使用扩展方法对代码的行为进行封装的例子：封装UIElement的“拖动”
很多情况下，我们需要对界面上的元素进行拖动，用鼠标在VS中biaji，biaji，biaji，点几个事件，然后再写出一堆代码，浪费时间不说，由IDE自动生成的那些代码实在是太难看，影响心情。本文使用扩展方法，对于这类行为需要进行封装，以使代码更简单简洁。
封装原则如下：
（1）要简单，最好是一行代码就搞定；
（2）要强大，能用于尽量多的类；
（3）要灵活，可适用于尽量多的场景。
在本文的末尾添加了修改版，修改版代码更简洁，操作更简单，且可以设置多个拖动逻辑。
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设计下面的扩展方法原型：
public static void SetDraggable(this UIElement element, IInputElement relativeTo, Action<DraggableContext> moveCallback = null, Action<DraggableContext> beforeDragCallback = null, Action<DraggableContext> afterDragCallback = null)
element 是拖动的界面元素，relativeTo 是参照物，moveCallback 是移动过程中的回调方法。DraggableContext 是一个类，包装了调用方所需要的信息。beforeDragCallback 是拖动前的处理，假设要拖动一个TextBlock，拖动前可以将它改变颜色，以示醒目。afterDragCallback 是拖动结束后的处理。
DraggableContext 的代码为：
public class DraggableContext
{
public DependencyObject Owner { get; private set; }
public IInputElement RelativeTo { get; private set; }
public Point StartPoint { get; internal set; }
public Point EndPoint { get; internal set; }public Point Offset
{
get { return new Point { X = EndPoint.X - StartPoint.X, Y = EndPoint.Y - StartPoint.Y }; }
}private Boolean _dragging;
public Boolean Dragging
{
get { return _dragging; }
internal set
{
if (value != _dragging)
{
_dragging = value;
if (value == true)
{
if (BeforeDragCallback != null)
BeforeDragCallback(this);
}
else
{
if (AfterDragCallback != null)
AfterDragCallback(this);
}
}
}
}public Action<DraggableContext> MoveCallback { get; private set; }
public Action<DraggableContext> BeforeDragCallback { get; private set; }
public Action<DraggableContext> AfterDragCallback { get; private set; }
public DraggableContext(DependencyObject owner, IInputElement relativeTo, Action<DraggableContext> moveCallback = null, Action<DraggableContext> beforeDragCallback = null, Action<DraggableContext> afterDragCallback = null)
{
Owner = owner;
RelativeTo = relativeTo;
MoveCallback = moveCallback;
BeforeDragCallback = beforeDragCallback;
AfterDragCallback = afterDragCallback;
}public override int GetHashCode()
{
return Owner.GetHashCode();
}
}
然后，还需要一个Dictionary，来储存所有的调用者：
private static Dictionary<Object, DraggableContext> _dicDragContext = new Dictionary<Object, DraggableContext>();
然后，是对拖动逻辑的实现：
public static void SetDraggable(this UIElement element, IInputElement relativeTo, Action<DraggableContext> moveCallback = null, Action<DraggableContext> beforeDragCallback = null, Action<DraggableContext> afterDragCallback = null)
{
if (element == null) throw new ArgumentNullException("element");_dicDragContext[element] = new DraggableContext(element, relativeTo, moveCallback, beforeDragCallback, afterDragCallback);
element.MouseLeftButtonDown += new System.Windows.Input.MouseButtonEventHandler(element_MouseLeftButtonDown);
element.MouseLeftButtonUp += new System.Windows.Input.MouseButtonEventHandler(element_MouseLeftButtonUp);
element.MouseLeave += new System.Windows.Input.MouseEventHandler(element_MouseLeave);
element.MouseMove += new System.Windows.Input.MouseEventHandler(element_MouseMove);
}private static void element_MouseLeftButtonUp(object sender, System.Windows.Input.MouseButtonEventArgs e)
{
_dicDragContext[sender].Dragging = false;
}private static void element_MouseLeftButtonDown(object sender, System.Windows.Input.MouseButtonEventArgs e)
{
DraggableContext ctx = _dicDragContext[sender];
ctx.Dragging = true;
ctx.StartPoint = e.GetPosition(ctx.RelativeTo);
ctx.EndPoint = ctx.StartPoint;
}private static void element_MouseLeave(object sender, System.Windows.Input.MouseEventArgs e)
{
_dicDragContext[sender].Dragging = false;
}private static void element_MouseMove(object sender, System.Windows.Input.MouseEventArgs e)
{
DraggableContext ctx = _dicDragContext[sender];
if (ctx.Dragging == true)
{
ctx.Dragging = true;
ctx.EndPoint = e.GetPosition(ctx.RelativeTo);
if (ctx.MoveCallback != null)
{
ctx.MoveCallback(ctx);
}
ctx.StartPoint = ctx.EndPoint;
}
}
最后，还需要提供一个扩展方法清除对对象和事件的引用，以避免出现内存泄漏。这个方法需要显示调用。
public static void UnsetDraggable(this UIElement element)
{
element.MouseLeftButtonDown -= new System.Windows.Input.MouseButtonEventHandler(element_MouseLeftButtonDown);
element.MouseLeftButtonUp -= new System.Windows.Input.MouseButtonEventHandler(element_MouseLeftButtonUp);
element.MouseLeave -= new System.Windows.Input.MouseEventHandler(element_MouseLeave);
element.MouseMove -= new System.Windows.Input.MouseEventHandler(element_MouseMove);if (_dicDragContext.ContainsKey(element)) _dicDragContext.Remove(element);
}
完整代码如下：
代码
2 using System.Collections.Generic;
3 using System.Linq;
4 using System.Text;
5 using System.Windows;
6 using System.Windows.Controls;
7
8 namespace Orc.Util
9 {
10 public class DraggableContext
11 {
12 public DependencyObject Owner { get; private set; }
13 public IInputElement RelativeTo { get; private set; }
14 public Point StartPoint { get; internal set; }
15 public Point EndPoint { get; internal set; }
16
17 public Point Offset
18 {
19 get { return new Point { X = EndPoint.X - StartPoint.X, Y = EndPoint.Y - StartPoint.Y }; }
20 }
21
22 private Boolean _dragging;
23
24 public Boolean Dragging
25 {
26 get { return _dragging; }
27 internal set
28 {
29 if (value != _dragging)
30 {
31 _dragging = value;
32 if (value == true)
33 {
34 if (BeforeDragCallback != null)
35 BeforeDragCallback(this);
36 }
37 else
38 {
39 if (AfterDragCallback != null)
40 AfterDragCallback(this);
41 }
42 }
43 }
44 }
45
46 public Action<DraggableContext> MoveCallback { get; private set; }
47
48 public Action<DraggableContext> BeforeDragCallback { get; private set; }
49
50 public Action<DraggableContext> AfterDragCallback { get; private set; }
51
52 public DraggableContext(DependencyObject owner, IInputElement relativeTo, Action<DraggableContext> moveCallback = null, Action<DraggableContext> beforeDragCallback = null, Action<DraggableContext> afterDragCallback = null)
53 {
54 Owner = owner;
55 RelativeTo = relativeTo;
56 MoveCallback = moveCallback;
57 BeforeDragCallback = beforeDragCallback;
58 AfterDragCallback = afterDragCallback;
59 }
60
61 public override int GetHashCode()
62 {
63 return Owner.GetHashCode();
64 }
65
66 public static void MoveOnCanvas(DraggableContext ctx)
67 {
68 Canvas cvs = ctx.RelativeTo as Canvas;
69 if (cvs == null) throw new NotSupportedException("RelativeTo 必须是 Canvas");
70 ctx.Owner.SetValue(Canvas.TopProperty, (double)(ctx.Owner.GetValue(Canvas.TopProperty)) + ctx.Offset.X);
71 ctx.Owner.SetValue(Canvas.LeftProperty, (double)(ctx.Owner.GetValue(Canvas.LeftProperty)) + ctx.Offset.Y);
72 }
73 }
74
75 public static class ClassHelper
76 {
77 private static Dictionary<Object, DraggableContext> _dicDragContext = new Dictionary<Object, DraggableContext>();
78
79 public static void SetDraggable(this UIElement element, IInputElement relativeTo, Action<DraggableContext> moveCallback = null, Action<DraggableContext> beforeDragCallback = null, Action<DraggableContext> afterDragCallback = null)
80 {
81 if (element == null) throw new ArgumentNullException("element");
82
83 _dicDragContext[element] = new DraggableContext(element, relativeTo, moveCallback, beforeDragCallback, afterDragCallback);
84 element.MouseLeftButtonDown += new System.Windows.Input.MouseButtonEventHandler(element_MouseLeftButtonDown);
85 element.MouseLeftButtonUp += new System.Windows.Input.MouseButtonEventHandler(element_MouseLeftButtonUp);
86 element.MouseLeave += new System.Windows.Input.MouseEventHandler(element_MouseLeave);
87 element.MouseMove += new System.Windows.Input.MouseEventHandler(element_MouseMove);
88 }
89
90 private static void element_MouseLeftButtonUp(object sender, System.Windows.Input.MouseButtonEventArgs e)
91 {
92 _dicDragContext[sender].Dragging = false;
93 }
94
95 private static void element_MouseLeftButtonDown(object sender, System.Windows.Input.MouseButtonEventArgs e)
96 {
97 DraggableContext ctx = _dicDragContext[sender];
98 ctx.Dragging = true;
99 ctx.StartPoint = e.GetPosition(ctx.RelativeTo);
100 ctx.EndPoint = ctx.StartPoint;
101 }
102
103 private static void element_MouseLeave(object sender, System.Windows.Input.MouseEventArgs e)
104 {
105 _dicDragContext[sender].Dragging = false;
106 }
107
108 private static void element_MouseMove(object sender, System.Windows.Input.MouseEventArgs e)
109 {
110 DraggableContext ctx = _dicDragContext[sender];
111 if (ctx.Dragging == true)
112 {
113 ctx.Dragging = true;
114 ctx.EndPoint = e.GetPosition(ctx.RelativeTo);
115 if (ctx.MoveCallback != null)
116 {
117 ctx.MoveCallback(ctx);
118 }
119 ctx.StartPoint = ctx.EndPoint;
120 }
121 }
122
123 public static void UnsetDraggable(this UIElement element)
124 {
125 element.MouseLeftButtonDown -= new System.Windows.Input.MouseButtonEventHandler(element_MouseLeftButtonDown);
126 element.MouseLeftButtonUp -= new System.Windows.Input.MouseButtonEventHandler(element_MouseLeftButtonUp);
127 element.MouseLeave -= new System.Windows.Input.MouseEventHandler(element_MouseLeave);
128 element.MouseMove -= new System.Windows.Input.MouseEventHandler(element_MouseMove);
129
130 if (_dicDragContext.ContainsKey(element)) _dicDragContext.Remove(element);
131 }
132 }
133 }
134
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下面，通过两个案例，看一下这样的封装能带来什么好处。
案例1：在一个ScrollViewer[scroll]中有1副Image[imgMain]，当这个图像过大时，需要支持拖动，拖动时，鼠标由箭头变成手的样子，拖动完毕再变回来。界面见下图：
代码如下：
private void UserControl_Loaded(object sender, RoutedEventArgs e)
{
this.imgMain.SetDraggable(this, this.Scroll, UseHandCursor, UseArrowCursor);
}private void UseHandCursor(DraggableContext ctx)
{
(ctx.Owner as FrameworkElement).Cursor = Cursors.Hand;
}private void UseArrowCursor(DraggableContext ctx)
{
(ctx.Owner as FrameworkElement).Cursor = Cursors.Arrow;
}private void Scroll(DraggableContext ctx)
{
this.scroll.ScrollToHorizontalOffset(this.scroll.HorizontalOffset - ctx.Offset.X);
this.scroll.ScrollToVerticalOffset(this.scroll.VerticalOffset - ctx.Offset.Y);
}
如果使用 Lambda 表达式，则更简单。
案例2：还是和上面类似场景，在一个Canvas[cvsFont]上有很多文字（TextBlock[tb]），有的文字压着线了，需要手动拖好。当选中文字时，文字显示红色，拖动完毕，变成黑色。
相关代码为：
tb.SetDraggable(this.cvsFont,
(DraggableContext ctx) =>
{
FontContext pos = tb.DataContext as FontContext;
pos.X += ctx.Offset.X / ImageScale;
pos.Y += ctx.Offset.Y / ImageScale;
tb.SetValue(Canvas.TopProperty, pos.Y * ImageScale);
tb.SetValue(Canvas.LeftProperty, pos.X * ImageScale);
},
(DraggableContext ctx) =>
{
tb.Foreground = new SolidColorBrush(Colors.Red);
},
(DraggableContext ctx) =>
{
tb.Foreground = new SolidColorBrush(Colors.Black);
}
);
FontContext 存储了当缩放比为1时，tb 相对 cvsFont 的位置。这些TextBlock的生命周期比较短，它们排队领盒饭时，需要Unset一下：
tb.UnsetDraggable();
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可以将常用的场景封装成方法，比如说，在DraggableContext类中添加下面的回调方法，将元素在Canvas上移动的逻辑抽象出来：
public static void MoveOnCanvas(DraggableContext ctx)
{
Canvas cvs = ctx.RelativeTo as Canvas;
if (cvs == null) throw new NotSupportedException("RelativeTo 必须是 Canvas");
ctx.Owner.SetValue(Canvas.TopProperty, (double)(ctx.Owner.GetValue(Canvas.TopProperty)) + ctx.Offset.X);
ctx.Owner.SetValue(Canvas.LeftProperty, (double)(ctx.Owner.GetValue(Canvas.LeftProperty)) + ctx.Offset.Y);
}
如果一个元素放置在Canvas上，且只需要支持拖动，不需要其它的逻辑，则一句话就搞定了：
xxx.SetDraggable(cvsXXX,DraggableContext.MoveOnCanvas);
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2010年11月13日对代码进行修改，修改后的代码更简洁，使用更简单，不用手动Unset，且可以挂接多个逻辑。
代码如下：
代码
2 using System.Collections.Generic;
3 using System.Linq;
4 using System.Text;
5 using System.Windows;
6 using System.Windows.Controls;
7
8 namespace Orc.Util
9 {
10 public class DraggableContext
11 {
12 public DependencyObject Owner { get; private set; }
13 public IInputElement RelativeTo { get; private set; }
14 public Point StartPoint { get; internal set; }
15 public Point EndPoint { get; internal set; }
16
17 public Point Offset
18 {
19 get { return new Point { X = EndPoint.X - StartPoint.X, Y = EndPoint.Y - StartPoint.Y }; }
20 }
21
22 private Boolean _dragging;
23
24 public Boolean Dragging
25 {
26 get { return _dragging; }
27 internal set
28 {
29 if (value != _dragging)
30 {
31 _dragging = value;
32 if (value == true)
33 {
34 if (BeforeDragCallback != null)
35 BeforeDragCallback(this);
36 }
37 else
38 {
39 if (AfterDragCallback != null)
40 AfterDragCallback(this);
41 }
42 }
43 }
44 }
45
46 internal void element_MouseLeftButtonUp(object sender, System.Windows.Input.MouseButtonEventArgs e)
47 {
48 this.Dragging = false;
49 }
50
51 internal void element_MouseLeftButtonDown(object sender, System.Windows.Input.MouseButtonEventArgs e)
52 {
53 this.Dragging = true;
54 this.StartPoint = e.GetPosition(this.RelativeTo);
55 this.EndPoint = this.StartPoint;
56 }
57
58 internal void element_MouseLeave(object sender, System.Windows.Input.MouseEventArgs e)
59 {
60 this.Dragging = false;
61 }
62
63 internal void element_MouseMove(object sender, System.Windows.Input.MouseEventArgs e)
64 {
65 if (this.Dragging == true)
66 {
67 this.Dragging = true;
68 this.EndPoint = e.GetPosition(this.RelativeTo);
69 if (this.MoveCallback != null)
70 {
71 this.MoveCallback(this);
72 }
73 this.StartPoint = this.EndPoint;
74 }
75 }
76
77 public Action<DraggableContext> MoveCallback { get; private set; }
78
79 public Action<DraggableContext> BeforeDragCallback { get; private set; }
80
81 public Action<DraggableContext> AfterDragCallback { get; private set; }
82
83 public DraggableContext(DependencyObject owner, IInputElement relativeTo, Action<DraggableContext> moveCallback = null, Action<DraggableContext> beforeDragCallback = null, Action<DraggableContext> afterDragCallback = null)
84 {
85 Owner = owner;
86 RelativeTo = relativeTo;
87 MoveCallback = moveCallback;
88 BeforeDragCallback = beforeDragCallback;
89 AfterDragCallback = afterDragCallback;
90 }
91
92 public override int GetHashCode()
93 {
94 return Owner.GetHashCode();
95 }
96
97 public static void MoveOnCanvas(DraggableContext ctx)
98 {
99 Canvas cvs = ctx.RelativeTo as Canvas;
100 if (cvs == null) throw new NotSupportedException("RelativeTo 必须是 Canvas");
101 ctx.Owner.SetValue(Canvas.TopProperty, (double)(ctx.Owner.GetValue(Canvas.TopProperty)) + ctx.Offset.X);
102 ctx.Owner.SetValue(Canvas.LeftProperty, (double)(ctx.Owner.GetValue(Canvas.LeftProperty)) + ctx.Offset.Y);
103 }
104 }
105
106 public static class ClassHelper
107 {
108 public static void SetDraggable(this UIElement element, IInputElement relativeTo, Action<DraggableContext> moveCallback = null, Action<DraggableContext> beforeDragCallback = null, Action<DraggableContext> afterDragCallback = null)
109 {
110 if (element == null) throw new ArgumentNullException("element");
111 DraggableContext ctx = new DraggableContext(element, relativeTo, moveCallback, beforeDragCallback, afterDragCallback);
112 element.MouseLeftButtonDown += new System.Windows.Input.MouseButtonEventHandler(ctx.element_MouseLeftButtonDown);
113 element.MouseLeftButtonUp += new System.Windows.Input.MouseButtonEventHandler(ctx.element_MouseLeftButtonUp);
114 element.MouseLeave += new System.Windows.Input.MouseEventHandler(ctx.element_MouseLeave);
115 element.MouseMove += new System.Windows.Input.MouseEventHandler(ctx.element_MouseMove);
116 }
117 }
118 }
