx(self.selectedIdx + delta)
+}
+
+// to be called when the model might have shrunk so that our selection is not not out of bounds
+func (self *ListCursor) RefreshSelectedIdx() {
+	self.SetSelectedLineIdx(self.selectedIdx)
+}
+
+func (self *ListCursor) GetItemsLength() int {
+	return self.list.GetItemsLength()
+}