cts Centralized Task Scheduling

Allows applications to hook into system events such as being plugged into a power socket or when the CPU is idle

Schedule Background Activity
Schedule Background Networking
Defer Tasks with XPC Activity

Grand Central Dispatch (GCD) in OS X manages concurrent and asynchronous execution of tasks so they can be queued and scheduled for execution as resoures become available. Schedual Background Activity

Create a Scheduler
NSBackgroundActivityScheduler API similar to an NSTimer object.

NSBackgroundActivityScheduler *activity = [[NSBackgroundActivityScheduler alloc] 
            initWithIdentifier:@"com.example.MyApp.updatecheck"];

Configure Scheduler Properties
```
activity.tolerance = 10 * 60;activity.interval = 30 * 60;activity.repeats = YES;
```
QualityOfService : The default value is NSQualityOfServiceBackground leave it.

Schedule Activity with scheduleWithBlock

[activity
scheduleWithBlock:^(NSBackgroundActivityCompletionHandler completion) {
   // Perform the activity
   self.completion(NSBackgroundActivityResultFinished);
}];

Detect Whether to Defer Activity
While a lengthy activity is running, the activity may now requiring deferral. For example, power source is disconnected. Activity should call shouldDefer value of YES indicates that the block should finish up and invoke its completion handler .
```
if ([activity.shouldDefer]) {
   // Wrap up processing and prepare to defer activity
   self.completion(NSBackgroundActivityResultDeferred);
} else {
   // Continue processing
   self.completion(NSBackgroundActivityResultFinished);
};
```
Stop Activity
```
[activity invalidate];
```

Specify Nondeferrable Background Activities

NSOperationQueue *myQueue = [[NSOperationQueue alloc] init];
id myBackgroundActivity = [[NSProcessInfo processInfo]
      beginActivityWithOptions: NSActivityBackground
      reason: @"Doing important background work"];
[myQueue addOperationWithBlock:^{
   // Do important background work here
   [[NSProcessInfo processInfo] endActivity:myBackgroundActivity];
}];