Revision as of 11:16, 8 January 2021

See also: Converters in UWP/WinX

Converters are used to prepare application data to format a UI defined by a XAML markup. These converters make it possible to show elements only when specific conditions (like data being available) are met. This allows the creation of a good UI with XAML code only, improving code readability while also decreasing code complexity.

This page explains how to properly use converters for customizing the UBIK Xamarin User Interface. Furthermore, all available converters are listed to provide a good reference.

Definition

In Xamarin, before converters can be used, they need to be defined in the page's Resources tag. Make sure to include the UBIK.CPL.Converters namespace in the namespace definitions! The following example shows how the NullToBoolConverter can be defined:

The key NullToBool makes this converter accessible from the customizing in this page.

Definition With Properties

In the following example, which is the same example from above, the StringFormatConverter will be additionally defined. To add more formatting possibilities, it has two extra properties, which need to be prefilled during initialization. Henceforth, these properties can be prefilled with static values or with static references.

Whereever this converter is used within the file it was defined in, Formatter will always have its Parameter1 set to 5 and its Parameter2 to the number of children of the currently viewed UBIK object. If this behavior is not intended, consider defining another StringFormatConverter with a different Key in the same way as shown above.
See below to find out how to use this converter!

Usage

A converter can only be used in conjunction with a {Binding}. The following is an easy example, showing how the NullToBool converter, defined above, can be used.

If MyBindableProperty is null or an empty string, the converter will return true, making the Label visible!

Parameter

Some converters accept a ConverterParameter, that passes additional information. Closely read the description of the available converters to find out which accept or even require a parameter to work properly. In Ubik, some converters accept a string parameter, consisting of multiple individual parameters separated by |.

An example of this behavior is the ContainsToBoolConverter, which checks if the current value is contained within a collection of values (passed as the parameter):

The label will only be visible if the MyValue property (which is expected to parse as an Integer in this example) is one of the values of the parameter.
Again, to get the converter working, don't forget to define it in the page's resources!

FallbackValue

In some rare cases, a converter might not return anything desired (like null) if some condition doesn't work out as it should (e.g. value is null). To still be able to get a usable return value, it's possible to define a FallbackValue.

Example Usage of StringFormatConverter

This converter can take up to two additional parameters during definition. Please see Definition With Properties for more information!

It's important to note that special characters, like {, } and ,, need to be escaped using a \, else the expression will be wrongly interpreted (and nothing happens)!

The text of the ConverterParameter will be analyzed, and any occurrance (including duplicates) of {0}, {1} and {2} replaced with the binding value, Formatter.Parameter1 and Formatter.Parameter2 respectively!

Make sure to note, that Xamarin Forms has a native StringFormat option as well, that is probably way faster than using a converter for it. Furthermore, it supports formatting options!

Data Source

Although technically not a converter, the SfDataSourceExt control is currently on the way of replacing the SfDataSourceConverter, due to improved performance. The usage is pretty simple: Similar to the converters, it needs to be defined in the Page Resources. Then, a Key and Expression, as well as an ItemsSource have to be defined. Last, but not least, the the converter can be used within the ItemsSource property of a Syncfusion SfListView. See the following example:

Thanks to the given expression, filtering is applied once the data source is evaluated. It's important to have XML special characters correctly escaped, like the example shows.

List of Available Converters

The following list contains all currently available converters for UBIK Xamarin customizing.

Two-way converters can be used with two-way bindings, e.g. a text-box displaying an editable value. If the value updates in the model, the text-box text changes. If the user edits the value, the value in the model behind also gets updated. As a clarification, Value refers to the Binding Value, and parameter to the ConverterParameter!

Converter	2-Way	Output Type	Input Type	Parameter Type	Description
BooleanConverter	✓	Boolean	Boolean		Interprets the boolean value and returns it. If the value cannot be interpreted, false is returned.
BooleanInvertConverter	✓	Boolean	Boolean		Interprets and converts a boolean into its inverted value. If the value cannot be interpreted, false is returned.
BooleanToFontAttributeConverter	✗	FontAttribute	Boolean		Converts a boolean into a font attribute value. In detail, if the value is true, the parameter is interpreted (Bold, Italic, None) and returned. The default returned font-attribute is None.
ByteToImageSourceValueConverter	✗	ImageSource	Byte[ ], Byte Stream		Converts a byte stream value into an image source.
ChildAreaTemplateConverter	✗	ChildAreaTemplate	ContentViewModel		Returns a ChildAreaTemplate from a ContentViewModel or null if the ViewModel cannot be interpreted.
ChildItemSelectionModeToSfListViewSelectionModeConverter	✓	ListViewSelectionMode	ChildItemSelectionMode		Converts a ChildItemSelectionMode to a Syncfusion ListViewSelectionMode. Default is "Single".
ClassificationToBoolConverter	✗	Boolean	ContentViewModel		Returns a boolean indicating whether the given ContentViewModel is successfully classified.
ContainsToBoolConverter	✗	Boolean	String, Guid, Object		Checks if the delivered value is contained within a collection of values (delivered in the parameter, seperated with `\|`). If the value is contained, true will be returned, else false. The functionality is similar to the ContainsToVisibilityConverter in the WinX project.
ContainsToInvertedBoolConverter	✗	Boolean	String, Guid, Object		Same as the ContainsToBoolConverter, but with inverted output.
ContentAreaTeamplateConverter	✗	UBIKContentArea	ContentViewModel		This converter is not yet finished and just returns the UBIKContentArea if the value is a ContentViewModel.
DateTimeOffsetToDateConverter	✓	DateTime	DateTimeOffset		Converts a DateTimeOffset to the correct DateTime, applied to the current date and time. If the value cannot be interpreted, a new DateTime, generated from the device's current time (`DateTime.Now`) is returned.
DateTimeToFromNowStringConverter	✗	String	DateTime		Returns a DateTime into a human-readable and easily understandable string message (the last applying one will be taken): Less than 5 minutes in the past: Just now Less than 10 minutes in the past: A few minutes ago Less than 1 hour in the past: An hour ago Less than 23 Hours in the past: Some hours ago Less than 1 day in the past: Yesterday More than 1 day in the past: The DateTime value formatted according to the device's localization rules. Not interpretable: ???
DebugConverter	✓	Object	Object		A converter returning the given value for debug reasons.
DistanceToStringConverter	✗	String	Double		Returns the given value interpreted as length. Values are returned in kilometers with two comma digits (e.g. 2.84km), values smaller than 1km are returned as meters and NaN will be returned as infinity. If the value cannot be interpreted, an empty string will be returned.
EqualityToBoolConverter	✗	Boolean	Object, String	Object, String	Returns true if the given value and parameter are equal to each other, false otherwise.
EqualityToInvertedBoolConverter	✗	Boolean	Object, String	Object, String	Returns true if the given value and parameter are not equal to each other, false otherwise.
SelectionChangedEventArgsConverter	✗	ListSelectionChangedEventArgs	ItemSelectionChangedEventArgs		Converts an Syncfusion ItemSelectionChangedEventArgs to a ListSelectionChangedEventArgs. The use-case of this converter is highly specific.
FilterCriterionToValueConverter	✓	String	FilterCriterion		Converts a FilterCriterion to its value. The functionality is similar to UWP's FilterCriterionToValueConverter.
GuidPropertyValueConverter	✓	String	Guid	Binding, where source is ContentViewModel	Returns the first item of the parameter's Source-ViewModel that matches the given GUID value. If none is found, null is returned.
IntToColorConverter	✗	Color	Integer		Converts an ARGB integer value to a color. If the value cannot be interpreted, the converter tries to parse the parameter as a color to return it. If everything fails, transparent is returned.
ItemCountToBoolConverter	✗	Boolean	Integer	Integer	This converter has a property to set the boolean return value, called LesserOrEqualReturnValue. This property-value will be returned if the given value is smaller than or equal to the threshold (parameter, defaults to 0). If the value is bigger than the threshold, the inverted LesserOrEqualReturnValue will be returned. In any other case (like the value cannot be interpreted), false is returned, so it's a good idea to set the converter's property in a way that the return is only true if you need it.
ItemCountToOverflowConverter	✗	String	Integer	Integer	Creates a human-readable text indicating how many items are available. The value will be interpreted as the total item count and the parameter as overflow threshold (defaults to 99). If there are more items than the overflow, the overflow value with a + sign will be returned (e.g. 99+). If not, the value itself will be returned. If everything fails, null is returned.
NullToBoolConverter	✓	Boolean	Object, String		Null or an empty string value get converted to true, and everything else to false.
NullToInvertedBoolConverter	✓	Boolean	Object, String		Null or an empty string value get converted to false, and everything else to true.
PathToImageSourceValueConverter	✗	Image Source	String		Reads the path provided as parameter and creates an image source from it.
PercentageToProgressConverter	✗	Double	Integer, Double		The numeric value (int or double), interpreted as percentage from 0 to 100, get converted to a progress level (0 to 1). Values outside this range will be contained.
PropertyNameExistsToBoolConverter	✗	Boolean (Nullable)	ContentViewModel, IContent		Converts a property to a boolean based on its existence. If it exists, true will be returned, else false or null (if something couldn't be properly interpreted).
PropertyNameExistsToInvertedBoolConverter	✗	Boolean (Nullable)	ContentViewModel, IContent		Converts a property to a boolean based on its existence. If it exists, false will be returned, else true or null (if something couldn't be properly interpreted).
RootAreaTemplateConverter	✗	ContentView	Object		If the provided value is not null, the UBIKRootArea template will be returned.
SelectiveItemToValueConverter	✓	Boolean, String, Integer, Double, DateTime, Guid, FileReferenceData, GeoData, Object	Boolean, String, Integer, Double, DateTime, Guid, FileReferenceData, GeoData, Object	Binding, where source is ContentViewModel	Converts a item of a selective list to its value. If none is found, the passed value will be returned.
SelectiveListToItemsConverter	✗	List of PropertyItems	IPropertyItem		Returns all items from a selective list.
SfDataSourceConverter	✗	DataSource (for ListView)	Double	String (Expression)	It's an advanced converter used for loading items and applying filters on it. It replaces to CollectionToViewConverter from the WinX.UWP project. The data source can be directly used with a SfListView. This converter is obsolete! Use SfDataSourceExt (see Data Source) instead!
StringContainsToBoolConverter	✗	Boolean	String	String	Returns a boolean indicating whether the parameter string is included in the value string.
StringContainsToInvertedBoolConverter	✗	Boolean	String	String	Same as the StringContainsToBoolConverter but with inverted output.
StringFormatConverter	✗	String	Object	String	Allows the creation of a *nicely formatted string* message*, similar to C#'s String.Format method. The value will be interpreted as the `{0}` element, and the parameter is the template string (e.g. `The total count is: {0}!`. Up to two additional parameters, named Parameter1* and Parameter2, can be defined at the converter declaration. Adding formatting options to the `{0}` notation is not possible. Attention, using the native StringFormat option is not only faster, but it also offers more customizability of the value! You might still want to use the StringFormatConverter in rare cases, though!
ToStringFormatConverter	✗	String	Double, Float, Integer, DateTime, String	String	Converts primitives or DateTime to string and allows the application of formatting options (as parameter). Adding a `.` to the end of the format-parameter allows truncation of all decimal places.
ToTypeConverter	✗	Object	Object	String	Attempts to convert value to the Type specified in parameter, returning the value.
TypeNameToBoolConverter	✗	Boolean	Object	String	Returns true if the type name of the value is present in the parameter-string (seperated by `\|`).
TypeNameToInvertedBoolConverter	✗	Boolean	Object	String	Inverted TypeNameToBoolConverter.
DataTemplateItemTemplateSelectorConverter	✗	ChildItemTemplateSelector	String	String	Chooses what ChildItemTemplateSelector to return. Having a parameter with the content "Small" returns the small item template selector.
DataTemplateItemsPanelConverter	✗	ItemsPanelTemplate	QueryDetailsPageViewModel		Chooses what ItemPanelTemplate to return. Having a parameter with the content "Small" returns the small item template selector.

@@ Line 239: / Line 239: @@
 | DataTemplateItemsPanelConverter || style="text-align: center;" | ✗ || ItemsPanelTemplate || Query<wbr/>Details<wbr/>Page<wbr/>ViewModel || || Chooses what '''ItemPanel<wbr/>Template''' to return. Having a parameter with the content "Small" returns the small item template selector.
 |}
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