我是一个VB的初学者，在看一个例程的时候，这个例程在“VB6实用程序100例”的第6章“06制作屏幕放大镜”中，调用了两个API函数：
SetWindowPos hWnd, hwnd_topmost, 0, 0, 0, 0, Flags
StretchBlt hdc, 0, 0, 200, 200, GetDC(0), sx, sy, 100, 100, Srccopy
其中的参数hWnd, 和 hdc 在程序中都找不到定义或者说明的地方，为什么？请各位知道的说明一二，我知道这个问题不复杂，但是我也不想花时间去查找原因了。在此先感谢下。
下面是它的源代码：
Option ExplicitPrivate Type POINTAPI
x As Long
y As Long
End TypeConst Srccopy = &HCC0020
Const Swp_nomove = &H2
Const Swp_nosize = &H1
Const Flags = Swp_nomove Or Swp_nosize
Const hwnd_topmost = -1Private Declare Function SetWindowPos Lib "user32" (ByVal hWnd As Long, ByVal hWndInsertAfter As Long, ByVal x As Long, ByVal y As Long, ByVal cx As Long, ByVal cy As Long, ByVal wFlags As Long) As Long
Private Declare Function GetCursorPos Lib "user32" (lpPoint As POINTAPI) As Long
Private Declare Function GetDC Lib "user32" (ByVal hWnd As Long) As Long
Private Declare Function StretchBlt Lib "gdi32" (ByVal hdc As Long, ByVal x As Long, ByVal y As Long, ByVal nWidth As Long, ByVal nHeight As Long, ByVal hSrcDC As Long, ByVal xSrc As Long, ByVal ySrc As Long, ByVal nSrcWidth As Long, ByVal nSrcHeight As Long, ByVal dwRop As Long) As Long
Dim pos As POINTAPI
Private Declare Function CreateEllipticRgn Lib "gdi32" (ByVal X1 As Long, ByVal Y1 As Long, ByVal X2 As Long, ByVal Y2 As Long) As Long
Private Declare Function SetWindowRgn Lib "user32" (ByVal hWnd As Long, ByVal hRgn As Long, ByVal bRedraw As Long) As Long
Private Sub Form_Load()
SetWindowPos hWnd, hwnd_topmost, 0, 0, 0, 0, Flags
Dim hr&, dl&
Dim usew&, useh&
usew& = Me.Width / Screen.TwipsPerPixelX
useh& = Me.Height / Screen.TwipsPerPixelY
hr& = CreateEllipticRgn(0, 0, usew, useh)
dl& = SetWindowRgn(Me.hWnd, hr, True)
End Sub
Private Sub start()
Dim sx As Integer
Dim sy As Integer
GetCursorPos pos
sx = IIf(pos.x < 50 Or pos.x > 590, IIf(pos.x < 50, 0, 540), pos.x - 50)
sy = IIf(pos.y < 50 Or pos.y > 430, IIf(pos.y < 50, 0, 380), pos.y - 50)
Caption = " 坐标" & sx & "," & sy & " 放大镜"
StretchBlt hdc, 0, 0, 200, 200, GetDC(0), sx, sy, 100, 100, Srccopy
End SubPrivate Sub Timer1_Timer()
start
End SubPrivate Sub Form_DblClick()
Unload Me
End Sub
SetWindowPos hWnd, hwnd_topmost, 0, 0, 0, 0, Flags
StretchBlt hdc, 0, 0, 200, 200, GetDC(0), sx, sy, 100, 100, Srccopy
其中的参数hWnd, 和 hdc 在程序中都找不到定义或者说明的地方，为什么？请各位知道的说明一二，我知道这个问题不复杂，但是我也不想花时间去查找原因了。在此先感谢下。
下面是它的源代码：
Option ExplicitPrivate Type POINTAPI
x As Long
y As Long
End TypeConst Srccopy = &HCC0020
Const Swp_nomove = &H2
Const Swp_nosize = &H1
Const Flags = Swp_nomove Or Swp_nosize
Const hwnd_topmost = -1Private Declare Function SetWindowPos Lib "user32" (ByVal hWnd As Long, ByVal hWndInsertAfter As Long, ByVal x As Long, ByVal y As Long, ByVal cx As Long, ByVal cy As Long, ByVal wFlags As Long) As Long
Private Declare Function GetCursorPos Lib "user32" (lpPoint As POINTAPI) As Long
Private Declare Function GetDC Lib "user32" (ByVal hWnd As Long) As Long
Private Declare Function StretchBlt Lib "gdi32" (ByVal hdc As Long, ByVal x As Long, ByVal y As Long, ByVal nWidth As Long, ByVal nHeight As Long, ByVal hSrcDC As Long, ByVal xSrc As Long, ByVal ySrc As Long, ByVal nSrcWidth As Long, ByVal nSrcHeight As Long, ByVal dwRop As Long) As Long
Dim pos As POINTAPI
Private Declare Function CreateEllipticRgn Lib "gdi32" (ByVal X1 As Long, ByVal Y1 As Long, ByVal X2 As Long, ByVal Y2 As Long) As Long
Private Declare Function SetWindowRgn Lib "user32" (ByVal hWnd As Long, ByVal hRgn As Long, ByVal bRedraw As Long) As Long
Private Sub Form_Load()
SetWindowPos hWnd, hwnd_topmost, 0, 0, 0, 0, Flags
Dim hr&, dl&
Dim usew&, useh&
usew& = Me.Width / Screen.TwipsPerPixelX
useh& = Me.Height / Screen.TwipsPerPixelY
hr& = CreateEllipticRgn(0, 0, usew, useh)
dl& = SetWindowRgn(Me.hWnd, hr, True)
End Sub
Private Sub start()
Dim sx As Integer
Dim sy As Integer
GetCursorPos pos
sx = IIf(pos.x < 50 Or pos.x > 590, IIf(pos.x < 50, 0, 540), pos.x - 50)
sy = IIf(pos.y < 50 Or pos.y > 430, IIf(pos.y < 50, 0, 380), pos.y - 50)
Caption = " 坐标" & sx & "," & sy & " 放大镜"
StretchBlt hdc, 0, 0, 200, 200, GetDC(0), sx, sy, 100, 100, Srccopy
End SubPrivate Sub Timer1_Timer()
start
End SubPrivate Sub Form_DblClick()
Unload Me
End Sub
我也懒得打太多字儿，就到这儿吧
Declare Function SetWindowPos Lib "user32" Alias "SetWindowPos" (ByVal hwnd As Long, ByVal hWndInsertAfter As Long, ByVal x As Long, ByVal y As Long, ByVal cx As Long, ByVal cy As Long, ByVal wFlags As Long) As Long
说明
这个函数能为窗口指定一个新位置和状态。它也可改变窗口在内部窗口列表中的位置。该函数与DeferWindowPos函数相似，只是它的作用是立即表现出来的（在vb里使用：针对vb窗体，如它们在win32下屏蔽或最小化，则需重设最顶部状态。如有必要，请用一个子类处理模块来重设最顶部状态
返回值
Long，非零表示成功，零表示失败。会设置GetLastError
参数表
参数 类型及说明
hwnd Long，欲定位的窗口
hWndInsertAfter Long，窗口句柄。在窗口列表中，窗口hwnd会置于这个窗口句柄的后面。也可能选用下述值之一：
HWND_BOTTOM 将窗口置于窗口列表底部
HWND_TOP 将窗口置于Z序列的顶部；Z序列代表在分级结构中，窗口针对一个给定级别的窗口显示的顺序
HWND_TOPMOST 将窗口置于列表顶部，并位于任何最顶部窗口的前面
HWND_NOTOPMOST 将窗口置于列表顶部，并位于任何最顶部窗口的后面
x Long，窗口新的x坐标。如hwnd是一个子窗口，则x用父窗口的客户区坐标表示
y Long，窗口新的y坐标。如hwnd是一个子窗口，则y用父窗口的客户区坐标表示
cx Long，指定新的窗口宽度
cy Long，指定新的窗口高度
wFlags Long，包含了旗标的一个整数
SWP_DRAWFRAME 围绕窗口画一个框
SWP_HIDEWINDOW 隐藏窗口
SWP_NOACTIVATE 不激活窗口
SWP_NOMOVE 保持当前位置（x和y设定将被忽略）
SWP_NOREDRAW 窗口不自动重画
SWP_NOSIZE 保持当前大小（cx和cy会被忽略）
SWP_NOZORDER 保持窗口在列表的当前位置（hWndInsertAfter将被忽略）
SWP_SHOWWINDOW 显示窗口
SWP_FRAMECHANGED 强迫一条WM_NCCALCSIZE消息进入窗口，即使窗口的大小没有改变
注解
窗口成为最顶级窗口后，它下属的所有窗口也会进入最顶级。一旦将其设为非最顶级，则它的所有下属和物主窗口也会转为非最顶级。Z序列用垂直于屏幕的一根假想Z轴量化这种从顶部到底部排列的窗口顺序
Declare Function StretchBlt Lib "gdi32" Alias "StretchBlt" (ByVal hdc As Long, ByVal x As Long, ByVal y As Long, ByVal nWidth As Long, ByVal nHeight As Long, ByVal hSrcDC As Long, ByVal xSrc As Long, ByVal ySrc As Long, ByVal nSrcWidth As Long, ByVal nSrcHeight As Long, ByVal dwRop As Long) As Long
说明
将一幅位图从一个设备场景复制到另一个。源和目标DC相互间必须兼容。这个函数会在设备场景中定义一个目标矩形，并在位图中定义一个源图象。源矩形会根据需要进行伸缩，以便与目标矩形的大小相符
返回值
Long，非零表示成功，零表示失败。会设置GetLastError
参数表
参数 类型及说明
hdc Long，目标设备场景
x,y Long，目标矩形左上角的x,y坐标，以逻辑坐标表示
nWidth,nHeight Long，目标矩形的宽度和高度，以逻辑坐标表示
hSrcDC Long，源设备场景。如光栅运算未指定一个源，则这个参数应为零
xSrc,ySrc Long，用源DC的逻辑坐标表示的源矩形左上角位置
nSrcWidth,nSrcHeight Long，分别指定用逻辑单位（以源DC为基础）传输的一幅图象的宽度和高度。如其中有一个参数的符号（指正负号）与对应的目标参数不符，位图就会在对应的轴上作镜像转换处理
dwRop Long，传输过程中进行的光栅运算。如刷子属于光栅运算的一部分，就使用选入目标DC的刷子
注解
可用GetDeviceCaps函数判断特定的设备场景是否支持此函数
不可选择对源位图进行剪切或旋转处理，源位图也不能是一个图元文件设备场景
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