Is there a keyboard shortcut to switch between XAML and Designer in VS 2012?

If Shift + F7 doesn't work right away then you may need to re-assign the context for which the keyboard shortcut applies. When I tried this in VS 2017 I had to go in and re-assign the shortcut by using the following steps:

• Tools > Options
• In the popup menu select Environment > Keyboard in the side bar
• Type View.ViewDesigner in the search bar
• Open the menu under Use new shortcut in:
• Select Text Editor from the menu, enter desired shortcut into the assignment bar, and click assign
• Select XAML Designer from the menu, enter desired shortcut into the assignment bar, and click assign
• Click OK to apply the changes and exit the options menu

NOTE: You must do both Text Editor and XAML Designer in order to get this to work well. The image below shows the result of the above steps.

For 99% of the times, you may want to set it to default

I don't know about other version, but in Visual Studio 2010, from the menu bar select Tools → Options..., from the dialog, select Text Editor → XAML → Miscellaneous, tick the Always open documents in full XAML view. Now everytime an XAML file is double clicked, it'll open in XAML (code) view, not design view.

I hope the image is not too big

Yupp there sure is, just use Shift+F7 ;)