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Enum Properties

Enum properties are special data properties that use Xomega Framework lookup data to look up their values and store them as a generic Header structure, as well as to provide a list of items that their value(s) can be selected from and validated against.

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Enum properties can be bound to a variety of UI controls that support value selection, such as drop-down lists, combo boxes, listboxes, checkbox groups, type-ahead fields, or even more sophisticated controls such as pick lists, where you select multiple values by moving items from one list to another.

The base class for all such properties is EnumProperty, which provides advanced functionality in terms of displaying and looking up the values, as well as filtering the lists, including when the selection list depends on the values of other properties, which is referred to as Cascading Selection in here.

Each EnumProperty has an associated LookupTable that it uses to look up the values. In order to associate your property with a lookup table from a lookup cache, you need to set the EnumType and optionally CacheType, if the cache is not global, as follows.

var enumProp = new EnumProperty(this, "status");
enumProp.EnumType = "status enum"; // lookup table type
enumProp.CacheType = LookupCache.User; // the default is LookupCache.Global
note

You can also use a local cache by setting the LocalCacheLoader instead. If you need to provide a custom lookup table, you can override the GetLookupTableAsync and GetLookupTable methods in your subclass.

Enum property value

The EnumProperty stores its value internally as a Header object, or as a list of headers, if the property is multi-valued. If the value you set is not a header of the proper type, then it will convert it to a string, and will use that string to look up the header in its lookup table.

Be default the look up will be by the Id, but you can also have it look up by any format that uniquely identifies the value by setting the KeyFormat field. For example, if the headers in your lookup table store alternative unique abbreviations in their additional attribute named "abbrev", then you can use it for value lookup on your property as follows.

// look up values by unique abbreviation from the "abbrev" attribute
enumProp.KeyFormat = string.Format(Header.AttrPattern, "abbrev");
note

If the property fails to look up the value by that string, it will construct a new Header with that string as an Id, which will make that header invalid. This allows you to always access the property values as headers, and use their IDs to access the real value.

Value display

The property values, as well as the headers in the selection items, are displayed as a string using their Text field by default. However, if you want to customize the way they are displayed as a string, then you can set the DisplayFormat field on your enum property. For example, you can display your values with both the Id and Text separated with a dash, as illustrated below.

// show status values as "ID - Text", e.g. "N - New"
statusProp.DisplayFormat = $"{Header.FieldId} - {Header.FieldText}"
note

For displaying values in the EditString format, which is used when you type in your values, the KeyFormat will be used, which defaults to the Id, but can be customized, as we described above.

Transport value

The property converts its value(s) to the Transport format when it needs to prepare them for being sent to the backend services. The base EnumProperty will use the Id of the Header as is, which means that the transport value(s) will be string-based.

For other typed values Xomega Framework provides a number of specialized enum properties that help you convert the values to the proper type. For example, if you store your enumerated status as an integer in the database, then you can use the EnumIntProperty.

Value validation

As we mentioned above, if the value you are setting cannot be found in the lookup table, the property will create and store an invalid header as its value. When you validate the property, it may report a validation error when the header is invalid.

In order to control how the value is validated, the EnumProperty allows you to set the LookupValidation field to one of the following values.

  • LookupValidationType.ActiveItem - the value should be an active item of the enum.
  • LookupValidationType.AnyItem - the value should be any item of the enum (default).
  • LookupValidationType.None - the value is not validated against the lookup table.

Value selection list

EnumProperty exposes a possible list of values to select from through its AsyncItemsProvider delegate, which is a function that returns the list of possible property items for the current property at any given time. This function accepts a DataRow when it's part of a data list object, as well as some user input to filter the items when the property is bound to a type-ahead UI control.

By default the AsyncItemsProvider is initialized to the GetItemsAsync method that returns all active items of the property's lookup table, and uses any applied custom filters and/or cascading selection restrictions. The method will load the look up table asynchronously, if it's not loaded yet.

note

The property also provides a synchronous version ItemsProvider, which returns only the cached version of the associated lookup table, and doesn't try to load it, if it's not available.

Sort order

By default the items returned for the selection list are ordered based on their display string, which is based on the configuration of the property's DisplayFormat. So, if you display the Text of each item, then those items will be ordered in the alphabetical order based on their text. If you display the Id, or a combination of the Id and text, then it will be sorted by the Id or by the specified format.

You can also provide a custom sort order by setting the SortField delegate of the property, which is a function that returns the value to sort by from each item header. For example, if you store the sort order of each item in a separate attribute, then you can specify the SortField function as follows.

enumProp.SortField = h => h["sort order"];

Filtered list

EnumProperty filters the value selection list from the lookup table using its delegate FilterFunc, which by default is initialized to the property's method IsAllowed. This method filters out inactive headers unless the header is one of the current property values. When cascading selection is set up, it also filters only the headers that match the current values of the cascading properties.

You can set a custom filtering function, which can also utilize the default behavior of the IsAllowed method, as follows.

myProp.FilterFunc = (hdr, row) => myProp.IsAllowed(hdr, row) && MyFilter(hdr, row);

Type-ahead

When the EnumProperty is bound to a type-ahead field, the list of available selection items will be filtered further based on the user input. The property will use its FilterTermFunc delegate to filter the headers that match the user input, which by default is initialized to the property's MatchesTerm function.

The MatchesTerm function checks if the input text matches the beginning of either the DisplayString or EditString formats of the header in a case-insensitive way. This is because the selected value shown in the text field will be in the EditString format (e.g. Id), while the values to select in the dropdown list will be in the DisplayString format (e.g. Text or "Id - Text").

tip

You can implement custom logic for matching the user input, such as matching any part of the string rather than just the beginning. You can do it by either overriding the MatchesTerm function in your subclass, or by just setting the FilterTermFunc delegate to your custom function.

caution

The FilterTermFunc just filters the list of the available items in the lookup table, and does not make any remote calls to get those items based on the user input. If you need to query remote services based on the user input, then you should set a custom AsyncItemsProvider function.

Refreshing the list

If the list of selection items for your EnumProperty has changed, either due to changes in the filtering conditions or when the data in the lookup table has changed, you should fire a property change event using the PropertyChange.Items change, in order to refresh the selection lists in the bound UI controls, as shown below.

// notify of selection list change synchronously
myProp.FirePropertyChange(new PropertyChangeEventArgs(PropertyChange.Items, null, null, row));

// notify of selection list change asynchronously
await myProp.FirePropertyChangeAsync(new PropertyChangeEventArgs(PropertyChange.Items, null, null, row));

Cascading selection

During cascading selection, selecting some value(s) in one property restricts the list of possible values in another property, which in turn may restrict the list of values in other dependent properties. For example, selecting a Car Make would restrict the list of Car Models to the selected make, and selecting a model could further restrict a list of Car Trims.

EnumProperty provides support for the following two types of cascading selection:

  • one where all selection data is cached as lookup tables, and restricting the lists happens on the client via a filter function;
  • the one where the property needs to make a remote call to get a list of possible values based on the values of other properties, and cache the result locally as a lookup table.

Cascading by attributes

When headers of the lookup table for your EnumProperty have some data stored in their additional attributes, and you have other properties that contain values corresponding to those attributes, then you can make the list of your enum property filtered based on the values in those other properties.

To make the EnumProperty filter its list by the value of an attribute matching the value of another property, you need to call the SetCascadingProperty method, and pass it the attribute name and the instance of the other property. You can do it for several combination of attributes an other properties, as needed.

For example, if the headers in a list of car models have the car make and the body style in the "make" and "style" attributes respectively, then you can have the list of models automatically filtered by the selected make(s) and style(s) as follows.

// cascade models based on selected make
carModelProperty.SetCascadingProperty("make", carMakeProperty);

// cascade models based on selected body styles
carModelProperty.SetCascadingProperty("style", bodyStyleProperty);

This will automatically subscribe to any changes in the carMakeProperty and bodyStyleProperty properties, and will refresh the list of car models using the MatchesCascadingProperties method as part of the FilterFunc delegate. It will also remove any currently selected model(s) that don't match the values of the cascading properties.

note

The MatchesCascadingProperties method can handle both single and multiple values in either the target property or in the attribute of each header. For multiple values it will check if any of the selected values match any of the attribute values.

Handling null values

You can control how to handle the null values in either the target property or in the additional attributes. To specify how to handle a null value in the target cascading property, you can set the CascadingMatchNulls flag to one of the following values.

  • true - if null cascading value matches only values with attributes set to null.
  • false - if null cascading value matches any value (default).

For example, if no car make is selected, then you can show either all car models (false) or no car model (true), requiring the user to select the car make first. The latter assumes that all car models have a non-null value in their "make" attribute.

Similarly, you can set the NullsMatchAnyCascading flag to control how to handle null values in the attributes, as follows.

  • true - if null attribute value matches any value of the cascading property.
  • false - if null attribute value matches only null value of the cascading property (default).

For example, if some car models don't have a body style attribute, e.g. because it's unknown, then you can either always display them, regardless of which body style is selected (true), or display them only when no body style is selected (false). In the latter case, it may also display other models with a known body style, depending on how you configure the CascadingMatchNulls flag.

tip

If you set CascadingMatchNulls = true and NullsMatchAnyCascading = false, then the bodyStyleProperty with a null value will show only the car models with unknown body style. In this case, you may want to consider displaying the null body style option as "Unknown" by setting the NullString field as follows.

bodyStyleProperty.NullString = "Unknown";

Locally cached items

When the list of all possible items is too large or too dynamic, it may be impractical to cache the entire data set and then filter it on the client. To get the data set that is specific to the context of the current screen, it makes more sense to call a remote service with some parameters that are coming from the current context.

However, when using EnumProperty you'd still need to convert the resulting data set of available items to a LookupTable, so that the property could use it to look up and validate the values. To do that you want to create a parameterized local cache loader that calls your remote service based on the current parameters.

When you have your local cache loader, the EnumProperty allows you to set it as the LocalCacheLoader, and will use its cache to get the lookup table, as opposed to using the global cache when you set the property's EnumType.

note

You can always manually initialize the local cache with the context parameters, which will load the data, by calling the SetParametersAsync method on your cache loader, and then manually refresh the list for the property.

If the values of the parameters are sourced from another property though, then for each parameter you can call the SetCacheLoaderParameters method on your EnumProperty, and pass it the parameter name and an instance of the property to get the value from. This will start listening for changes in that property, and will refresh the list of values in response to any value changes. It will also clear any values that are invalid based on the new list.

For example, imagine that your Order Details screen has a product stored in the productProperty, and a list of special offers for the selected product in the specialOfferProperty. Since we don't want to globally cache all special offers for all products, we'll create a local cache loader ProductSpecialOfferLoader that has a parameter "productId", which it uses to call the remote service for getting the product's special offers.

Then all you have to do is to set your cache loader to the LocalCacheLoader of the specialOfferProperty, and call SetCacheLoaderParameters to tell the property to source the value of the "productId" parameter from the productProperty, as follows.

// initialize the local cache loader for product special offers
specialOfferProperty.LocalCacheLoader = new ProductSpecialOfferLoader(serviceProvider);

// source the productId parameter from the value of productProperty
specialOfferProperty.SetCacheLoaderParameters("productId", productProperty);

Now whenever you select a new product, the specialOfferProperty will automatically call the cache loader with the new product Id, and will refresh the list of available special offers. It will also clear any previously selected special offers that are no longer valid.

Typed enum properties

When you need to persist the data in your property or send it over to remote services, the EnumProperty will convert its value(s) to the Transport format, which is the IDs of the headers that it stores internally. Since headers have their IDs as strings, the value sent to the services will be string-based by default.

If your persisted value has a different type, such as a number or a GUID, then you should use one of the following specialized enum properties that Xomega Framework provides, or create a custom subclass, if they don't cover your requirements.

Integer-based enums

If the underlying data type of your enum values is int, long, short or byte, then you should use the EnumIntProperty, EnumLongProperty, EnumShortProperty and EnumByteProperty respectively.

For multi-valued properties the transport value will be of type List<T>, e.g. List<int>.

caution

These enum properties assume that the values in their lookup tables have IDs of the proper type.

If the header's Id cannot be parsed as the correct type, the transport value will be null, or not included in the list when it's multi-valued, which may cause unwanted side effects.

EnumBoolProperty

Sometimes when your services return the value for your data property as a bool, you want to show and edit it as a list of two values, such as Yes / No. In this case, you can create a simple static lookup table with two values, and use it with an EnumBoolProperty for your property.

You can use your own text for the headers in that lookup table, but their IDs should match one of the following values.

EnumGuidProperty

If the underlying data type of your enum values is Guid, then you should use the EnumGuidProperty. For multi-valued properties the transport value will be of type List<Guid>.

caution

EnumGuidProperty assumes that the IDs of the values in its lookup tables can be parsed as a Guid.

If the header's Id cannot be parsed as a Guid, the transport value will be null, or not included in the list when it's multi-valued, which may cause unwanted side effects.