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Queue

A persistent background job queue for iOS.

While NSOperation and NSOperationQueue work well for some repetitive problems and NSInvocation for others, iOS doesn't really include a set of tools for managing large collections of arbitrary background tasks easily. EDQueue provides a high-level interface for implementing a threaded job queue using GCD and SQLLite3. All you need to do is handle the jobs within the provided delegate method and EDQueue handles the rest.

Getting Started

The easiest way to get going with EDQueue is to take a look at the included example application. The Xcode project file can be found in Project > queue.xcodeproj.

Setup

EDQueue needs both libsqlite3.0.dylib and FMDB for the storage engine. As always, the quickest way to take care of all those details is to use CocoaPods. EDQueue is implemented as a singleton as to allow jobs to be created from anywhere throughout an application. However, tasks are all processed through a single delegate method and thus it often makes the most sense to setup EDQueue within the application delegate:

YourAppDelegate.h

#import "EDQueue.h"
@interface YourAppDelegate : UIResponder <UIApplicationDelegate, EDQueueDelegate>

YourAppDelegate.m

- (void)applicationDidBecomeActive:(UIApplication *)application
{
    [[EDQueue sharedInstance] setDelegate:self];
    [[EDQueue sharedInstance] start];
}

- (void)applicationWillResignActive:(UIApplication *)application
{
    [[EDQueue sharedInstance] stop];
}

- (EDQueueResult)queue:(EDQueue *)queue processJob:(NSDictionary *)job
{
    sleep(1);           // This won't block the main thread. Yay!
    
    // Wrap your job processing in a try-catch. Always use protection!
    @try {
        if ([[job objectForKey:@"task"] isEqualToString:@"success"]) {
            return EDQueueResultSuccess;
        } else if ([[job objectForKey:@"task"] isEqualToString:@"fail"]) {
            return EDQueueResultFail;
        }
    }
    @catch (NSException *exception) {
        return EDQueueResultCritical;
    }
    
    return EDQueueResultCritical;
}

SomewhereElse.m

[[EDQueue sharedInstance] enqueueWithData:@{ @"foo" : @"bar" } forTask:@"nyancat"];

In order to keep things simple, the delegate method expects a return type of EDQueueResult which permits three distinct states:

  • EDQueueResultSuccess: Used to indicate that a job has completed successfully
  • EDQueueResultFail: Used to indicate that a job has failed and should be retried (up to the specified retryLimit)
  • EDQueueResultCritical: Used to indicate that a job has failed critically and should not be attempted again

Handling Async Jobs

As of v0.6.0 queue includes a delegate method suited for handling asyncronous jobs such as HTTP requests or Disk I/O:

- (void)queue:(EDQueue *)queue processJob:(NSDictionary *)job completion:(void (^)(EDQueueResult))block
{
    sleep(1);
    
    @try {
        if ([[job objectForKey:@"task"] isEqualToString:@"success"]) {
            block(EDQueueResultSuccess);
        } else if ([[job objectForKey:@"task"] isEqualToString:@"fail"]) {
            block(EDQueueResultFail);
        } else {
            block(EDQueueResultCritical);
        }
    }
    @catch (NSException *exception) {
        block(EDQueueResultCritical);
    }
}

Introspection

As of v0.7.0 queue includes a collection of methods to aid in queue introspection specific to each task:

- (Boolean)jobExistsForTask:(NSString *)task;
- (Boolean)jobIsActiveForTask:(NSString *)task;
- (NSDictionary *)nextJobForTask:(NSString *)task;

Methods

- (void)enqueueWithData:(id)data forTask:(NSString *)task;

- (void)start;
- (void)stop;
- (void)empty;

- (Boolean)jobExistsForTask:(NSString *)task;
- (Boolean)jobIsActiveForTask:(NSString *)task;
- (NSDictionary *)nextJobForTask:(NSString *)task;

Delegate Methods

- (EDQueueResult)queue:(EDQueue *)queue processJob:(NSDictionary *)job;
- (void)queue:(EDQueue *)queue processJob:(NSDictionary *)job completion:(void (^)(EDQueueResult result))block;

Result Types

EDQueueResultSuccess
EDQueueResultFail
EDQueueResultCritical

Properties

@property (weak) id<EDQueueDelegate> delegate;
@property (readonly) Boolean isRunning;
@property (readonly) Boolean isActive;
@property NSUInteger retryLimit;

Notifications

EDQueueDidStart
EDQueueDidStop
EDQueueDidDrain
EDQueueJobDidSucceed
EDQueueJobDidFail

iOS Support

EDQueue is designed for iOS 5 and up.

ARC

EDQueue is built using ARC. If you are including EDQueue in a project that does not use Automatic Reference Counting (ARC), you will need to set the -fobjc-arc compiler flag on all of the EDQueue source files. To do this in Xcode, go to your active target and select the "Build Phases" tab. Now select all EDQueue source files, press Enter, insert -fobjc-arc and then "Done" to enable ARC for EDQueue.