Jorge Canizales | 30697c9 | 2015-02-17 17:09:14 -0800 | [diff] [blame^] | 1 | #import <Foundation/Foundation.h> |
| 2 | |
| 3 | #import "GRXWriteable.h" |
| 4 | |
| 5 | typedef NS_ENUM(NSInteger, GRXWriterState) { |
| 6 | |
| 7 | // The writer has not yet been given a writeable to which it can push its |
| 8 | // values. To have an writer transition to the Started state, send it a |
| 9 | // startWithWriteable: message. |
| 10 | // |
| 11 | // An writer's state cannot be manually set to this value. |
| 12 | GRXWriterStateNotStarted, |
| 13 | |
| 14 | // The writer might push values to the writeable at any moment. |
| 15 | GRXWriterStateStarted, |
| 16 | |
| 17 | // The writer is temporarily paused, and won't send any more values to the |
| 18 | // writeable unless its state is set back to Started. The writer might still |
| 19 | // transition to the Finished state at any moment, and is allowed to send |
| 20 | // didFinishWithError: to its writeable. |
| 21 | // |
| 22 | // Not all implementations of writer have to support pausing, and thus |
| 23 | // trying to set an writer's state to this value might have no effect. |
| 24 | GRXWriterStatePaused, |
| 25 | |
| 26 | // The writer has released its writeable and won't interact with it anymore. |
| 27 | // |
| 28 | // One seldomly wants to set an writer's state to this value, as its |
| 29 | // writeable isn't notified with a didFinishWithError: message. Instead, sending |
| 30 | // finishWithError: to the writer will make it notify the writeable and then |
| 31 | // transition to this state. |
| 32 | GRXWriterStateFinished |
| 33 | }; |
| 34 | |
| 35 | // An object that conforms to this protocol can produce, on demand, a sequence |
| 36 | // of values. The sequence may be produced asynchronously, and it may consist of |
| 37 | // any number of elements, including none or an infinite number. |
| 38 | // |
| 39 | // GRXWriter is the active dual of NSEnumerator. The difference between them |
| 40 | // is thus whether the object plays an active or passive role during usage: A |
| 41 | // user of NSEnumerator pulls values off it, and passes the values to a writeable. |
| 42 | // A user of GRXWriter, though, just gives it a writeable, and the |
| 43 | // GRXWriter instance pushes values to the writeable. This makes this protocol |
| 44 | // suitable to represent a sequence of future values, as well as collections |
| 45 | // with internal iteration. |
| 46 | // |
| 47 | // An instance of GRXWriter can start producing values after a writeable is |
| 48 | // passed to it. It can also be commanded to finish the sequence immediately |
| 49 | // (with an optional error). Finally, it can be asked to pause, but the |
| 50 | // conforming instance is not required to oblige. |
| 51 | // |
| 52 | // Unless otherwise indicated by a conforming class, no messages should be sent |
| 53 | // concurrently to a GRXWriter. I.e., conforming classes aren't required to |
| 54 | // be thread-safe. |
| 55 | @protocol GRXWriter <NSObject> |
| 56 | |
| 57 | // This property can be used to query the current state of the writer, which |
| 58 | // determines how it might currently use its writeable. Some state transitions can |
| 59 | // be triggered by setting this property to the corresponding value, and that's |
| 60 | // useful for advanced use cases like pausing an writer. For more details, |
| 61 | // see the documentation of the enum. |
| 62 | @property(nonatomic) GRXWriterState state; |
| 63 | |
| 64 | // Start sending messages to the writeable. Messages may be sent before the method |
| 65 | // returns, or they may be sent later in the future. See GRXWriteable.h for the |
| 66 | // different messages a writeable can receive. |
| 67 | // |
| 68 | // If this writer draws its values from an external source (e.g. from the |
| 69 | // filesystem or from a server), calling this method will commonly trigger side |
| 70 | // effects (like network connections). |
| 71 | // |
| 72 | // This method might only be called on writers in the NotStarted state. |
| 73 | - (void)startWithWriteable:(id<GRXWriteable>)writeable; |
| 74 | |
| 75 | // Send didFinishWithError:errorOrNil immediately to the writeable, and don't send |
| 76 | // any more messages to it. |
| 77 | // |
| 78 | // This method might only be called on writers in the Started or Paused |
| 79 | // state. |
| 80 | // |
| 81 | // TODO(jcanizales): Consider adding some guarantee about the immediacy of that |
| 82 | // stopping. I know I've relied on it in part of the code that uses this, but |
| 83 | // can't remember the details in the presence of concurrency. |
| 84 | - (void)finishWithError:(NSError *)errorOrNil; |
| 85 | @end |
| 86 | |
| 87 | // A "proxy" class that simply forwards values, completion, and errors from its |
| 88 | // input writer to its writeable. |
| 89 | // It is useful as a superclass for pipes that act as a transformation of their |
| 90 | // input writer, and for classes that represent objects with input and |
| 91 | // output sequences of values, like an RPC. |
| 92 | @interface GRXWriter : NSObject<GRXWriter> |
| 93 | - (instancetype)initWithWriter:(id<GRXWriter>)writer NS_DESIGNATED_INITIALIZER; |
| 94 | @end |