在Canvas中，我们可以为它的子元素通过附加属性Canvas.Left和Canvas.Top来设置位置，那么子元素和Canvas之间的位置关系也就很清楚了，但是我们需要更一般化的方法，来获得某个子元素相对于桌面，某个父元素等的位置关系，那就需要引入Transform的概念了，也就是通过这个数学化的概念来达到我们想到达的目的。当然除了这个，还有计算的函数，那就是来自Visual的TransformToVisual(Visual visual)方法，在该方法的MSDN文档中有这样一个例子：

XAML代码：

<StackPanel Name="myStackPanel" Margin="8">
  <TextBlock Name="myTextBlock" Margin="4" Text="Hello, world" />
</StackPanel>

 核心代码：

// Return the general transform for the specified visual object.
GeneralTransform generalTransform1 = myStackPanel.TransformToVisual(myTextBlock);

// Retrieve the point value relative to the child.
Point currentPoint = generalTransform1.Transform(new Point(0, 0));

先说一下这个例子的目的是，求得myStackPanel相对于它的子元素myTextBlock的位置，我们可以猜想，若以myTextBlock的左上角为(0,0)，那么算上Margin值，这个相对的位置应该是(-4,-4)。

现在看代码，首先，使用myStackPanel.TransformToVisual获得一个用于封装Transform概念的GeneralTransform对象，这是个抽象的通用类，我们可以看一下它的继承关系：

Object

     DispatcherObject (abstract)

            DependencyObject

                   Freezable (abstract)

                          Animatable (abstract)

                                 GeneralTransform (abstract)

                                        GeneralTransformGroup

                                 Transform (abstract)

                                       TranslateTransform

                                       ScaleTransform

                                       SkewTransform

                                       RotateTransform

                                       MatrixTransform

                                       TransformGroup

我通过断点，发现它返回的是一个MatrixTransform类型，这就好办了，因为它们的内部都含有一个Matrix属性，用来表示Visual的Transform状况，看其中的值，就是{1,0,0,1,-4,-4}，其实就是如下的一个矩阵：

|1    0   0|

|0    1   0|

|-4  -4   1|

最后一列是固定的，所以省去，按行展开，就是{1,0,0,1,-4,-4}。我们知道这个3乘以3矩阵的每个元素的作用，最后一行的前两列用于表示位移，那么，用(0,0)乘以这个矩阵后得出的点是哪个点呢（自己算算就明白，其实这里就是平移产生效果），若你懒点，就直接用Transform方法，它会帮助我们得到这个结果，那就是把我们的点Point(0,0)放进去即可，得出的点就是(-4,-4)。

今天在提交应用的时候遇到了一个新问题：二进制文件已经Archive、Validate、Distribute，Itunes Connect上却显示Missing Screenshot。经查看，在Itunes Connect上3.5-Inch Retina Display Screenshots中也上传了应用截图，后来，经查看iTunes Connect Developer Guide以及开发者邮件，得知问题所在。

iTunes Connect Developer Guide中有这么一段话：

If your binary indicates that your app will run on the 4-inch Retina display, you are required to upload at least one of these screenshots.

Up to four additional optional screenshots can be uploaded. All subsequent screenshots will appear in numbered order on the App Store, in the same order in which they were uploaded in iTunes Connect. Only Newsstand apps can reorder their screenshots after they have been uploaded.

For best results, do not include the iPhone status bar in your screenshots, and follow these requirements:

640 x 1096 pixels for portrait (without status bar) minimum

640 x 1136 pixels for portrait (full screen) maximum

1136 x 600 pixels for landscape (without status bar) minimum

1136 x 640 pixels for landscape (full screen) minimum

72 ppi, RGB, flattened, no transparency

High-quality JPEG, TIFF or PNG image file format. ZIP-compressed TIFF files are not acceptable.

邮件中有这么一段：

If you deliver a new app binary that takes advantage of the new screen size for iPhone 5 or iPod touch (5th generation) without screenshots in the new dimensions, the status of your app will change to Missing Screenshot. Your app will not be reviewed until you upload screenshots with the correct dimensions. You will also need to upload screenshots for any added localizations you have set up on iTunes Connect.

那么，iTunes Connect是如何判断是否需要上传iPhone 5的应用截图的呢？

The only thing that indicates whether or not your app supports the 4-inch screen is whether you have a "Default-568h@2x.png" file or not. If you haven't added that, your app won't natively support the 4-inch screen (which is fine!) and you don't need to change any other settings or provide other screenshots. 

如果你的应用不兼容iPhone 5的话，那么可以果断删除Default-568h@2x.png这个图片，然后重新上传即可。

如果你的应用兼容iPhone 5的话，那么Default-568h@2x.png这个图片就不能删了，有了这个图片就必须提供至少一张iPhone 5的应用截图了，否则就会出现Missing Screenshot的错误，所以在4-Inch Retina Display Screenshots中上传一下iPhone 5的应用截图即可。