C# 實現滑鼠拖曳捲動頁面功能（二）：核心邏輯與 WinAPI 呼叫實作

狀態變數宣告

bool isDragging = false; // 是否按下滑鼠左鍵進入拖曳模式
Point dragStart;         // 拖曳起始點 (螢幕/控制項座標)
Point dragCurrent;       // 拖曳當前點

滑鼠事件處理

MouseDown：進入拖曳模式

private void viewer_MouseDown(object sender, MouseEventArgs e) {
    if (e.Button == MouseButtons.Left) {
        isDragging = true;
        dragStart = e.Location;
    }
}

MouseUp：結束拖曳

private void viewer_MouseUp(object sender, MouseEventArgs e) {
    isDragging = false;
}

MouseMove：核心捲動邏輯

private void viewer_MouseMove(object sender, MouseEventArgs e) {
    if (!isDragging) return;
    dragCurrent = e.Location;

    // 計算位移量 (除以 10 降低靈敏度，負號反向使內容跟隨滑鼠移動)
    int deltaX = -(dragCurrent.X - dragStart.X) / 10;
    int deltaY = -(dragCurrent.Y - dragStart.Y) / 10;

    // 取得捲軸範圍與當前位置
    GetScrollRange(viewer.Handle, 0, out int minH, out int maxH); // SB_HORZ
    GetScrollRange(viewer.Handle, 1, out int minV, out int maxV); // SB_VERT
    int posH = GetScrollPos(viewer.Handle, 0);
    int posV = GetScrollPos(viewer.Handle, 1);

    // 計算新位置並邊界檢查
    int newH = posH + deltaX;
    if (newH >= minH && newH <= maxH) {
        SetScrollPos(viewer.Handle, 0, newH, true);
        PostMessage(viewer.Handle, 0x114, (IntPtr)(4 + 0x10000 * newH), IntPtr.Zero); // WM_HSCROLL + SB_THUMBPOSITION
    }
    int newV = posV + deltaY;
    if (newV >= minV && newV <= maxV) {
        SetScrollPos(viewer.Handle, 1, newV, true);
        PostMessage(viewer.Handle, 0x115, (IntPtr)(4 + 0x10000 * newV), IntPtr.Zero); // WM_VSCROLL
    }
}

關鍵 WinAPI (P/Invoke 宣告)

[DllImport("user32.dll")] static extern int SetScrollPos(IntPtr hWnd, int nBar, int nPos, bool bRedraw);
[DllImport("user32.dll")] static extern int GetScrollPos(IntPtr hWnd, int nBar);
[DllImport("user32.dll")] static extern bool PostMessage(IntPtr hWnd, int Msg, IntPtr wParam, IntPtr lParam);
[DllImport("user32.dll")] static extern bool GetScrollRange(IntPtr hWnd, int nBar, out int lpMinPos, out int lpMaxPos);

const int SB_HORZ = 0, SB_VERT = 1;
const int WM_HSCROLL = 0x114, WM_VSCROLL = 0x115;
const int SB_THUMBPOSITION = 4;

關鍵點：PostMessage 送出捲動訊息才會真正觸發控制項內容重繪；僅呼叫 SetScrollPos 僅移動捲軸滑塊，內容不動。