回答編集履歴
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追記
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@@ -2,4 +2,143 @@
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ファイル名のみとしてListViewに追加。
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サムネイルは別スレッドでキャッシュを作成しつつ、ListViewへの描画はOwnerDrawにてアイテムを描画する時にキャッシュにあればキャッシュから、キャッシュにない場合はその場でサムネイル作成→キャッシュへ追加→描画
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とすれば高速化できないでしょうか。
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ちょっと古い記事ですが[サムネイル画像の生成と表示](https://clown.cube-soft.jp/entry/20101008/1286517159)が参考になるのではないかと思います。
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ちょっと古い記事ですが[サムネイル画像の生成と表示](https://clown.cube-soft.jp/entry/20101008/1286517159)が参考になるのではないかと思います。
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とりあえず動くものを作ってみました。
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言いたいことは伝わるのではないかと思います。
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```VBNET
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Imports System.Threading
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Public Class Form1
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Const thumbnailwidth As Integer = 100
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Const thumbnailheight As Integer = 80
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Private listitem As New List(Of ListViewItem)
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Private thumbnailCache As New Dictionary(Of String, Image)
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Private listlock As New Object
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Private cts As CancellationTokenSource
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Private cachetask As Task
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Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
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Dim imagelist1 As New ImageList
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imagelist1.ImageSize = New Size(thumbnailwidth, thumbnailheight)
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ListView1.LargeImageList = imagelist1
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ListView1.OwnerDraw = True
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End Sub
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Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
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Dim imageDir As String = "C:\Users\user\Pictures" ' 画像ディレクトリ
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Dim jpgFiles As IEnumerable = System.IO.Directory.EnumerateFiles(imageDir, "*.jpg", IO.SearchOption.AllDirectories)
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'サムネイルキャッシュ作成中であればキャンセル
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If cachetask?.Status = TaskStatus.Running Then
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cts.Cancel()
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cachetask.Wait()
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End If
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listitem.Clear()
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thumbnailCache.Clear()
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For Each file As String In jpgFiles
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Dim item = New ListViewItem(file)
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listitem.Add(item)
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Next
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ListView1.VirtualMode = True
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ListView1.VirtualListSize = listitem.Count
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'キャッシュ作成開始
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CreateCache()
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End Sub
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Private Sub ListView1_RetrieveVirtualItem(sender As Object, e As RetrieveVirtualItemEventArgs) Handles ListView1.RetrieveVirtualItem
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If e.Item Is Nothing Then e.Item = listitem(e.ItemIndex)
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End Sub
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Function createThumbnail(filename As String) As Image
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Dim canvas As New Bitmap(thumbnailwidth, thumbnailheight)
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Using original = Bitmap.FromFile(filename)
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Using g As Graphics = Graphics.FromImage(canvas)
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Using WhiteBrush = New SolidBrush(Color.White)
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g.FillRectangle(WhiteBrush, 0, 0, thumbnailwidth, thumbnailheight)
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End Using
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Dim fw As Double = CDbl(thumbnailwidth) / CDbl(original.Width)
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Dim fh As Double = CDbl(thumbnailheight) / CDbl(original.Height)
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Dim scale As Double = Math.Min(fw, fh)
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Dim w2 As Integer = CInt(original.Width * scale)
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Dim h2 As Integer = CInt(original.Height * scale)
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g.DrawImage(original, (thumbnailwidth - w2) \ 2, (thumbnailheight - h2) \ 2, w2, h2)
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End Using
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End Using
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Return canvas
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End Function
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Private Sub CreateCache()
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cts = New CancellationTokenSource
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cachetask = Task.Factory.StartNew(Sub()
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Try
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For Each item In listitem
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If thumbnailCache.ContainsKey(item.Text) = False Then
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Dim thumbnail As Image = createThumbnail(item.Text)
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SyncLock listlock
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If thumbnailCache.ContainsKey(item.Text) = False Then
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thumbnailCache.Add(item.Text, thumbnail)
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End If
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End SyncLock
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Threading.Thread.Sleep(0)
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cts.Token.ThrowIfCancellationRequested()
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End If
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Next
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Catch ex As OperationCanceledException
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Console.WriteLine("キャッシュ作成キャンセル")
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End Try
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End Sub, cts.Token)
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End Sub
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Private Sub ListView1_DrawItem(sender As Object, e As DrawListViewItemEventArgs) Handles ListView1.DrawItem
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Dim filiename As String = e.Item.Text
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Dim thumbnail As Image
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'サムネイルキャッシュに含まれている場合はキャッシュを
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'そうでない場合はサムネイルを作成、キャッシュに追加したのち描画する
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If thumbnailCache.ContainsKey(filiename) Then
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thumbnail = thumbnailCache(filiename)
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Else
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thumbnail = createThumbnail(filiename)
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SyncLock listlock
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If thumbnailCache.ContainsKey(filiename) = False Then
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thumbnailCache.Add(filiename, thumbnail)
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End If
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End SyncLock
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End If
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'アイテムの描画
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Dim imagerect As New Rectangle(New Point(e.Bounds.X + ((e.Bounds.Width - thumbnail.Width) / 2), e.Bounds.Y), New Size(thumbnail.Width, thumbnail.Height))
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e.DrawDefault = False
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e.DrawBackground()
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e.Graphics.DrawImage(thumbnail, imagerect)
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Dim stringFormat = New StringFormat()
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stringFormat.Alignment = StringAlignment.Center
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stringFormat.LineAlignment = StringAlignment.Center
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e.Graphics.DrawString(e.Item.Text, ListView1.Font, Brushes.Black, New RectangleF(e.Bounds.X, e.Bounds.Y + imagerect.Height + 5, e.Bounds.Width, e.Bounds.Height - imagerect.Height - 5), stringFormat)
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e.DrawFocusRectangle()
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End Sub
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End Class
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```
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