Abstract: Adds an array of numbers (very slowly) and returns the result.
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AdderOperation is an example of using NSOperation to do asynchronous processing
in a sane fashion. It follows the thread confinement model. That is, you
initialise the operation with an array of numbers to operate on, and it adds up
those numbers and vends the result through read-only properties.
The data used by the operation are made thread safe in a number of different ways:
o Some properties (for example, numbers) are set up at initialisation time and
can't be changed. These are immutable, thus can be safely shared between threads.
o Some properties must be set before the operation is queued and should be immutable
after that. Again, these are thread safe because there's no possibility of two
threads trying to mutate the same value at the same time. interNumberDelay
is an example of this.
o Some properties are set by the operation but shouldn't be read by the client
until the operation is finished. total and formattedTotal are examples of this.
o Some properties, like formatter, are private to the operation and are only ever
accessed by the thread running the operation.
o Global data, like sSequenceNumber, is protected by a lock.
It's obvious that adding a few numbers is going to happen very quickly, so we
artifically slow things down by sleeping for an extended period of time between
additions. This delay is controlled by the interNumberDelay, which defaults to
@interface AdderOperation : NSOperation
- (instancetype)initWithNumbers:(NSArray *)numbers;
// set up by the init method that can't be changed
@property (atomic, copy, readonly ) NSArray * numbers; // of NSNumber
@property (atomic, assign, readonly ) NSUInteger sequenceNumber;
// must be configured before the operation is started
@property (atomic, assign, readwrite) NSTimeInterval interNumberDelay; // defaults to 1.0
// only meaningful after the operation is finished
@property (atomic, assign, readonly ) NSInteger total;
@property (atomic, copy, readonly ) NSString * formattedTotal;