scrollable: extract scrolling behavior for reuse

Extract the vertical scrolling ability into its own Scrollable struct
that can be embedded and reused across any ui element that relies on
scrolling.

Signed-off-by: Koni Marti <koni.marti@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>

4 files changed, 101 insertions, 139 deletions

diff --git a/widgets/dirlist.go b/widgets/dirlist.go
index 19b45d4..cb22a61 100644
--- a/widgets/dirlist.go
+++ b/widgets/dirlist.go
@@ -41,6 +41,7 @@ type DirectoryLister interface {
 
 type DirectoryList struct {
 	ui.Invalidatable
+	Scrollable
 	aercConf   *config.AercConfig
 	acctConf   *config.AccountConfig
 	store      *lib.DirStore
@@ -48,7 +49,6 @@ type DirectoryList struct {
 	logger     *log.Logger
 	selecting  string
 	selected   string
-	scroll     int
 	spinner    *Spinner
 	worker     *types.Worker
 	skipSelect chan bool
@@ -261,16 +261,11 @@ func (dirlist *DirectoryList) Draw(ctx *ui.Context) {
 		return
 	}
 
-	dirlist.ensureScroll(ctx.Height())
-
-	needScrollbar := true
-	percentVisible := float64(ctx.Height()) / float64(len(dirlist.dirs))
-	if percentVisible >= 1.0 {
-		needScrollbar = false
-	}
+	dirlist.UpdateScroller(ctx.Height(), len(dirlist.dirs))
+	dirlist.EnsureScroll(findString(dirlist.dirs, dirlist.selecting))
 
 	textWidth := ctx.Width()
-	if needScrollbar {
+	if dirlist.NeedScrollbar() {
 		textWidth -= 1
 	}
 	if textWidth < 0 {
@@ -278,10 +273,10 @@ func (dirlist *DirectoryList) Draw(ctx *ui.Context) {
 	}
 
 	for i, name := range dirlist.dirs {
-		if i < dirlist.scroll {
+		if i < dirlist.Scroll() {
 			continue
 		}
-		row := i - dirlist.scroll
+		row := i - dirlist.Scroll()
 		if row >= ctx.Height() {
 			break
 		}
@@ -299,13 +294,13 @@ func (dirlist *DirectoryList) Draw(ctx *ui.Context) {
 		ctx.Printf(0, row, style, dirString)
 	}
 
-	if needScrollbar {
+	if dirlist.NeedScrollbar() {
 		scrollBarCtx := ctx.Subcontext(ctx.Width()-1, 0, 1, ctx.Height())
-		dirlist.drawScrollbar(scrollBarCtx, percentVisible)
+		dirlist.drawScrollbar(scrollBarCtx)
 	}
 }
 
-func (dirlist *DirectoryList) drawScrollbar(ctx *ui.Context, percentVisible float64) {
+func (dirlist *DirectoryList) drawScrollbar(ctx *ui.Context) {
 	gutterStyle := tcell.StyleDefault
 	pillStyle := tcell.StyleDefault.Reverse(true)
 
@@ -313,44 +308,11 @@ func (dirlist *DirectoryList) drawScrollbar(ctx *ui.Context, percentVisible floa
 	ctx.Fill(0, 0, 1, ctx.Height(), ' ', gutterStyle)
 
 	// pill
-	pillSize := int(math.Ceil(float64(ctx.Height()) * percentVisible))
-	percentScrolled := float64(dirlist.scroll) / float64(len(dirlist.dirs))
-	pillOffset := int(math.Floor(float64(ctx.Height()) * percentScrolled))
+	pillSize := int(math.Ceil(float64(ctx.Height()) * dirlist.PercentVisible()))
+	pillOffset := int(math.Floor(float64(ctx.Height()) * dirlist.PercentScrolled()))
 	ctx.Fill(0, pillOffset, 1, pillSize, ' ', pillStyle)
 }
 
-func (dirlist *DirectoryList) ensureScroll(h int) {
-	selectingIdx := findString(dirlist.dirs, dirlist.selecting)
-	if selectingIdx < 0 {
-		// dir not found, meaning we are currently adding / removing a dir.
-		// we can simply ignore this until we get redrawn with the new
-		// dirlist.dir content
-		return
-	}
-
-	maxScroll := len(dirlist.dirs) - h
-	if maxScroll < 0 {
-		maxScroll = 0
-	}
-
-	if selectingIdx >= dirlist.scroll && selectingIdx < dirlist.scroll+h {
-		if dirlist.scroll > maxScroll {
-			dirlist.scroll = maxScroll
-		}
-		return
-	}
-
-	if selectingIdx >= dirlist.scroll+h {
-		dirlist.scroll = selectingIdx - h + 1
-	} else if selectingIdx < dirlist.scroll {
-		dirlist.scroll = selectingIdx
-	}
-
-	if dirlist.scroll > maxScroll {
-		dirlist.scroll = maxScroll
-	}
-}
-
 func (dirlist *DirectoryList) MouseEvent(localX int, localY int, event tcell.Event) {
 	switch event := event.(type) {
 	case *tcell.EventMouse:
diff --git a/widgets/dirtree.go b/widgets/dirtree.go
index 52195d8..cf4575f 100644
--- a/widgets/dirtree.go
+++ b/widgets/dirtree.go
@@ -40,7 +40,7 @@ func (dt *DirectoryTree) UpdateList(done func([]string)) {
 		dt.buildTree()
 		dt.listIdx = findString(dt.dirs, dt.selecting)
 		dt.Select(dt.selecting)
-		dt.scroll = 0
+		dt.Scrollable = Scrollable{}
 	})
 }
 
@@ -60,7 +60,8 @@ func (dt *DirectoryTree) Draw(ctx *ui.Context) {
 		return
 	}
 
-	dt.ensureScroll(ctx.Height())
+	dt.UpdateScroller(ctx.Height(), n)
+	dt.EnsureScroll(dt.countVisible(dt.list[:dt.listIdx]))
 
 	needScrollbar := true
 	percentVisible := float64(ctx.Height()) / float64(n)
@@ -78,10 +79,10 @@ func (dt *DirectoryTree) Draw(ctx *ui.Context) {
 
 	rowNr := 0
 	for i, node := range dt.list {
-		if i < dt.scroll || !isVisible(node) {
+		if i < dt.Scroll() || !isVisible(node) {
 			continue
 		}
-		row := rowNr - dt.scroll
+		row := rowNr - dt.Scroll()
 		if row >= ctx.Height() {
 			break
 		}
@@ -105,41 +106,9 @@ func (dt *DirectoryTree) Draw(ctx *ui.Context) {
 		ctx.Printf(0, row, style, dirString)
 	}
 
-	if needScrollbar {
+	if dt.NeedScrollbar() {
 		scrollBarCtx := ctx.Subcontext(ctx.Width()-1, 0, 1, ctx.Height())
-		dt.drawScrollbar(scrollBarCtx, percentVisible)
-	}
-}
-
-func (dt *DirectoryTree) ensureScroll(h int) {
-	selectingIdx := dt.countVisible(dt.list[:dt.listIdx])
-	if selectingIdx < 0 {
-		// dir not found, meaning we are currently adding / removing a dir.
-		// we can simply ignore this until we get redrawn with the new
-		// dirlist.dir content
-		return
-	}
-
-	maxScroll := dt.countVisible(dt.list) - h
-	if maxScroll < 0 {
-		maxScroll = 0
-	}
-
-	if selectingIdx >= dt.scroll && selectingIdx < dt.scroll+h {
-		if dt.scroll > maxScroll {
-			dt.scroll = maxScroll
-		}
-		return
-	}
-
-	if selectingIdx >= dt.scroll+h {
-		dt.scroll = selectingIdx - h + 1
-	} else if selectingIdx < dt.scroll {
-		dt.scroll = selectingIdx
-	}
-
-	if dt.scroll > maxScroll {
-		dt.scroll = maxScroll
+		dt.drawScrollbar(scrollBarCtx)
 	}
 }
 
diff --git a/widgets/msglist.go b/widgets/msglist.go
index ae0d211..f4cea70 100644
--- a/widgets/msglist.go
+++ b/widgets/msglist.go
@@ -20,10 +20,10 @@ import (
 
 type MessageList struct {
 	ui.Invalidatable
+	Scrollable
 	conf          *config.AercConfig
 	logger        *log.Logger
 	height        int
-	scroll        int
 	nmsgs         int
 	spinner       *Spinner
 	store         *lib.MessageStore
@@ -70,16 +70,13 @@ func (ml *MessageList) Draw(ctx *ui.Context) {
 		}
 	}
 
-	ml.ensureScroll()
-
-	needScrollbar := true
-	percentVisible := float64(ctx.Height()) / float64(len(store.Uids()))
-	if percentVisible >= 1.0 {
-		needScrollbar = false
+	ml.UpdateScroller(ml.height, len(store.Uids()))
+	if store := ml.Store(); store != nil && len(store.Uids()) > 0 {
+		ml.EnsureScroll(store.SelectedIndex())
 	}
 
 	textWidth := ctx.Width()
-	if needScrollbar {
+	if ml.NeedScrollbar() {
 		textWidth -= 1
 	}
 	if textWidth < 0 {
@@ -105,7 +102,7 @@ func (ml *MessageList) Draw(ctx *ui.Context) {
 					return nil
 				}
 				counter--
-				if counter > len(store.Uids())-1-ml.scroll {
+				if counter > len(store.Uids())-1-ml.Scroll() {
 					//skip messages which are higher than the viewport
 					return nil
 				}
@@ -142,7 +139,7 @@ func (ml *MessageList) Draw(ctx *ui.Context) {
 		}
 	} else {
 		uids := store.Uids()
-		for i := len(uids) - 1 - ml.scroll; i >= 0; i-- {
+		for i := len(uids) - 1 - ml.Scroll(); i >= 0; i-- {
 			uid := uids[i]
 			msg := store.Messages[uid]
 			fmtCtx := format.Ctx{
@@ -159,9 +156,9 @@ func (ml *MessageList) Draw(ctx *ui.Context) {
 		}
 	}
 
-	if needScrollbar {
+	if ml.NeedScrollbar() {
 		scrollbarCtx := ctx.Subcontext(ctx.Width()-1, 0, 1, ctx.Height())
-		ml.drawScrollbar(scrollbarCtx, percentVisible)
+		ml.drawScrollbar(scrollbarCtx)
 	}
 
 	if len(store.Uids()) == 0 {
@@ -241,7 +238,7 @@ func (ml *MessageList) drawRow(textWidth int, ctx *ui.Context, uid uint32, row i
 
 	var style tcell.Style
 	// current row
-	if row == ml.store.SelectedIndex()-ml.scroll {
+	if row == ml.store.SelectedIndex()-ml.Scroll() {
 		style = uiConfig.GetComposedStyleSelected(config.STYLE_MSGLIST_DEFAULT, msg_styles)
 	} else {
 		style = uiConfig.GetComposedStyle(config.STYLE_MSGLIST_DEFAULT, msg_styles)
@@ -265,7 +262,7 @@ func (ml *MessageList) drawRow(textWidth int, ctx *ui.Context, uid uint32, row i
 	return false
 }
 
-func (ml *MessageList) drawScrollbar(ctx *ui.Context, percentVisible float64) {
+func (ml *MessageList) drawScrollbar(ctx *ui.Context) {
 	gutterStyle := tcell.StyleDefault
 	pillStyle := tcell.StyleDefault.Reverse(true)
 
@@ -273,9 +270,8 @@ func (ml *MessageList) drawScrollbar(ctx *ui.Context, percentVisible float64) {
 	ctx.Fill(0, 0, 1, ctx.Height(), ' ', gutterStyle)
 
 	// pill
-	pillSize := int(math.Ceil(float64(ctx.Height()) * percentVisible))
-	percentScrolled := float64(ml.scroll) / float64(len(ml.Store().Uids()))
-	pillOffset := int(math.Floor(float64(ctx.Height()) * percentScrolled))
+	pillSize := int(math.Ceil(float64(ctx.Height()) * ml.PercentVisible()))
+	pillOffset := int(math.Floor(float64(ctx.Height()) * ml.PercentScrolled()))
 	ctx.Fill(0, pillOffset, 1, pillSize, ' ', pillStyle)
 }
 
@@ -328,7 +324,7 @@ func (ml *MessageList) Clicked(x, y int) (int, bool) {
 	if store == nil || ml.nmsgs == 0 || y >= ml.nmsgs {
 		return 0, false
 	}
-	return y + ml.scroll, true
+	return y + ml.Scroll(), true
 }
 
 func (ml *MessageList) Height() int {
@@ -364,7 +360,7 @@ func (ml *MessageList) storeUpdate(store *lib.MessageStore) {
 
 func (ml *MessageList) SetStore(store *lib.MessageStore) {
 	if ml.Store() != store {
-		ml.scroll = 0
+		ml.Scrollable = Scrollable{}
 	}
 	ml.store = store
 	if store != nil {
@@ -402,39 +398,6 @@ func (ml *MessageList) Select(index int) {
 	ml.Invalidate()
 }
 
-func (ml *MessageList) ensureScroll() {
-	store := ml.Store()
-	if store == nil || len(store.Uids()) == 0 {
-		return
-	}
-
-	h := ml.Height()
-
-	maxScroll := len(store.Uids()) - h
-	if maxScroll < 0 {
-		maxScroll = 0
-	}
-
-	selectedIndex := store.SelectedIndex()
-
-	if selectedIndex >= ml.scroll && selectedIndex < ml.scroll+h {
-		if ml.scroll > maxScroll {
-			ml.scroll = maxScroll
-		}
-		return
-	}
-
-	if selectedIndex >= ml.scroll+h {
-		ml.scroll = selectedIndex - h + 1
-	} else if selectedIndex < ml.scroll {
-		ml.scroll = selectedIndex
-	}
-
-	if ml.scroll > maxScroll {
-		ml.scroll = maxScroll
-	}
-}
-
 func (ml *MessageList) drawEmptyMessage(ctx *ui.Context) {
 	uiConfig := ml.aerc.SelectedAccountUiConfig()
 	msg := uiConfig.EmptyMessage
diff --git a/widgets/scrollable.go b/widgets/scrollable.go
new file mode 100644
index 0000000..0e8bb5a
--- /dev/null
+++ b/widgets/scrollable.go
@@ -0,0 +1,68 @@
+package widgets
+
+// Scrollable implements vertical scrolling
+type Scrollable struct {
+	scroll int
+	height int
+	elems  int
+}
+
+func (s *Scrollable) Scroll() int {
+	return s.scroll
+}
+
+func (s *Scrollable) PercentVisible() float64 {
+	if s.elems <= 0 {
+		return 1.0
+	}
+	return float64(s.height) / float64(s.elems)
+}
+
+func (s *Scrollable) PercentScrolled() float64 {
+	if s.elems <= 0 {
+		return 1.0
+	}
+	return float64(s.scroll) / float64(s.elems)
+}
+
+func (s *Scrollable) NeedScrollbar() bool {
+	needScrollbar := true
+	if s.PercentVisible() >= 1.0 {
+		needScrollbar = false
+	}
+	return needScrollbar
+}
+
+func (s *Scrollable) UpdateScroller(height, elems int) {
+	s.height = height
+	s.elems = elems
+}
+
+func (s *Scrollable) EnsureScroll(selectingIdx int) {
+	if selectingIdx < 0 {
+		return
+	}
+
+	maxScroll := s.elems - s.height
+	if maxScroll < 0 {
+		maxScroll = 0
+	}
+
+	if selectingIdx >= s.scroll && selectingIdx < s.scroll+s.height {
+		if s.scroll > maxScroll {
+			s.scroll = maxScroll
+		}
+		return
+	}
+
+	if selectingIdx >= s.scroll+s.height {
+		s.scroll = selectingIdx - s.height + 1
+	} else if selectingIdx < s.scroll {
+		s.scroll = selectingIdx
+	}
+
+	if s.scroll > maxScroll {
+		s.scroll = maxScroll
+	}
+
+}