/*

    File:       NetworkManager.h

    Contains:   A singleton to manage the core network interactions.

    Written by: DTS

    Copyright:  Copyright (c) 2010 Apple Inc. All Rights Reserved.

    Disclaimer: IMPORTANT: This Apple software is supplied to you by Apple Inc.

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                The Apple Software is provided by Apple on an "AS IS" basis. 

                APPLE MAKES NO WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

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*/

#import <Foundation/Foundation.h>

@interface NetworkManager : NSObject

{

    NSThread *                      _networkRunLoopThread;

    NSOperationQueue *              _queueForNetworkManagement;

    NSOperationQueue *              _queueForNetworkTransfers;

    NSOperationQueue *              _queueForCPU;

    CFMutableDictionaryRef          _runningOperationToTargetMap;

    CFMutableDictionaryRef          _runningOperationToActionMap;

    CFMutableDictionaryRef          _runningOperationToThreadMap;

    NSUInteger                      _runningNetworkTransferCount;

}

+ (NetworkManager *)sharedManager;

    // Returns the network manager singleton.

    //

    // Can be called from any thread.

- (NSMutableURLRequest *)requestToGetURL:(NSURL *)url;

    // Returns a mutable request that's configured to do an HTTP GET operation 

    // for the specified URL.  This sets up any request properties that should be 

    // common to all network requests, most notably the user agent string.

    //

    // Can be called from any thread.

// networkInUse is YES if any network transfer operations are in progress; you can only 

// call the getter from the main thread.

@property (nonatomic, assign, readonly ) BOOL           networkInUse;               // observable, always changes on main thread

// Operation dispatch

// We have three operation queues to separate our various operations.  There are a bunch of 

// important points here:

//

// o There are separate network management, network transfer and CPU queues, so that network 

//   operations don't hold up CPU operations, and vice versa.

//

// o The width of the network management queue (that is, the maxConcurrentOperationCount value) 

//   is unbounded, so that network management operations always proceed.  This is fine because 

//   network management operations are all run loop based and consume very few real resources.

//

// o The width of the network transfer queue is set to some fixed value, which controls the total 

//   number of network operations that we can be running simultaneously.

//

// o The width of the CPU operation queue is left at the default value, which typically means 

//   we start one CPU operation per available core (which on iOS devices means one).  This 

//   prevents us from starting lots of CPU operations that just thrash the scheduler without 

//   getting any concurrency benefits.

//

// o When you queue an operation you must supply a target/action pair that is called when 

//   the operation completes without being cancelled.

//

// o The target/action pair is called on the thread that added the operation to the queue.

//   You have to ensure that this thread runs its run loop.

//

// o If you queue a network operation and that network operation supports the runLoopThread 

//   property and the value of that property is nil, this sets the run loop thread of the operation 

//   to the above-mentioned internal networking thread.  This means that, by default, all 

//   network run loop callbacks run on this internal networking thread.  The goal here is to 

//   minimise main thread latency.

// 

//   It's worth noting that this is only true for network operation run loop callbacks, and is

//   /not/ true for target/action completions.  These are called on the thread that queued 

//   the operation, as described above.

//

// o If you cancel an operation you must do so using -cancelOperation:, lest things get 

//   very confused.

//

// o Both -addXxxOperation:finishedTarget:action: and -cancelOperation: can be called from 

//   any thread.

//

// o If you always cancel the operation on the same thread that you used to queue the operation 

//   (and therefore the same thread that will run the target/action completion), you can be 

//   guaranteed that, after -cancelOperation: returns, the target/action completion will 

//   never be called.

//

// o To simplify clean up, -cancelOperation: does nothing if the supplied operation is nil 

//   or if it's not currently queued.

//

// We don't do any prioritisation of operations, although that would be a relatively 

// simple extension.  For example, you could have one network transfer queue for gallery XML 

// files and another for thumbnail downloads, and tweak their widths appropriately.  And 

// don't forget, within a queue, a client can affect the priority of an operation using 

// -[NSOperation setThreadPriority:] and -[NSOperation setQueuePriority:].

- (void)addNetworkManagementOperation:(NSOperation *)operation finishedTarget:(id)target action:(SEL)action;

- (void)addNetworkTransferOperation:(NSOperation *)operation finishedTarget:(id)target action:(SEL)action;

- (void)addCPUOperation:(NSOperation *)operation finishedTarget:(id)target action:(SEL)action;

- (void)cancelOperation:(NSOperation *)operation;

@end