chap8_3.htm

着重介绍基于C++的web编程技术

    <p>{</p>
    <p>CPen penStroke;</p>
    <p>if (!penStroke.CreatePen(PS_SOLID, m_nPenWidth, RGB(0,0,0)))</p>
    <p>return FALSE;</p>
    <p>CPen* pOldPen = pDC-&gt;SelectObject(&amp;penStroke);</p>
    <p>pDC-&gt;MoveTo(m_pointArray[0]);</p>
    <p>for (int i=1; i &lt; m_pointArray.GetSize(); i++)</p>
    <p>{</p>
    <p>pDC-&gt;LineTo(m_pointArray[i]);</p>
    <p>}</p>
    <p>pDC-&gt;SelectObject(pOldPen);</p>
    <p>return TRUE;</p>
    <p>}</p>
    <p></font><font SIZE="3"><b>　</p>
    <p></b><font color="#3973DE">鼠标绘图</font></font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">鼠标绘图基本过程是：用户按下鼠标左键时开始绘图，在鼠标左键按下且移动过程中不断画线跟踪鼠标位置，当松开鼠标左键结束绘图。因此，需要处理三个消息：</font><font FACE="Times New Roman" SIZE="3">WM_LBUTTONDOWN</font><font SIZE="3">、</font><font FACE="Times New Roman" SIZE="3">WM_MOUSEMOVE</font><font SIZE="3">、</font><font FACE="Times New Roman" SIZE="3">WM_LBUTTONUP</font><font SIZE="3">。用</font><font FACE="Times New Roman" SIZE="3">ClassWizard</font><font SIZE="3">为上述三个消息生成消息处理函数，并在其中手工加入代码，修改后的成员函数如下：</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><b><font SIZE="3">　</p>
    <p ALIGN="JUSTIFY">清单</font><font FACE="Times New Roman" SIZE="3">8.8 </font><font SIZE="3">鼠标消息处理函数</font><font FACE="Times New Roman" SIZE="3">OnLButtonDown()</b></p>
    <p>void CDrawView::OnLButtonDown(UINT nFlags, CPoint point) </p>
    <p>{</p>
    <p>// TODO: Add your message handler code here and/or call default</p>
    <p>// Pressing the mouse button in the view window starts a new stroke</p>
    <p>// CScrollView changes the viewport origin and mapping mode.</p>
    <p>// It's necessary to convert the point from device coordinates</p>
    <p>// to logical coordinates, such as are stored in the document.</p>
    <p>CClientDC dc(this);</p>
    <p>OnPrepareDC(&amp;dc);</p>
    <p>dc.DPtoLP(&amp;point);</p>
    <p>m_pStrokeCur = GetDocument()-&gt;NewStroke();</p>
    <p>// Add first point to the new stroke</p>
    <p>m_pStrokeCur-&gt;m_pointArray.Add(point);</p>
    <p>SetCapture(); // Capture the mouse until button up.</p>
    <p>m_ptPrev = point; // Serves as the MoveTo() anchor point for the</p>
    <p>// LineTo() the next point, as the user drags the</p>
    <p>// mouse.</p>
    <p>return;</p>
    <p>}</p>
    <p></font><font SIZE="3">　</font><font FACE="Times New Roman" SIZE="3"></p>
    <p></font><font SIZE="3">在鼠标左键按下，首先获得鼠标按下的位置坐标。由于它是设备坐标，因此先用</font><font FACE="Times New Roman" SIZE="3">DPToLP</font><font SIZE="3">将它转换为逻辑坐标。在此之前，要用</font><font FACE="Times New Roman" SIZE="3">OnPrepareDC()</font><font SIZE="3">对视图坐标原点进行调整。然后用</font><font FACE="Times New Roman" SIZE="3">CDrawDoc</font><font SIZE="3">的</font><font FACE="Times New Roman" SIZE="3">NewStroke()</font><font SIZE="3">成员函数创建一个笔划对象，并将笔划对象加入到笔划链表中。然后，将当前点坐标加入道笔划对象内部的点数组中。以后，当鼠标移动时，</font><font FACE="Times New Roman" SIZE="3">OnMouseMove</font><font SIZE="3">就不断修改该笔划对象的内部数据成员（加入新的点到笔划对象的数组中）。另外，为了用</font><font FACE="Times New Roman" SIZE="3">LineTo</font><font SIZE="3">画出线条，需要将当前鼠标位置保存到</font><font FACE="Times New Roman" SIZE="3">m_ptPrev</font><font SIZE="3">中，以便出现一个新的点时，画一条从</font><font FACE="Times New Roman" SIZE="3">m_ptPrev</font><font SIZE="3">到新的点的直线。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p></font><font SIZE="3">但是，由于用户的鼠标可以在屏幕上任意移动。当鼠标移出窗口外时，窗口无法收到鼠标消息。此时，如果松开了鼠标左键，应用程序由于无法接受到该条消息而不会终止当前笔划，这样就造成了错误。如何避免这种情况发生呢？解决的办法是要让窗口在鼠标移出窗口外时仍然能接受到鼠标消息。幸好，</font><font FACE="Times New Roman" SIZE="3">Windows</font><font SIZE="3">提供了一个</font><font FACE="Times New Roman" SIZE="3">API</font><font SIZE="3">函数</font><font FACE="Times New Roman" SIZE="3">SetCapture()</font><font SIZE="3">解决了这一问题。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p>CWnd::SetCapture()</font><font SIZE="3">用于捕获鼠标：无论鼠标光标位置在何处，都会将鼠标消息送给调用它的那一个窗口。在用完后，需要用</font><font FACE="Times New Roman" SIZE="3">ReleaseCapture()</font><font SIZE="3">释放窗口对鼠标的控制，否则其他窗口将无法接收到鼠标消息。这一工作当然最好在鼠标左键松开</font><font FACE="Times New Roman" SIZE="3">OnLButtonUp()</font><font SIZE="3">时来做。<b></p>
    <p>清单</font><font FACE="Times New Roman" SIZE="3">8.9 OnLButtonUp</font><font SIZE="3">消息处理函数</font></b><font FACE="Times New Roman" SIZE="3"></p>
    <p>void CDrawView::OnLButtonUp(UINT nFlags, CPoint point) </p>
    <p>{</p>
    <p>// TODO: Add your message handler code here and/or call default</p>
    <p></font><font SIZE="3">　</font><font FACE="Times New Roman" SIZE="3"></p>
    <p>// Mouse button up is interesting in the draw application</p>
    <p>// only if the user is currently drawing a new stroke by dragging</p>
    <p>// the captured mouse.</p>
    <p>if (GetCapture() != this)</p>
    <p>return; // If this window (view) didn't capture the mouse,</p>
    <p>// then the user isn't drawing in this window.</p>
    <p>CDrawDoc* pDoc = GetDocument();</p>
    <p>CClientDC dc(this);</p>
    <p>// CScrollView changes the viewport origin and mapping mode.</p>
    <p>// It's necessary to convert the point from device coordinates</p>
    <p>// to logical coordinates, such as are stored in the document.</p>
    <p>OnPrepareDC(&amp;dc); // set up mapping mode and viewport origin</p>
    <p>dc.DPtoLP(&amp;point);</p>
    <p>CPen* pOldPen = dc.SelectObject(pDoc-&gt;GetCurrentPen());</p>
    <p>dc.MoveTo(m_ptPrev);</p>
    <p>dc.LineTo(point);</p>
    <p>dc.SelectObject(pOldPen);</p>
    <p>m_pStrokeCur-&gt;m_pointArray.Add(point);</p>
    <p>// Tell the stroke item that we're done adding points to it.</p>
    <p>// This is so it can finish computing its bounding rectangle.</p>
    <p>m_pStrokeCur-&gt;FinishStroke();</p>
    <p>// Tell the other views that this stroke has been added</p>
    <p>// so that they can invalidate this stroke's area in their</p>
    <p>// client area.</p>
    <p>pDoc-&gt;UpdateAllViews(this, 0L, m_pStrokeCur);</p>
    <p>ReleaseCapture(); // Release the mouse capture established at</p>
    <p>// the beginning of the mouse drag.</p>
    <p>return;</p>
    <p>}</p>
    <p></font><font SIZE="3">　</font><font FACE="Times New Roman" SIZE="3"></p>
    <p>OnLButtonUp</font><font SIZE="3">首先检查鼠标是否被当前窗口所捕获，如果不是则返回。然后画出笔划最后两点之间的极短的直线段。接着，调用</font><font FACE="Times New Roman" SIZE="3">CStroke::FinishStroke()</font><font SIZE="3">，请求</font><font FACE="Times New Roman" SIZE="3">CStroke</font><font SIZE="3">对象计算它的最小矩形。然后调用</font><font FACE="Times New Roman" SIZE="3">pDoc-&gt;UpdateAllViews(this, 0L, m_pStrokeCur)</font><font SIZE="3">通知其他视图更新显示。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p></font><font SIZE="3">当一个视图修改了文档内容并更新显示时，一般的其它的对应于同一文档的视图也需要相应更新，这通过调用文档的成员函数</font><font FACE="Times New Roman" SIZE="3">UpdateAllViews</font><font SIZE="3">完成。</font><font FACE="Times New Roman" SIZE="3"></p>
    <blockquote>
      <blockquote>
        <p>void UpdateAllViews( CView* pSender, LPARAM lHint = 0L, CObject* pHint = </p>
        <p>NULL );</p>
        <b>
      </blockquote>
    </blockquote>
    <p></b>UpdateAllViews</font><font SIZE="3">带三个参数：</font><font FACE="Times New Roman" SIZE="3">pSender</font><font SIZE="3">指向修改文档的视图。由于该视图已经作了更新，所以不再需要更新。比如，在上面的例子中，</font><font FACE="Times New Roman" SIZE="3">OnLButtonUp</font><font SIZE="3">已经绘制了视图，因此不需要再次更新。如果为</font><font FACE="Times New Roman" SIZE="3">NULL</font><font SIZE="3">，则文档对应的所有视图都被更新。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p>lHint</font><font SIZE="3">和</font><font FACE="Times New Roman" SIZE="3">pHint</font><font SIZE="3">包含了更新视图时所需的附加信息。在本例中，其他视图只需要重画当前绘制中的笔划，因此</font><font FACE="Times New Roman" SIZE="3">OnLButtonUp</font><font SIZE="3">把当前笔划指针传给</font><font FACE="Times New Roman" SIZE="3">UpdateAllViews</font><font SIZE="3">函数。该函数调用文档所对应的除</font><font FACE="Times New Roman" SIZE="3">pSender</font><font SIZE="3">外的所有视图的</font><font FACE="Times New Roman" SIZE="3">OnUpdate</font><font SIZE="3">函数，并将</font><font FACE="Times New Roman" SIZE="3">lHint</font><font SIZE="3">和</font><font FACE="Times New Roman" SIZE="3">pHint</font><font SIZE="3">传给</font><font FACE="Times New Roman" SIZE="3">OnUpdate</font><font SIZE="3">函数通知更新附加信息。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p>OnLButtonUp</font><font SIZE="3">最后释放对鼠标的控制，这样别的应用程序窗口就可以获得鼠标消息了。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p></font><font SIZE="3">结合上面讲到的知识，读者不难自行理解下面的</font><font FACE="Times New Roman" SIZE="3">OnMouseMove</font><font SIZE="3">函数。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p>void CDrawView::OnMouseMove(UINT nFlags, CPoint point) </p>
    <p>{</p>
    <p>// TODO: Add your message handler code here and/or call default</p>
    <p>// Mouse movement is interesting in the Scribble application</p>
    <p>// only if the user is currently drawing a new stroke by dragging</p>
    <p>// the captured mouse.</p>
    <p>if (GetCapture() != this)</p>
    <p>return; // If this window (view) didn't capture the mouse,</p>
    <p>// then the user isn't drawing in this window.</p>
    <p>CClientDC dc(this);</p>
    <p>// CScrollView changes the viewport origin and mapping mode.</p>
    <p>// It's necessary to convert the point from device coordinates</p>
    <p>// to logical coordinates, such as are stored in the document.</p>
    <p>OnPrepareDC(&amp;dc);</p>
    <p>dc.DPtoLP(&amp;point);</p>
    <p>m_pStrokeCur-&gt;m_pointArray.Add(point);</p>
    <p>// Draw a line from the previous detected point in the mouse</p>
    <p>// drag to the current point.</p>
    <p>CPen* pOldPen = dc.SelectObject(GetDocument()-&gt;GetCurrentPen());</p>
    <p>dc.MoveTo(m_ptPrev);</p>
    <p>dc.LineTo(point);</p>
    <p>dc.SelectObject(pOldPen);</p>
    <p>m_ptPrev = point;</p>
    <p>return;</p>
    <p>}</p>
    <p></font><font SIZE="3">至此，绘图程序的文档、视图全部设计完了，现在编译运行程序。程序启动后，在空白窗口中徒手绘图，如图</font><font FACE="Times New Roman" SIZE="3">8-7</font><font SIZE="3">所示。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p align="center"></font><img src="T8_7.gif" alt="T8_7.tif (267124 bytes)" WIDTH="470" HEIGHT="335"><font FACE="Times New Roman" SIZE="4"><b></p>
    </b></font><p align="center"><font SIZE="3">图</font><font FACE="Times New Roman" SIZE="3">8-7 </font><font SIZE="3">多文档绘图程序窗口</font></p>
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