在进行文档编写、教程制作或其他需要截图的场景时，一个方便的屏幕截图工具是非常有用的。本文将介绍如何使用VB编程语言实现一个简单的屏幕截图工具，并展示其API声明和代码实现。
屏幕截图工具可以帮助用户快速捕捉屏幕上的特定区域，并将其保存为图像文件。这种工具在很多场合都非常实用，比如在编写技术文档、制作教学视频或进行软件测试时。
要使用这个工具，首先需要运行相应的代码。在运行代码后，用户需要按下鼠标的主按钮，此时鼠标光标会变成一个“+”号。用户需要将这个“+”号移动到需要截图的区域的左上角，然后按住鼠标的次按钮并拖动出一个矩形区域。释放按钮后，会弹出一个“另存为”对话框，用户可以选择保存截图，或者点击“取消”。无论用户是否选择保存，截图都会被复制到剪贴板中，这样用户就可以将其粘贴到任何打开的文档中。
在VB中，要实现屏幕截图功能，需要声明一些Windows API函数。以下是一些必要的API声明：
Private Declare Function SetCapture Lib "user32" (ByVal hWnd As IntPtr) As Int32
Private Declare Function ReleaseCapture Lib "user32" () As Int32
Private Declare Auto Function CreateDC Lib "gdi32" Alias "CreateDC" _
(ByVal lpDriverName As String, _
ByVal lpDeviceName As String, _
ByVal lpOutput As String, _
ByVal lpInitData As IntPtr) As IntPtr
Private Declare Function SelectObject Lib "gdi32" _
(ByVal dc As IntPtr, ByVal hObject As Int32) As Int32
Private Declare Function GetStockObject Lib "gdi32" _
(ByVal nIndex As Int32) As Int32
Private Declare Function SetROP2 Lib "gdi32" _
(ByVal dc As IntPtr, ByVal nDrawMode As Int32) As Int32
Private Declare Function Rectangle Lib "gdi32" _
(ByVal dc As IntPtr, ByVal x1 As Int32, ByVal y1 As Int32, _
ByVal X2 As Int32, ByVal Y2 As Int32) As Int32
Private Declare Function DeleteDC Lib "gdi32" _
(ByVal dc As IntPtr) As Int32
Private Const NULL_BRUSH As Int32 = 5
Private Const R2_NOT As Int32 = 6
Private Const R2_NOTXORPEN As Int32 = 10
Private Structure POINTAPI
Dim X As Int32
Dim Y As Int32
End Structure
需要注意的是，POINTAPI结构体可能不在.NET框架中，需要用户自行定义。
以下是使用VB绘制矩形的代码示例：
' 获取光标位置
ptNow.X = Cursor.Position.X
ptNow.Y = Cursor.Position.Y
' 绘制矩形
Rectangle(dc, ptAnchor.X, ptAnchor.Y, ptNow.X, ptNow.Y)
' 移除之前的矩形
Rectangle(dc, ptAnchor.X, ptAnchor.Y, ptOld.X, ptOld.Y)
Dim Image As Bitmap
Const SRCCOPY As Integer = &HCC0020
Dim sdlgImage As New SaveFileDialog
' 获取矩形大小
nWidth = Math.Abs(ptAnchor.X - ptNow.X)
nHeight = Math.Abs(ptAnchor.Y - ptNow.Y)
With picImage
' 调整图片框大小
.Width = nWidth
.Height = nHeight
Dim g As Graphics = .CreateGraphics
' 创建合适大小的空图像
Image = New Bitmap(nWidth, nHeight, g)
' 从图像创建新的图形
g = Graphics.FromImage(Image)
' 获取窗口句柄
Dim deviceContext2 As IntPtr = g.GetHdc
' 将屏幕图像复制到Image变量
BitBlt(deviceContext2, 0, 0, nWidth, nHeight, dc, ptAnchor.X, ptAnchor.Y, SRCCOPY)
' 释放资源
g.ReleaseHdc(deviceContext2)
' 将图像放入图片框
.Image = Image
' 这可能是不必要的
.Refresh()
.Visible = True
' 将图像放入剪贴板
Clipboard.SetDataObject(.Image)
End With
Try
With sdlgImage
.FileName = "Image"
.Filter = "Bitmap (*.bmp)|*.bmp|JPEG (*.jpg, *.jpeg)|*.jpg;*.jpeg|GIF (*.gif)|*.gif|TIFF (*.tif, *.tiff)|*.tif;*.tiff|PNG (*.png)|*.png"
.AddExtension = True
.OverwritePrompt = True
.CheckPathExists = True
.ValidateNames = True
.Title = "Save Image"
If .ShowDialog() = DialogResult.OK Then
Dim bmp As New Bitmap(picImage.Image)
Dim fmt As Imaging.ImageFormat
Select Case .FilterIndex
Case 1
fmt = Imaging.ImageFormat.Bmp
Case 2
fmt = Imaging.ImageFormat.Jpeg
Case 3
fmt = Imaging.ImageFormat.Gif
Case 4
fmt = Imaging.ImageFormat.Tiff
Case 5
fmt = Imaging.ImageFormat.Png
Case Else
fmt = Imaging.ImageFormat.Bmp
End Select
bmp.Save(.FileName, fmt)
End If
End With
Catch e As Exception
MessageBox.Show(e.Message, "Saving Image")
End Try