# Delay

Delay is an easy-to-use micro-library for delaying code execution in a cancellable manner. It uses Grand Central Dispatch and blocks.

## Installation

[![Carthage compatible](https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat)](https://github.com/Carthage/Carthage)

### 1. Update your Cartfile

**Swift 3 (latest version) – master**

```
git "git@source.ind.ie:project/delay.git"
```

For Swift 2.3, use the swift-2.3 branch:

```
git "git@source.ind.ie:project/delay.git" "swift-2.3"
```

### 2. Add the framework to your Xcode project

[Follow the instructions on Carthage’s readme](https://github.com/Carthage/Carthage#adding-frameworks-to-an-application).

## Demos

1. Run the `./install` script to install the dependencies for the demo apps.
2. Run the `./dev` script and play with the iOS and macOS demo apps.

## Usage

Delay has three main use cases:

  * Delay execution of a block to the next stack frame (next frame of the runloop)
  * Delay execution by N seconds
  * Delay execution in a cancellable manner

### Execute on next stack frame

```swift
_ = delay(0.0)
{
  // Do something.
}
```

### Execute after an arbitrary number of seconds

```swift
_ = delay(42.0)
{
  // Do something.
}
```
### Cancel before execution

```swift
let cancellableBlock = delay(42.0)
{
  // Do something.
}
cancellableBlock.cancel()
```

### Idiom: throttle user input

```swift
var cancellableCommand:CancellableDelayedCommand?

// …

// Throttle expensive operation so it is performed at
// most every second, no matter how often a signal is received.
cancellableCommand = cancellableCommand?.reset() ?? delay(1.0)
{
  // Perform expensive operation:
  // …
}
```

For example, you can use this to implement auto-complete without flooding the lookup method on every keystroke.

```swift
var textDidChangeHandler:NotificationHandler?
var cancellableAutoCompleteCommand:CancellableDelayedCommand?

// …

override func viewWillAppear()
{
  textDidChangeHandler = handle(NSControlTextDidChangeNotification, from: myTextInput)
  {
    /* with */ notification in

    let text = self.myTextInput.stringValue

    // Throttle auto-complete lookups to every 1/3rd of a second.
    cancellableAutoCompleteCommand = cancellableAutoCompleteCommand?.reset() ?? delay(0.3)
    {
      // Perform expensive operation: look-up text for auto-complete
      // …
    }
  }
}

override func viewWillDisappear()
{
  cancellableAutoCompleteCommand.cancel()
  textDidChangeHandler.remove()
}
```

Read more about the library [in this blog post](https://ind.ie/labs/blog/delay) on [Ind.ie Labs](https://ind.ie/labs).

## On performance

Note that Delay is optimised for ease of authoring, beauty of interface, and clarity of intent. I haven’t run into performance issues and hence haven’t felt the need to run any benchmarks at the moment but [Evgenii Rtishchev’s purely-block-based CancelBlocks](https://github.com/katleta3000/CancelBlocks/blob/master/CancelBlocks.swift), on which my solution is based, is a lighter alternative should you need it.

## Credits

Delay based on the work of [Evgenii Rtishchev](https://github.com/katleta3000/CancelBlocks) and [Chris Brind](http://stackoverflow.com/questions/24034544/dispatch-after-gcd-in-swift/24318861#24318861) (with thanks to [Cezary Wojcik](http://stackoverflow.com/a/24034838/253485).)

Copyright © Aral Balkan.
Released with ♥ by [Ind.ie](https://ind.ie) under the MIT License.