Observable List At A Glance

Dynamic data v4 has been taken out of beta and officially released.

For source code see Dynamic Data on GitHub
For install see Dynamic Data On Nuget

Dynamic data has always had an observable cache and now after a busy 4 months development and 188 commits there is an observable list. I also could not resist throwing in some aggregation operations. These new features are outlined in this post.

Getting Started

Create an observable list like this:

var myInts= new SourceList<int>();

There are direct edit methods, for example

myInts.AddRange(Enumerable.Range(0, 10000)); 
myInts.Add(99999); 
myInts.Remove(99999);

Each amend operation will produced a change notification. A much more efficient option is to batch edit which produces a single notification.

myInts.Edit(innerList =>
{
   innerList.Clear();
   innerList.AddRange(Enumerable.Range(0, 10000));
});

If SourceList is to be exposed publicly it can be made read only

IObservableList<int> readonlyInts = myInts.AsObservableList();

which hides the edit methods.

The list changes can be observed by calling myInts.Connect(). This creates an observable change set for which there are dozens of list specific operators. The changes are transmitted as an Rx observable so are fluent and composable.

Chain together operators

The following filters the observable list to include only odd numbers and applies a sum operation.

int total = 0;
var sumOfOddNumbers = myInts.Connect()
.Filter(i => i%2 == 1)
.Sum(i => i)
.Subscribe(result => total = result);

The result is a true observable. When values are added, replaced or removed from myInts , the result will always reflect these changes.

Create a derived list

Any observable change set can be converted into an observable list so for example


var myOddNumberList = myInts.Connect()
.Filter(i => i%2 == 1)
.AsObservableList();

produces a read only observable list of odd numbers.

Dynamic data compliments Rx but since Rx does not deal with collections I have created many list specific operators to account for adds, updates, replaces and moves. There are detail below.

Observable List Operators (35 operators and counting)

The following table is a brief description of all the observable operators which can be applied to the observable list. I will use to as a basic of further detailed documentation. This will take some time but will be done in near future.

Operator Description
Adapt Injects side effects using IChangeSetAdaptor interface.
AsObservableList Converts an observable change set into a read only observable list / hides the edit methods of a source list.
Bind Ensures a specified observable collection reflects the underling data
BufferIf Conditional buffer which accepts a boolean observable to turn buffering on or off
Clone Populate an IList instance with the latest changes.
Convert Light weight conversion / casting of a type as specified by a value selector
DeferUntilLoaded Delay subscription until the source is populated with data.
DisposeMany Dispose each item when removed from the stream. Also disposes all items when the stream is completed.
DistinctValues Return a changeset of distinct values as specified by a value selector
ExpireAfter Time based expiry for a source list
Filter The semantic equivalent of Rx.Where. There is an overload to allow the predicate to be changed
FlattenBufferResult A changeset is already a collection and therefore becomes a nested collection when Rx buffer operatons are applied. Use this to flatten the result back to a single change set
GroupOn Semantic equivalent of Rx.GroupBy operator
LimitSizeTo Limits the size of the source cache to the specified limit, providing a notification when an item is removed due to the size limit being exceeded
MergeMany Dynamically merges the observable which is selected from each item in the stream, and unmerges the item when it is no longer part of the stream.
NotEmpty Prevents a zero count change set notification
OnItemRemoved Callback for each item as and when it is being removed from the stream
Page Applies paging to the the data source
PopulateInto Populates an ISourceList instance with the latest changes
QueryWhenChanged The latest copy of the underlying data is exposed for querying i) after each modification to the underlying data ii) upon subscription
RefCount Change set equivalent to Publish().RefCount(). The source is cached so long as there is at least 1 subscriber.
SkipInitial Defer the subscription until there is underlying data loaded and skip first set of changes
Sort Allow sort operators using a specified comparer. There is an overload to allow the sort to be changed
SubscribeMany Subscribes to each item when it is added to the stream and unsubcribes when it is removed. All items will be unsubscribed when the stream is disposed
ToCollection Converts the changeset into a fully formed collection. Each change in the source results in a new read only collection
Top Limits the size of the result set to the specified number of items
Transform The semantic equivalent of Rx.Select
TransformMany Equivalent to a select many transform
Virtualise Virtualises the data. Parameters are provided and changed using a virtualising controller
WhenAnyChanged Property changed observation of each item in the underlying collection which provides a notification when any item has changed. The notification includes the sender and value of the changed property.
WhenAnyValueChanged Property changed observation of each item in the underlying collection which provides a notification when any item has changed. The notification only includes the value of the changed property.
WhereReasonsAre Includes changes for the specified reasons only
WhereReasonsAreNot Excludes changes for the specified reasons
WithKey Applies a key to each item. This changes the change set from a list change set to a cache changeset

This list has less operators than the observable cache but will be expanded in future releases.

Aggregation Operators

The following aggregation are also new to Dynamic Data version 4. The apply to both observable list and observable cache. With the exception of Count() which requires no parameter, the value to continually compute on is specified as a value selector Func valueSelector

Operator Syntax
Count Counts the number of items in the underlying data
Sum The sum of values matching the specified value selector
Avg The average value matching the specified value selector
Maximum The maximum value matching the specified value selector
Minimum The minimum value matching the specified value selector
StdDev The standard deviation of all the values matching the specified value selector
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