When the Core is created by the Environment, it must be as light-weight as possible and must not take any decisions on actions without the UI/Environment explicitly stating so.
Examples of actions that are not allowed are:
- Opening of files
- Connections to networks
- Synchronization of favorites
- Starting of Threads
The Core is however free to (and should) take follow-up decisions to achieve a goal requested by the UI/Environment.
Examples of allowed decisions are:
- Switch-overs to new servers due to the current server being offline
- Fall-backs to alternative persistent storage methods due to lack of capabilities or permissions
At least on the Android platform, we can expect the Core to be created on the event dispatcher. The Core creation must therefor be slimmed to a bare minimum of object creation to ensure that it’s quick and efficient and can be run on for example the event dispatcher without problems.
Additionally the Core cannot predict what parts of the Core will actually be in use by the application. Therefor we will leave it up to the UI or Environment to request actions as appropriate.
Core method callbacks to UI
When the Core is created by the UI, the UI will provide the Core with a stub implementation that decides which thread will invoke the callbacks to the UI. The Core will guarantee that the provided method will be used for all callbacks to the UI.
For the UI, there is much that can be gained if the callback is performed in the event dispatcher thread. Populating a view will be faster and easier to do since there is no need for the UI to reschedule such a task back onto the dispatcher.
However, there are also situations where the event dispatcher is not available or even bad to use due to demands on platforms or products. One example is if the Core will be used on a BlackBerry background task where there is no event dispatcher at all.
Because of this, the decision on how the callbacks will be made will be up to the UI. The Core will in turn guarantee that the provided method will be used for all callbacks to the UI.
Core method invocations
No exposed methods in the Core may block. Any calls that end up in blocking code (such as I/O operations or server communication) must be done asynchronous with use of callbacks to the UI. Any exposed synchronous methods must return quickly.
Each method will as parameter take the listener that will be invoked upon completion (or failure) of the request. Example code:
Since the calls to the Core will be very much driven by the UI and the user interaction, we can expect most (if not all) Core calls to be invoked by the event dispatcher.
Since a blocking call on the event dispatcher will result in a non-responsive UI, blocking methods will not be present in the Core interfaces against the UI.
Please note that the internal methods in the Core and the Core ↔ Platform interaction may still contain blocking calls, as long as the UI never know they are there.
Objects and data shared between UI and Core
All objects shared between the UI and Core must either be immutable or one-shot objects, with a preference for immutable objects.
If a mutable object is used, it must be considered gone once it has passed into the realm of the other part.
All objects passed to the other part must be considered ”easy to use” without too much in-depth knowledge of the inner workings. The owning module (package) is allowed to exploit the inner structure of data objects as long as concurrency correctness is preserved. Only in rare, performance critical, and well documented, circumstances should the object change state after it has passed the line of demarcation. This is subject to normal best-practices for encapsulation.
Due to the nature of the Core being disconnected completely from the UI, we will not be able to rely on the notion that the other part knows what the first part is doing.
We must also be prepared for a situation where the Core (or parts of it) will be shipped to a third party who will use it without any knowledge of how the Core works internally.
Due to this any data that is passed between the Core and the UI must be in a state that allows the other party to freely use it as it sees best.
All methods in the Core that runtime will be disallowed due to permission restrictions in the currently running platform will still be available for use, but will result in the callback containing an error (SecurityException for synchronous methods).
Since lack of permissions can influence what the user can actually do, it will be the responsibility of the UI to ”grey out” or hide any actions not allowed by the current level of permissions.
An interface for checking the current level of permissions will be made available via the Platform component.
While the pre-installed versions of our clients will often have the permission problem ”removed” by carrier signing, the problem will still remain for both non-carrier clients and (at least) the BlackBerry platform which does not offer the possibility to remove security by signing.