For the past few days, I’ve been investigating the self-assignment principle for Windows Runtime properties:<\/p>\n
One corollary to this was that setting a property twice to the same value has no effect<\/a>. Another is that the default value of a property must be a legal value<\/a>.<\/p>\n Another corollary of this principle is this:<\/p>\n Consider the following API design proposal:<\/p>\n The This design violates the self-assignment principle.<\/p>\n Suppose we have this code fragment:<\/p>\n The above fragment updates the widget background color and leaves the position unchanged.<\/p>\n We then introduce a harmless self-assignment:<\/p>\n This time, since there was an assignment to One way to fix this is to have a special “no change” sentinel value. For example, you might say that a color of Transparent means “do not change the color” and a position of (NaN, NaN) means “do not change the position”. Those special values would also be the default values for the properties.<\/p>\n One downside of sentinel values is that there might not be a good choice of sentinel value. For example, what if you want a Widget to have a transparent background? Another is that there may not be an obvious<\/i> choice for sentinel value. We sort of had to invent a weird sentinel value of (NaN, NaN) for the “no point” point. And it might be hard to generate a NaN value in some languages.<\/p>\n The Windows Runtime supports nullable types, so you can make the properties nullable, where the null value means “leave unchanged.” This is more natural and discoverable.<\/p>\n Another solution is to change the properties to methods. Methods are allowed to have side effects.<\/p>\n\n
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Widget\u00adUpdate\u00adOptions<\/code> class is a parameter passed to the Widget.Update<\/code> method, and it describes how you want to update a widget. The Widget\u00adUpdate\u00adOptions<\/code> class has two properties:<\/p>\nruntimeclass WidgetUpdateOptions\r\n{\r\n Windows.UI.Color BackgroundColor;\r\n Windows.Foundation.Point Position;\r\n}\r\n<\/pre>\n\n
BackgroundColor<\/code>: If you set this property, then the widget’s background color is updated to the color specified.<\/li>\nPosition<\/code>: If you set this property, then the widget’s position is updated to the position specified.<\/li>\n<\/ul>\n<\/blockquote>\nvar options = new WidgetUpdateOptions();\r\noptions.BackgroundColor = Color.Blue;\r\nwidget.Update(options);\r\n<\/pre>\n
var options = new WidgetUpdateOptions();\r\noptions.BackgroundColor = Color.Blue;\r\noptions.Position = options.Position;<\/span>\r\nwidget.Update(options);\r\n<\/pre>\noptions.Position<\/code>, the call to widget.Update(options)<\/code> updates not just the background color, but it also updates the position, probably to (0,0). Oops.<\/p>\nruntimeclass WidgetUpdateOptions\r\n{\r\n Windows.Foundation.IReference<Windows.UI.Color> BackgroundColor;\r\n Windows.Foundation.IReference<Windows.Foundation.Point> Position;\r\n}\r\n<\/pre>\n\n
BackgroundColor<\/code>: If you set this property to a non-null value, then the widget’s background color is updated to the color specified.<\/li>\nPosition<\/code>: If you set this property to a non-null value, then the widget’s position is updated to the position specified.<\/li>\n<\/ul>\nruntimeclass WidgetUpdateOptions\r\n{\r\n void UpdateBackgroundColor(Windows.UI.Color backgroundColor);\r\n void UpdatePosition(Windows.Foundation.Point position);\r\n}\r\n<\/pre>\n