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QLoop Reference


QLAnchor

QLAnchor<Input>

  • type: class
  • conforms to: AnyAnchor

Creating

  • init( )

  • init(onChange: (Input?)->() )

  • init(onChange: (Input?)->(), onError: (Error)->() )

  • init(earlyRepeaters: QLAnchor.Repeater, ... )

  • init(lateRepeaters: QLAnchor.Repeater, ... )


Instance Variables

  • value: Input?

  • error: Error?

  • onChange: (Input?)->()

  • onError: (Error)->()

  • inputSegment: AnySegment?


Configuration

By default, QLAnchor always remembers the last value or error it received, but it releases them in production builds. This behavior can be disabled by setting the anchor global config releaseValues to false:

QLCommon.Config.Anchor.releaseValues = false


Discussion

An anchor is what facilitates the contract binding segments. To bind to an anchor essentially means to respond to its onChange(_) and/or onError(_) events.

An anchor :

  • can only receive a value or an error
  • may only have one subscriber
  • may have any number of input providers
  • can only retain one segment at a time
  • onChange and onError default to the main thread

QLAnchor implements a type of semaphore that makes use of synchronous dispatch queues around its value and error nodes. Inputs can safely arrive on any thread, and the events are guaranteed to arrive in serial fashion, although their order is not.

Repeaters

Repeaters offer a way to fork multiple streams off of the main path.

When an Anchor has repeaters applied, then it will echo any value and error changes to each of them.

By default, it forwards all changes to all repeaters. In order to make it conditional, include an EchoFilter. Return false from the EchoFilter to block that repeater from receiving the change.

EchoFilter
  • (Input?, QLAnchor) -> (Bool)

Default filter returns true. You can evaluate the input value and decide whether or not the particular anchor (repeater) should receive the new value.

To identify the anchor, you will need to do so using the object reference.

example:

let progressRepeater = viewController.progressAnchor
let finalRepeater = viewController.downloadCompleteAnchor

let baseAnchor = QLAnchor<DownloadStatus>(
    echoFilter: ({ obj, repeater in
        return (obj.isProgress && repeater === progressRepeater)
            || (obj.isFinal && repeater === finalRepeater)
    }),
    repeaters: progressRepeater, finalRepeater
)