Popup弹出后，因业务需求设置了StaysOpen=true后，移动窗口位置或者改变窗口大小，Popup的位置不会更新。
如何更新位置？
获取当前Popup的Target绑定UserControl所在窗口，位置刷新时，时时更新Popup的位置即可。
1.添加一个附加属性
1 ///2 /// Popup位置更新3 /// 4 public static readonly DependencyProperty PopupPlacementTargetProperty =5 DependencyProperty.RegisterAttached("PopupPlacementTarget", typeof(DependencyObject), typeof(PopupHelper), new PropertyMetadata(null, OnPopupPlacementTargetChanged));
2.窗口移动后触发popup更新
首先，有个疑问，popup首次显示时，为何显示的位置是正确的呢？
通过查看源码，发现，其实popup也是有内置更新popup位置的！
而通过查看UpdatePosition代码，其方法确实是更新popup位置的。源码如下：
1 private void UpdatePosition() 2 { 3 if (this._popupRoot.Value == null) 4 return; 5 PlacementMode placement = this.Placement; 6 Point[] targetInterestPoints = this.GetPlacementTargetInterestPoints(placement); 7 Point[] childInterestPoints = this.GetChildInterestPoints(placement); 8 Rect bounds = this.GetBounds(targetInterestPoints); 9 Rect rect1 = this.GetBounds(childInterestPoints); 10 double num1 = rect1.Width * rect1.Height; 11 int num2 = -1; 12 Vector offsetVector1 = new Vector((double)this._positionInfo.X, (double)this._positionInfo.Y); 13 double num3 = -1.0; 14 PopupPrimaryAxis popupPrimaryAxis = PopupPrimaryAxis.None; 15 CustomPopupPlacement[] customPopupPlacementArray = (CustomPopupPlacement[])null; 16 int num4; 17 if (placement == PlacementMode.Custom) 18 { 19 CustomPopupPlacementCallback placementCallback = this.CustomPopupPlacementCallback; 20 if (placementCallback != null) 21 customPopupPlacementArray = placementCallback(rect1.Size, bounds.Size, new Point(this.HorizontalOffset, this.VerticalOffset)); 22 num4 = customPopupPlacementArray == null ? 0 : customPopupPlacementArray.Length; 23 if (!this.IsOpen) 24 return; 25 } 26 else 27 num4 = Popup.GetNumberOfCombinations(placement); 28 for (int i = 0; i < num4; ++i) 29 { 30 bool flag1 = false; 31 bool flag2 = false; 32 Vector offsetVector2; 33 PopupPrimaryAxis axis; 34 if (placement == PlacementMode.Custom) 35 { 36 offsetVector2 = (Vector)targetInterestPoints[0] + (Vector)customPopupPlacementArray[i].Point; 37 axis = customPopupPlacementArray[i].PrimaryAxis; 38 } 39 else 40 { 41 Popup.PointCombination pointCombination = this.GetPointCombination(placement, i, out axis); 42 Popup.InterestPoint targetInterestPoint = pointCombination.TargetInterestPoint; 43 Popup.InterestPoint childInterestPoint = pointCombination.ChildInterestPoint; 44 offsetVector2 = targetInterestPoints[(int)targetInterestPoint] - childInterestPoints[(int)childInterestPoint]; 45 flag1 = childInterestPoint == Popup.InterestPoint.TopRight || childInterestPoint == Popup.InterestPoint.BottomRight; 46 flag2 = childInterestPoint == Popup.InterestPoint.BottomLeft || childInterestPoint == Popup.InterestPoint.BottomRight; 47 } 48 Rect rect2 = Rect.Offset(rect1, offsetVector2); 49 Rect rect3 = Rect.Intersect(this.GetScreenBounds(bounds, targetInterestPoints[0]), rect2); 50 double num5 = rect3 != Rect.Empty ? rect3.Width * rect3.Height : 0.0; 51 if (num5 - num3 > 0.01) 52 { 53 num2 = i; 54 offsetVector1 = offsetVector2; 55 num3 = num5; 56 popupPrimaryAxis = axis; 57 this.AnimateFromRight = flag1; 58 this.AnimateFromBottom = flag2; 59 if (Math.Abs(num5 - num1) < 0.01) 60 break; 61 } 62 } 63 if (num2 >= 2 && (placement == PlacementMode.Right || placement == PlacementMode.Left)) 64 this.DropOpposite = !this.DropOpposite; 65 rect1 = new Rect((Size)this._secHelper.GetTransformToDevice().Transform((Point)this._popupRoot.Value.RenderSize)); 66 rect1.Offset(offsetVector1); 67 Rect screenBounds = this.GetScreenBounds(bounds, targetInterestPoints[0]); 68 Rect rect4 = Rect.Intersect(screenBounds, rect1); 69 if (Math.Abs(rect4.Width - rect1.Width) > 0.01 || Math.Abs(rect4.Height - rect1.Height) > 0.01) 70 { 71 Point point1 = targetInterestPoints[0]; 72 Vector vector1 = targetInterestPoints[1] - point1; 73 vector1.Normalize(); 74 if (!this.IsTransparent || double.IsNaN(vector1.Y) || Math.Abs(vector1.Y) < 0.01) 75 { 76 if (rect1.Right > screenBounds.Right) 77 offsetVector1.X = screenBounds.Right - rect1.Width; 78 else if (rect1.Left < screenBounds.Left) 79 offsetVector1.X = screenBounds.Left; 80 } 81 else if (this.IsTransparent && Math.Abs(vector1.X) < 0.01) 82 { 83 if (rect1.Bottom > screenBounds.Bottom) 84 offsetVector1.Y = screenBounds.Bottom - rect1.Height; 85 else if (rect1.Top < screenBounds.Top) 86 offsetVector1.Y = screenBounds.Top; 87 } 88 Point point2 = targetInterestPoints[2]; 89 Vector vector2 = point1 - point2; 90 vector2.Normalize(); 91 if (!this.IsTransparent || double.IsNaN(vector2.X) || Math.Abs(vector2.X) < 0.01) 92 { 93 if (rect1.Bottom > screenBounds.Bottom) 94 offsetVector1.Y = screenBounds.Bottom - rect1.Height; 95 else if (rect1.Top < screenBounds.Top) 96 offsetVector1.Y = screenBounds.Top; 97 } 98 else if (this.IsTransparent && Math.Abs(vector2.Y) < 0.01) 99 {100 if (rect1.Right > screenBounds.Right)101 offsetVector1.X = screenBounds.Right - rect1.Width;102 else if (rect1.Left < screenBounds.Left)103 offsetVector1.X = screenBounds.Left;104 }105 }106 int x = DoubleUtil.DoubleToInt(offsetVector1.X);107 int y = DoubleUtil.DoubleToInt(offsetVector1.Y);108 if (x == this._positionInfo.X && y == this._positionInfo.Y)109 return;110 this._positionInfo.X = x;111 this._positionInfo.Y = y;112 this._secHelper.SetPopupPos(true, x, y, false, 0, 0);113 }
那么，我们有什么办法调用这个私有方法呢？我相信大家都想，找到popup源码开发者，爆了他Y的！
有一种方法，叫反射，反射可以获取类的任一个字段或者属性。
反射，可以参考：https://www.cnblogs.com/vaevvaev/p/6995639.html
通过反射，我们获取到UpdatePosition方法，并调用执行。
1 var mi = typeof(Popup).GetMethod("UpdatePosition", BindingFlags.NonPublic | BindingFlags.Instance);2 mi.Invoke(pop, null);
下面是详细的属性更改事件实现：
1 private static void OnPopupPlacementTargetChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) 2 { 3 Popup pop = d as Popup; 4 5 //旧值取消LocationChanged监听 6 if (e.OldValue is DependencyObject previousPlacementTarget) 7 { 8 Window window = Window.GetWindow(previousPlacementTarget); 9 if (window != null)10 {11 window.LocationChanged -= WindowLocationChanged;12 }13 }14 15 //新值添加LocationChanged监听16 if (e.NewValue is DependencyObject newPlacementTarget)17 {18 Window window = Window.GetWindow(newPlacementTarget);19 if (window != null)20 {21 window.LocationChanged -= WindowLocationChanged;22 window.LocationChanged += WindowLocationChanged;23 }24 }25 void WindowLocationChanged(object s1, EventArgs e1)26 {27 if (pop != null && pop.IsOpen)28 {29 //通知更新相对位置30 var mi = typeof(Popup).GetMethod("UpdatePosition", BindingFlags.NonPublic | BindingFlags.Instance);31 mi.Invoke(pop, null);32 }33 }34 } 值得注意的是，原有的绑定目标源要记得取消LocationChanged事件订阅，新的绑定目标源保险起见，也要提前注销再添加事件订阅。
另：通知popup位置更新，也可能通过如下的黑科技：
1 //通知更新相对位置2 var offset = pop.HorizontalOffset;3 pop.HorizontalOffset = offset + 1;4 pop.HorizontalOffset = offset;
为何改变一下HorizontalOffset就可行呢？因为上面最终并没有改变HorizontalOffset的值。。。
原来。。。好吧，先看源码
1 ///获取或设置目标原点和弹出项对齐之间的水平距离点。 2 ///3 /// 目标原点和 popup 对齐点之间的水平距离。 4 /// 有关目标原点和 popup 对齐点的信息，请参阅 Popup 放置行为。 5 /// 默认值为 0。 6 /// 7 [Bindable(true)] 8 [Category("Layout")] 9 [TypeConverter(typeof (LengthConverter))]10 public double HorizontalOffset11 {12 get13 {14 return (double) this.GetValue(Popup.HorizontalOffsetProperty);15 }16 set17 {18 this.SetValue(Popup.HorizontalOffsetProperty, (object) value);19 }20 }21 22 private static void OnOffsetChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)23 {24 ((Popup) d).Reposition();25 }
是的，最终调用了Reposition，而Reposition方法中有调用UpdatePosition更新popup位置。
所以以上，更新HorizontalOffset，是更新popup位置的一种捷径。
3.界面设置绑定目标源
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