Case Study

Loading Extensions

Since extensions can be uninstalled at any time, it makes sense that they have a Lifetime. If an extension is uninstalled, its Lifetime is terminated. Any registered actions will be run and any nested Lifetimes will also be terminated.

Settings Files

When an extension is first loaded, the ExtensionSettingsLoader class looks to see if it contains any settings files. If so, it calls into the Settings subsystem to load the files. The files are loaded with a Lifetime instance, and the Settings subsystem will register a callback that removes the settings file when that Lifetime terminates.

However, we also want to remove settings files when the extension is disabled. So, instead of calling the Settings subsystem with the extension's own Lifetime, ExtensionSettingsLoader creates a new Lifetime that is valid for the duration that an extension is enabled. Since this is a consecutive, non-overlapping duration, the settings loader uses an instance of SequentialLifetimes to manage this.

When the extension is enabled, SequentialLifetimes.Next is called to create a new Lifetime, and this can be passed to the Settings subsystem when loading the files. When the extension is disabled, SequentialLifetimes.TerminateCurrent causes this settings file Lifetime to be terminated, running the Settings subsystem's callback, which removes the files.

Since SequentialLifetimes creates a nested Lifetime with the extension's own Lifetime as the parent, when the extension is uninstalled, this parent Lifetime terminates, which terminates the nested Lifetime, and the settings files are automatically removed.

Enforcing Thread Affinity

To further complicate matters, there is a thread affinity issue here. When adding a settings file, it must be done on the main thread, since the list of settings layers are used as-is for binding to WPF (a design flaw, admittedly) and if they're not added on the main thread, WPF throws an exception. This requires the call into the Settings subsystem to be enqueued to run on the main thread.

However, we now run the risk of a race condition - if we enqueue an action to add the settings file on the main thread, it's possible that the extension is disabled or even uninstalled before the settings files can be loaded.

Fortunately, we can use a Lifetime when enqueuing the add action to the UI thread. We can use the Lifetime created by the SequentialLifetimes for this. If the extension is disabled, or uninstalled, the SequentialLifetimes class will terminate the Lifetime and the queued action is not executed.

Removing Settings Files

Removing the settings files is also complicated by the thread affinity issue.

Lifetime instances are not multi-threaded. That is, their callbacks are executed on the same thread that terminated the Lifetime. If there are nested Lifetime instances, then all callbacks of both the parent and the children are are executed on the same thread.

So, we can't add the settings files with a Lifetime that is a child of either the extension's own Lifetime, or the SequentialLifetimes class; if either of these terminate, then the callback is executed on the same thread that terminated it. This means the Settings subsystem will try to remove the settings on the same thread that either uninstalled or disabled the extension, and this is not necessarily the UI thread.

Lifetime doesn't provide any threading facilities, so we need to manage this ourselves. The answer is fairly straightforward though - create a new, unnested Lifetime (it's parent is EternalLifetime.Instance) and use it to load the settings file. This Lifetime is later explicitly terminated on the UI thread, by registering a callback with the SequentialLifetimes instance that calls LifetimeDefinition.Terminate.

The final thing to consider is the Lifetime used to enqueue the terminate action to the UI thread. Since this only happens when a Lifetime is terminated, we can't use any existing Lifetime instances (they've been terminated - the action wouldn't happen!), so in this case, we use EternalLifetime.Instance - we always want this to happen, because no-one else can terminate that unnested Lifetime. We also know that the action will be executed, and removed from the EternalLifetime instance, so there's no memory leak.

Summary

This means we have three Lifetime objects on the go.

• Firstly, the extension's own Lifetime. This is the main parent Lifetime. If it's terminated, all other Lifetime objects are also terminated.

• Secondly, we have a Lifetime created by the SequentialLifetimes object every time the extension is enabled, and terminated when it's disabled. This is used to enqueue a call to the main thread, and also used to register a callback to explicitly terminate the third Lifetime.

• The third Lifetime is an un-nested Lifetime that is passed to the Settings subsystem. Terminating this is what causes the settings files to be unloaded. This Lifetime is explicitly terminated on the UI thread. The terminate action is enqeued to the UI thread with the EternalLifetime to ensure it actually happens.

We now have a system of adding and removing the settings files when the extension's Enabled property changes. We also add and remove the settings files on the main thread, and don't add the settings files if the extension has been disabled before the main thread was available to run the command. Finally, everything is removed when the extension is uninstalled.

Consider implementing these same requirements with IDisposable.

• Firstly, we would need to know when the extension was being uninstalled, which IDisposable wouldn't give us. (We would need to register with some event, presumably exposed by the extension itself)

• Secondly, we would have to manage state to know if the files were currently loaded or not in the Settings subsystem and update this when the extension's enabled state changed.

• Thirdly, we would have to manage enqueuing to the main thread, and work around the race condition of disabling the extension before adding the files.

• Finally, we would have to explicitly tell the Settings subsystem to unload the settings files when either the extension was removed, or made disabled.

Lifetime manages a lot of state for us in this situation.