C++IdiomsinBREW,Part.docVIP

C++ Idioms in BREW, Part 2By  HYPERLINK /feedback.php/http://ws/brew/article.php/3370971 Radu Braniste Introduction This installment is entirely dedicated to a central BREW design pattern; Ive called it Beware of the Watchdog. Maybe this is the most significant distinction between BREW and other mobile platforms (a notable exception being C++ Blackberry): for the sake of simplicity, BREW doesnt hide all the details of the operating system; developers are forced to take additional responsibility. After describing the problem, the article presents a C++ idiom designed to solve the problem in an automatic way. Beware of the Watchdog Problem: In BREW programming, it is very important not to use a busy loop or poll for data Context: The Rex OS uses a watchdog to check the health of the running processes (threads). The watchdog has to be petted periodically to signal a healthy state. Forces: A linear, synchronous programming model is easier to understand and use. But, BREW offers a centrally scheduled, asynchronous, event-based model. On mobile, cooperative multitasking systems, tasks have to avoid tight loops but there is no real constraint (specifically time constraints) in yielding. In BREW, a developers responsibility is to pet the watchdog constantly (forced yield model). Solution [1]: Periodically yield control to the system during long operations such as tight loops The time spent inside the loop should be at most one cycle of the watchdog Use non-blocking calls or techniques. Example: Lets start with a very simple example, in pseudo-code: while(whileBody ()) {} doSomethingElse(); This is everyday programming, based on some well-established assumptions and expectations: whileBody() returns false or throws during execution. After exiting, the loop control is passed to the next instruction (doSomethingElse()), in a linear fashion. But, look what happens when moving from our well-behaved platform to BREW. First of all, the initial code might fail misera