Plugins with user interface

A while ago I wrote about plugins and now I received a mail from somebody asking about plugins that extend the UI of an application. From one side, these "plugins" are the cornerstone of Android application design: activity invocation can be considered a "UI plugin mechanism". But what if somebody wants to plug in UI elements into an application's screen so that the UI elements provided by the plugin appear on the application's own screen? Limited screen estate makes these sorts of plugins less relevant than in case of PC applications but there is a precedent: widgets plug into the Launcher's screen this way. I decided therefore to play with the idea and created a prototype.

Click here to download the application.

You will find 3 subdirectories in the package (respluginapp, resplugin1, resplugin2), each is a separate Android project. Compile and install respluginapp and launch the application. You will see something like this:
Now compile and install resplugin1 and resplugin2. You will notice that the screen of respluginapp dynamically changes as you install the plugin packages. Eventually it will look like this:



The prototype idea is simple. We have 4 rows on the screen. Each row has a default content (some text) but if a plugin is installed, the default row content is replaced by the UI elements provided by the plugin.

Sounds simple but it isn't.

• Getting the layout from the newly installed plugin package and recognizing the plugin package is the easy part. The plugin package is recognized because it exposes a service with aexp.intent.action.PICK_RESPLUGIN intent action. That service will be used to handle plugin events. There is already a hack here: the name of a layout is available only to the application in the package because the resource compiler generates R.java that maps layout names to layout IDs. As the plugin host application cannot access this R.java, it has no access to layout names. Our plugin host application accesses the plugin's row layout by ID (0x7f030000 is hardcoded). This ID is allocated to the layout whose name is the first alphabetically among the layouts. As our plugins have only one layout in the package, it is not really an issue but this hack can cause quite a nightmare in larger projects.
• The next hack happens with widget IDs. The resource compiler assigns IDs sequentially. Unfortunately it uses the same ID base for the plugin host application and the plugins so chances are that the IDs in the plugin host application and in the layouts of the plugins overlap. The plugin host application solves it by adding a row-specific offset to the IDs of the UI elements when they are loaded from the layout provided by the plugin package. But the downside is that from this moment, IDs used by the plugin host application and IDs used by the plugin will be different and the plugin host application has to constantly convert back and forth when it sends events to the plugin event handler service.
• While the UI elements reside in the plugin host application's screen, the event handling is provided by a service exposed by the plugin. The event handling looks like the following: the event is intercepted in the plugin host application, the state of the plugin UI elements is serialized, the plugin event handler service is invoked that processes the event using the serialized plugin UI element state and spits out actions, how the plugin UI elements should be updated. Only some basic functionality is implemented in this prototype. Only button click events are intercepted and the state capture/update only works for TextViews and descendants. But you get the idea how complex it would be to do it properly. While fiddling with this part, I ran into a dead end street because I thought RemoteViews could be used to implement the serialization/deserialization. RemoteViews, however, does not support EditText so this approach had to be abandoned.

Is this something you should do? Well, I am not sure. Pulling UI events between host applications and services are not trivial and also it can be slow because by default the plugins are in a different package therefore in a different Linux process (see further explanation here). Our button click handlers probably do not consume so much CPU time but it can be worse with complex and frequent events. Also, the serialization mechanism presented here is kind of naive, check out the RemoteViews source to check, how deep you can go.

On the other hand, applications that change their screen layout when you install plugins are kind of cool as the Android widgets demonstrate. Decide yourself.

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Plugins

I blogged 2 years ago that Android is a surprisingly modern application platform, I even said that it was the most modern application platform. This is partly due to Android's advanced component features. These features are not always evident to the naked eye so I present here a standard exercise to demonstrate them.

Click here to download the example program.

In this exercise we will implement a plugin framework for an example program. Plugins are components that connect to the main application over a uniform interface and are dynamically deployable. The download package contains two Android applications. The first, pluginapp is the example application that will use the plugins. The second, plugin1 is a plugin package that contains two plugins for pluginapp. Plugin1 package is not visible among the executable applications, it does not expose any activity that may be launched by the end user. If deployed, however, its plugins appear in pluginapp. Plugin deployment/undeployment is dynamic, if you uninstall plugin1 package while pluginapp is running, pluginapp notices the changes and the plugins disappear from the plugin list.




In order to implement these functionalities, the following features of the Android framework were used.

• Each plugin is a service. Plugin1 exposes two of them. In order to identify our plugins, I defined a specific Intent (aexp.intent.action.PICK_PLUGIN) and I attached an intent filter listening to this intent in plugin1's AndroidManifest.xml. In order to select a particular plugin, I abused the category field again. One plugin can be accessed therefore by binding a service with an intent whose action is aexp.intent.action.PICK_PLUGIN and whose category equals to the category listed in AndroidManifest.xml of the package that exposes the plugin service.
• Plugins are discovered using the Android PackageManager. Pluginapp asks the PackageManager to return the list of plugins that are bound to aexp.intent.action.PICK_PLUGIN action. Pluginapp then retrieves the category from this list and can produce an intent that is able to bindthe selected plugin.
• Pluginapp updates the plugin list dynamically by listening to ACTION_PACKAGE_ADDED, ACTION_PACKAGE_REMOVED and ACTION_PACKAGE_REPLACED intents. Whenever any of these intents are detected, it refreshes the plugin list.

Even though Android is not as sophisticated as OSGi when it comes to component framework, it has every necessary element. It has a searchable service registry, packages can be queried for the services they expose and there are deployment events.

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