Resetting NSOperation - ios

Question

My network requests need to happen in a FIFO serial fashion. If one fails due to a network issue (i.e. offline, timeout) I need the failed request to retry before continuing the queue. How can I accomplish this?
When the operation fails, I set executing and finished both to - assuming that would work as a 'reset' for the NSOperation. But the queue never resumes.
Is there something I'm missing?

Answer 1

You could build a network operation that performs retries, and only sets isFinished when the network completion block is called, and it determines that no retry is needed:
class NetworkOperationWithRetry: AsynchronousOperation {
var session: NSURLSession
var request: NSURLRequest
var task: NSURLSessionTask?
var networkCompletionHandler: ((NSData?, NSURLResponse?, NSError?) -> ())?
init(session: NSURLSession = NSURLSession.sharedSession(), request: NSURLRequest, networkCompletionHandler: (NSData?, NSURLResponse?, NSError?) -> ()) {
self.session = session
self.request = request
self.networkCompletionHandler = networkCompletionHandler
}
override func main() {
attemptRequest()
}
func attemptRequest() {
print("attempting \(request.URL!.lastPathComponent)")
task = session.dataTaskWithRequest(request) { data, response, error in
if error?.domain == NSURLErrorDomain && error?.code == NSURLErrorNotConnectedToInternet {
print("will retry \(self.request.URL!.lastPathComponent)")
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 5 * Int64(NSEC_PER_SEC)), dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0)) {
self.attemptRequest()
}
return
}
print("finished \(self.request.URL!.lastPathComponent)")
self.networkCompletionHandler?(data, response, error)
self.networkCompletionHandler = nil
self.completeOperation()
}
task?.resume()
}
override func cancel() {
task?.cancel()
super.cancel()
}
}
And you could call it like so:
let queue = NSOperationQueue()
queue.maxConcurrentOperationCount = 1 // I wouldn't generally do this, but just to illustrate that it's honoring operation queue dependencies/concurrency settings
let requests = urlStrings.map { NSURLRequest(URL: NSURL(string: $0)!) }
requests.forEach { request in
queue.addOperation(NetworkOperationWithRetry(request: request) { data, response, error in
// do something with the `data`, `response`, and `error`
})
}
Now, that's only retrying on NSURLErrorNotConnectedToInternet, but you could change that logic to be whatever you want. Likewise, I'd probably be inclined to add some "max retries" logic. Also, if the lack of Internet connectivity is really the issue, rather than retrying until Internet connectivity is achieved, I'd probably be inclined to set the operation's isReady notification tied into Reachability.
By the way, the above uses the following AsynchronousOperation:
/// Asynchronous Operation base class
///
/// This class performs all of the necessary KVN of `isFinished` and
/// `isExecuting` for a concurrent `NSOperation` subclass. So, to developer
/// a concurrent NSOperation subclass, you instead subclass this class which:
///
/// - must override `main()` with the tasks that initiate the asynchronous task;
///
/// - must call `completeOperation()` function when the asynchronous task is done;
///
/// - optionally, periodically check `self.cancelled` status, performing any clean-up
/// necessary and then ensuring that `completeOperation()` is called; or
/// override `cancel` method, calling `super.cancel()` and then cleaning-up
/// and ensuring `completeOperation()` is called.
public class AsynchronousOperation : NSOperation {
override public var asynchronous: Bool { return true }
private let stateLock = NSLock()
private var _executing: Bool = false
override private(set) public var executing: Bool {
get {
return stateLock.withCriticalScope { _executing }
}
set {
willChangeValueForKey("isExecuting")
stateLock.withCriticalScope { _executing = newValue }
didChangeValueForKey("isExecuting")
}
}
private var _finished: Bool = false
override private(set) public var finished: Bool {
get {
return stateLock.withCriticalScope { _finished }
}
set {
willChangeValueForKey("isFinished")
stateLock.withCriticalScope { _finished = newValue }
didChangeValueForKey("isFinished")
}
}
/// Complete the operation
///
/// This will result in the appropriate KVN of isFinished and isExecuting
public func completeOperation() {
if executing {
executing = false
}
if !finished {
finished = true
}
}
override public func start() {
if cancelled {
finished = true
return
}
executing = true
main()
}
override public func main() {
fatalError("subclasses must override `main`")
}
}
extension NSLock {
/// Perform closure within lock.
///
/// An extension to `NSLock` to simplify executing critical code.
///
/// - parameter block: The closure to be performed.
func withCriticalScope<T>(#noescape block: Void -> T) -> T {
lock()
let value = block()
unlock()
return value
}
}

Answer 2

I used this custom class to manage NSOperation. I fiddled a bit with the code from GCDWebServer and came out with this stub. Most of it is self explanatory.
#interface SROperation : NSOperation <NSCopying>
///-------------------------------
/// #name error reporting
///-------------------------------
/**
The error, if any, that occurred in the lifecycle of the request.
*/
#property (nonatomic, strong) NSError *error;
///-----------------------------------------
/// #name the operation properties
///-----------------------------------------
/**
a dictionary.
*/
#property (nonatomic, strong) NSDictionary*aDictionary;
///----------------------------------
/// #name Pausing / Resuming Requests
///----------------------------------
+(instancetype)operationWithDictionary:(NSDictionary*)dict;
/**
Pauses the execution of the request operation.
A paused operation returns `NO` for `-isReady`, `-isExecuting`, and `-isFinished`. As such, it will remain in an `NSOperationQueue` until it is either cancelled or resumed. Pausing a finished, cancelled, or paused operation has no effect.
*/
- (void)pause;
/**
Whether the request operation is currently paused.
#return `YES` if the operation is currently paused, otherwise `NO`.
*/
#property (NS_NONATOMIC_IOSONLY, getter=isPaused, readonly) BOOL paused;
/**
Resumes the execution of the paused request operation.
Pause/Resume behavior varies depending on the underlying implementation for the operation class. In its base implementation, resuming a paused requests restarts the original request. However, since HTTP defines a specification for how to request a specific content range, `AFHTTPRequestOperation` will resume downloading the request from where it left off, instead of restarting the original request.
*/
- (void)resume;
#end
SROperation.m
#import "SROperation.h"
typedef NS_ENUM(NSInteger, SROperationState) {
SROperationPausedState = -1,
SROperationReadyState = 1,
SROperationExecutingState = 2,
SROperationFinishedState = 3,
};
static NSString * const kSROperationLockName = #"your.application.bundle.id.operationlock"
static inline BOOL SRStateTransitionIsValid(SROperationState fromState, SROperationState toState, BOOL isCancelled) {
switch (fromState) {
case SROperationReadyState:
switch (toState) {
case SROperationPausedState:
case SROperationExecutingState:
return YES;
case SROperationFinishedState:
return isCancelled;
default:
return NO;
}
case SROperationExecutingState:
switch (toState) {
case SROperationPausedState:
case SROperationFinishedState:
return YES;
default:
return NO;
}
case SROperationFinishedState:
return NO;
case SROperationPausedState:
return toState == SROperationReadyState;
default: {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wunreachable-code"
switch (toState) {
case SROperationPausedState:
case SROperationReadyState:
case SROperationExecutingState:
case SROperationFinishedState:
return YES;
default:
return NO;
}
}
#pragma clang diagnostic pop
}
}
static inline NSString * SRKeyPathFromSROperationState(SROperationState state) {
switch (state) {
case SROperationReadyState:
return #"isReady";
case SROperationExecutingState:
return #"isExecuting";
case SROperationFinishedState:
return #"isFinished";
case SROperationPausedState:
return #"isPaused";
default: {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wunreachable-code"
return #"state";
#pragma clang diagnostic pop
}
}
}
#interface SROperation ()
#property (readwrite, nonatomic, assign) SROperationState state;
#property (readwrite, nonatomic, strong) NSRecursiveLock *lock;
#end
#implementation SROperation
+(instancetype)operationWithDictionary:(NSDictionary*)dict {
return [[self alloc] initWithDictionary:dict];
}
-(instancetype)initWithDictionary:(NSDictionary*)aDictionary {
self = [self init];
if (self)
{
self.aDictionary = aDictionary;
self.queuePriority = NSOperationQueuePriorityVeryHigh;
}
return self;
}
-(instancetype)init
{
self = [super init];
if(self) {
_state = SROperationReadyState;
_lock = [[NSRecursiveLock alloc] init];
_lock.name = kSROperationLockName;
}
return self;
}
#pragma mark -
- (void)setState:(SROperationState)state {
if (!SRStateTransitionIsValid(self.state, state, [self isCancelled])) {
return;
}
[self.lock lock];
NSString *oldStateKey = SRKeyPathFromSROperationState(self.state);
NSString *newStateKey = SRKeyPathFromSROperationState(state);
[self willChangeValueForKey:newStateKey];
[self willChangeValueForKey:oldStateKey];
_state = state;
[self didChangeValueForKey:oldStateKey];
[self didChangeValueForKey:newStateKey];
[self.lock unlock];
}
- (void)pause {
if ([self isPaused] || [self isFinished] || [self isCancelled]) {
return;
}
[self.lock lock];
if ([self isExecuting]) {
dispatch_async(dispatch_get_main_queue(), ^{
NSNotificationCenter *notificationCenter = [NSNotificationCenter defaultCenter];
[notificationCenter postNotificationName:SROperationDidFinishNotification object:self userInfo:nil];
});
}
self.state = SROperationPausedState;
[self.lock unlock];
}
-(void)operationDidPause
{
}
- (BOOL)isPaused {
return self.state == SROperationPausedState;
}
- (void)resume {
if (![self isPaused]) {
return;
}
[self.lock lock];
self.state = SROperationReadyState;
[self start];
[self.lock unlock];
}
- (BOOL)isReady {
return self.state == SROperationReadyState && [super isReady];
}
- (BOOL)isExecuting {
return self.state == SROperationExecutingState;
}
- (BOOL)isFinished {
return self.state == SROperationFinishedState;
}
- (void)start {
[self.lock lock];
if ([self isCancelled]) {
[self cancelConnection];
} else if ([self isReady]) {
self.state = SROperationExecutingState;
[self operationDidStart];
}
[self.lock unlock];
}
- (void)operationDidStart {
[self.lock lock];
if (![self isCancelled]) {
// YOUR ACTUAL OPERATION CODE
// finish
self.state = SROperationFinishedState;
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SROperationDidFinishNotification object:nil];
});
}
[self.lock unlock];
}
- (void)cancel {
[self.lock lock];
if (![self isFinished] && ![self isCancelled]) {
[super cancel];
if ([self isExecuting]) {
}
}
[self.lock unlock];
}
#pragma mark - NSObject
- (NSString *)description {
[self.lock lock];
NSString *description = [NSString stringWithFormat:#"<%#: %p, state: %#, cancelled: %#>", NSStringFromClass([self class]), self, SRKeyPathFromSROperationState(self.state), ([self isCancelled] ? #"YES" : #"NO")];
[self.lock unlock];
return description;
}
#pragma mark - NSCopying
- (id)copyWithZone:(NSZone *)zone {
SROperation *operation = [(SROperation *)[[self class] allocWithZone:zone] init];
// copy more properties
return operation;
}
}