iOS 7 Programming Cookbook

7.13. Creating Dependency Between Operations

Problem

You want to start a certain task only after another task has finished executing.

Solution

If operation B has to wait for operation A before it can run the task associated with it, operation B has to add operation A as its dependency using the addDependency: instance method of NSOperation, as shown here:

[self.firstOperation addDependency:self.secondOperation];

Both the firstOperation and the secondOperation properties are of type NSInvocationOperation, as we will see in this recipe’s Discussion. In this example code, the first operation will not be executed by the operation queue until after the second operation’s task is finished.

Discussion

An operation will not start executing until all the operations on which it depends have successfully finished executing the tasks associated with them. By default, an operation, after initialization, has no dependency on other operations.

If we want to introduce dependencies to the example code described in Recipe 7.12, we can slightly modify the application delegate’s implementation and use the addDependency: instance method to have the first operation wait for the second operation:

#import "AppDelegate.h"

@interface AppDelegate ()
@property (nonatomic, strong) NSInvocationOperation *firstOperation;
@property (nonatomic, strong) NSInvocationOperation *secondOperation;
@property (nonatomic, strong) NSOperationQueue *operationQueue;
@end

@implementation AppDelegate

- (void) firstOperationEntry:(id ...