Interface IFlagTracker

An interface for tracking changes in feature flag configurations.

Namespace: LaunchDarkly.Sdk.Server.Interfaces

Assembly: LaunchDarkly.ServerSdk.dll

Syntax

public interface IFlagTracker

Remarks

An implementation of this interface is returned by FlagTracker. Application code never needs to implement this interface.

Methods

FlagValueChangeHandler(string, Context, EventHandler<FlagValueChangeEvent>)

Creates a handler for receiving notifications when a specific feature flag's value has changed for a specific evaluation context.

Declaration

EventHandler<FlagChangeEvent> FlagValueChangeHandler(string flagKey, Context context, EventHandler<FlagValueChangeEvent> handler)

Parameters

Type	Name	Description
string	flagKey	the flag key to be evaluated
Context	context	the evaluation context
EventHandler<FlagValueChangeEvent>	handler	a handler that will receive a FlagValueChangeEvent

Returns

Type	Description
EventHandler<FlagChangeEvent>	a handler to be added to FlagChanged

Remarks

When you call this method, it first immediately evaluates the feature flag. It then returns a new event handler which you can add to the FlagChanged event. Whenever the specified feature flag changes, it re-evaluates the flag for the same user, and calls your handler if and only if the resulting value has changed. In other words, this method filters the more general FlagChangeEvent events to produce more specific FlagValueChangeEvent events.

All feature flag evaluations require an instance of Context. If the feature flag you are tracking does not have any targeting rules, you must still pass a dummy context such as Context.New("for-global-flags"). If you do not want the user to appear on your dashboard, use the Anonymous property: Context.Builder("for-global-flags").Anonymous(true).Build().

Examples

var flagKey = "my-important-flag";
var contextForFlagEvaluation = Context.New("context-for-evaluation");
var listenForNewValue = client.FlagTracker.FlagValueChangeHandler(
    flagKey,
    contextForFlagEvaluation,
    (sender, changeArgs) =>
        {
            System.Console.WriteLine("flag '" + changeArgs.Key
                + "' changed for " + contextForFlagEvaluation.Key
                + " from " + changeArgs.OldValue
                + " to " + changeArgs.NewValue);
        });
client.FlagTracker.FlagChanged += listenForNewValue;

Events

FlagChanged

An event for receiving notifications of feature flag changes in general.

Declaration

event EventHandler<FlagChangeEvent> FlagChanged

Event Type

Type	Description
EventHandler<FlagChangeEvent>

Remarks

This event is raised whenever the SDK receives any change to any feature flag's configuration, or to a segment that is referenced by a feature flag. If the updated flag is used as a prerequisite for other flags, the SDK assumes that those flags may now behave differently and sends flag change events for them as well.

Note that this does not necessarily mean the flag's value has changed for any particular evaluation context, only that some part of the flag configuration was changed so that it may return a different value than it previously returned for some context. If you want to track flag value changes, use FlagValueChangeHandler(string, Context, EventHandler<FlagValueChangeEvent>).

Change events only work if the SDK is actually connecting to LaunchDarkly (or using the file data source). If the SDK is only reading flags from a database (ExternalUpdatesOnly) then it cannot know when there is a change, because flags are read on an as-needed basis.

Notifications will be dispatched on a background task. It is the listener's responsibility to return as soon as possible so as not to block subsequent notifications.

Examples

client.FlagTracker.FlagChanged += (sender, eventArgs) => { System.Console.WriteLine("a flag has changed: " + eventArgs.Key); };