The Various Agenda Models in .NET

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12.1 The Various Agenda Models
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Just as with the Contacts model, the Agenda Model is built on top of a database. However, there is not just one Agenda Model but three: The entry model (CAgnEntryModel) is the base model and provides access to an Agenda database. This model is not intended for user interface applications, but for more batch oriented tasks. Repeating entries (of which more below) are represented by a single entry object. The indexed model (CAgnIndexedModel) extends the entry model. It includes indices of the data in the database which support ltering and so it is easier to access a subset of the data than when using the entry model. The instance model (CAgnModel) extends the indexed model and provides a separate object for each instance of a repeating entry. This makes it suitable for use by a user interface (which is more concerned with showing all instances for a speci c date than with repeating patterns). The terms entry and instance used here are de ned in the next section. The Agenda Models support observers to allow an application to be informed when entries are added, altered or deleted. The Agenda Models support ltering to allow access to a subset of entries. The Agenda Models
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USING THE AGENDA MODEL
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support progress monitoring on long-running operations. The only such operation that we will be concerned with is opening a database with the indexed model. Alongside the Agenda Models, the Alarm Server keeps track of all pending alarms and noti es the user when any alarms are due. We will not deal with the Alarm Server directly, but we will see how to manipulate alarmed entries.
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Types of Agenda Entries
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The Agenda database (Figure 12.1) contains a number of types of entry: appointments (CAgnAppt) to-dos (CAgnTodo) events (CAgnEvent) anniversaries (CAgnAnniv).
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The Agenda database
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An appointment has a start date and time, an end date and time, and a range of properties. A to-do entry is a task and has a due date and priority, among other attributes. An event is like an appointment but lasts for one or more whole days. An anniversary is an event that repeats on an annual basis.
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12.3 Repeating Entries
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One of the features supported by the Agenda Model is repeating entries. An appointment or event may be a one-off, but it may also be a repeating entry a weekly meeting, a monthly event, etc. The Agenda Model allows such an entry to be de ned and the repetition con gured. This is very ef cient for storage and also allows a change to be applied to all instances of an appointment or event, but it is not ideal for displaying a day at a time or for changing one instance of a series. Therefore, the Agenda Model differentiates between entries and instances. For a repeating entry, the entry de nes the whole series, whereas an instance applies to only one date. When using the entry model or the indexed model, we are only presented with entries; if we want to access instances then we need to use the instance model, which is why it is aimed at user interfaces. In the instance model each repeating entry is converted into an instance for each date that applies. Non-repeating entries are also converted into instances, though only one instance each, of course. Because an instance is an entry on a date, an instance identi er contains an entry identi er and a date. This is worth remembering because most identi ers in Symbian OS are simply four-byte integers. One point to bear in mind is that the data of a repeating entry must be consistent across the whole of the series. Therefore, changing instance data will mean either altering all instances to have the same data or splitting the series into more than one series. If the data is changed for one instance only or if one instance is deleted, then the series is split to leave a separate series before the changed/deleted entry and a separate series after the change. If an instance is changed along with all entries before or after, the series is split into two. The Agenda Model will take care of this for us, but it means that changing or deleting an instance may affect instances for other dates. This is re ected in the return codes for some operations.
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