然后我们要定义ViewMode。先建立一个enum: Public Enum VMode FitSize = 0 StretchImage = 1 TrueSize = 2 End Enum
然后建立一个变量存放ViewMode属性: Private vViewMode As VMode = VMode.TrueSize
当ViewMode变化的时候,控件有必要向外界发出一个事件: Public Event ViewModeChanged(ByVal Mode As VMode)
现在可以写ViewMode属性了: Public Property ViewMode() As VMode Get Return vViewMode End Get Set(ByVal Value As VMode) Dim changed As Boolean = False If Value <> vViewMode Then changed = True vViewMode = Value If changed Then ShowPic() RaiseEvent ViewModeChanged(Value) End Set End Property
这里,ShowPic()是尚未定义的方法,作用就是显示图片。 另外,既然是MyPictureBox,那就要有PictureBox的样。所以要加上一个Image属性: Public Property Image() As Image Get Return PictureBox1.Image End Get Set(ByVal Value As Image) If Value Is Nothing Then ''''判断一下Value是不是空值。 PictureBox1.BorderStyle = BorderStyle.None Exit Property Else PictureBox1.BorderStyle = BorderStyle.FixedSingle End If Value = New Bitmap(Value) PictureBox1.SizeMode = PictureBoxSizeMode.StretchImage PictureBox1.Image = Value ImageRate = Value.Width / Value.Height ShowPic() End Set End Property
这里ImageRate是为了避免重复计算的开销的定义的一个变量,用来存储图像的宽高比,类型是Single。可以在前面加上它的定义。 现在我们来写ShowPic。为了代码结构分明,ShowPic内容很简单: Private Sub ShowPic() If PictureBox1.Image Is Nothing Then Exit Sub If PictureBox1.Visible = False Then PictureBox1.Visible = True ResizeImage() End Sub
之所以要把ResizeImage单独写开来是因为这部分还有其他地方要重用。ResizeImage的作用是根据当前的ViewMode值,来改变PictureBox1的SizeMode属性,并决定是否对图片进行缩放: Private Sub ResizeImage() If PictureBox1.Image Is Nothing Then Exit Sub
If vViewMode = VMode.FitSize Then If PictureBox1.Image.Width > Me.Width Or PictureBox1.Image.Height > Me.Height Then StretchImage() Else If Not PictureBox1.SizeMode = PictureBoxSizeMode.AutoSize Then PictureBox1.SizeMode = PictureBoxSizeMode.AutoSize End If ElseIf vViewMode = VMode.StretchImage Then StretchImage() Else If Not PictureBox1.SizeMode = PictureBoxSizeMode.AutoSize Then PictureBox1.SizeMode = PictureBoxSizeMode.AutoSize End If
LocateImage() End Sub
这一段判断有些复杂,其实应该可以写得更好一点的。StretchImage和LocateImage的作用从名字上大概就能看出来了。先看看StretchImage。它是真正缩放图片的方法。其实也不复杂: Private Sub StretchImage() PictureBox1.SizeMode = PictureBoxSizeMode.StretchImage If MeRate < ImageRate Then PictureBox1.Width = Me.Width PictureBox1.Height = PictureBox1.Width / ImageRate Else PictureBox1.Height = Me.Height PictureBox1.Width = PictureBox1.Height * ImageRate End If End Sub
这里MeRate用来保存MyPictureBox本身的宽高比。类型是Single,现在可以在前面加上它的定义。显然,StrechImage实际上就是改变PictureBox1的宽高。计算好图片框的大小之后,要保持图片在中间,需要计算图片框的位置。LocateImage就是作这个用的: Private Sub LocateImage() If PictureBox1.Width < Me.Width Then PictureBox1.Left = (Me.Width - PictureBox1.Width) / 2 Else PictureBox1.Left = 0 End If
If PictureBox1.Height < Me.Height Then PictureBox1.Top = (Me.Height - PictureBox1.Height) / 2 Else PictureBox1.Top = 0 End If End Sub
当MyPictureBox大小改变的时候,图片的大小和位置就要重新计算。没错,应该写在Resize事件里面。不过且慢。如果不加控制的话,在用户拖动改变MyPictureBox大小的过程中,因为不停的计算图片的大小和位置,CPU的占用率会达到100%。这是很不友好的,甚至可以用“霸道”来形容。也许我们应该开一个线程,在线程当中用Sleep来避免这种情况。但是这写起来又太复杂了点。所以我们用Timer这种折中的办法。回到设计器视图,拖过来一个Timer。设置Interval值为25。在Timer1_Tick当中写上: Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick If vViewMode = VMode.StretchImage Or vViewMode = VMode.FitSize Then ResizeImage() Else LocateImage() End If Timer1.Stop() End Sub
然后再在Resize事件中写上: Private Sub MyPictureBox_Resize(ByVal sender As Object, ByVal e As System.EventArgs) Handles MyBase.Resize MeRate = Me.Width / Me.Height Timer1.Start()