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Should the Payload Be Immutable
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Some arguments might be passed by reference in a notification. For example, languages like C#, Java, and Delphi Object Pascal pass all objects by reference. Doing so opens the possibility of the notification handler modifying those objects, possibly causing undesirable side effects later in the caller. There is no conceptual reason why the objects passed to event handlers should be immutable when dealing with unicast events. Such objects are merely the arguments of a procedure call. With multicast events, things are different. Why Assume a notification carries a by-reference argument P and that the sender expects recipients to change the state of P while handling the notification. In a
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CHAPTER 4 NOTIFICATION PAYLOADS
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multicast situation, the first handler might set P to one state, but a subsequent handler might set P to a different state. After multicasting the notification to all subscribers, P would have the state set by the last event handler. The states saved by the previous handlers would be lost. Such a system would probably behave erratically, depending on the multicast order to subscribers. As a practical example, consider a multicast notification used to allow subscribers to cancel a Closing operation that is about to take place. For the purposes of this discussion, it doesn t matter what the Closing operation is about to close. What counts is the fact that event subscribers are given the chance of vetoing an operation in progress by returning a certain value. The notification might have the following C# signature: void OnClosing(ref bool doCancel); An event handler would set doCancel to true to veto the Closing operation. If there were two handlers, one called after the other, the second handler would overwrite the first one s doCancel value. When dealing with multicast events, you must take special precautions to safeguard any byreference parameters set by notification handlers. Such parameters are passed by address, so the callee has the opportunity to replace the object being pointed at with a new one. Fortunately, the solution is simple and involves the use of envelopes, described at the end of this chapter. An envelope is nothing more than a holder of objects. Envelopes allow event handlers to put values into it, without disturbing values already in it. The envelope then can carry back to the sender all the values set by each event handler.
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String-Based Payloads
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It is very common to use payloads containing nothing but strings. After all, strings can be used to represent many different scalar values, like Booleans, numbers, and dates. When a payload uses only strings to encode scalar types, the notification recipient incurs no type coupling to custom types. For Internet-based distributed systems, UTF-8 character encoding is the most efficient for Western languages. UTF-16 is required for most Asian character sets. As an example of a string-based payload, consider a component that might display book pages to the user. When the user changes the page, the component might fire an event. The payload could provide information about the selected page, such as the page title, page number, and document author, using the payload shown in Figure 4-2.
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Property Name "PageTitle" Property Value "Baking a Potato" "14" "Peter Hughes"
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Message
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"PageNumber" "DocumentAuthor"
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Figure 4-2. A message with a string payload If the payload contains variable-length property names and values, a header would need to describe the position of each payload element. There are several design variations that can improve performance. If the payload always contains the same properties and in the same order, which is a very common scenario, then the property names could be omitted. If the values have varying lengths, the payload might need a header to indicate where each property value begins. The header could help recipients access each value randomly. If the property values have assigned a fixed size, a header isn t necessary. The receiver can access any property value randomly, because it knows the location of all the values. The drawback is that
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