Using Combine

https://heckj.github.io/swiftui-notes/
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An intermediate to advanced book focusing on how to use Apple's Combine framework, a declarative...
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### Manually Connect a Timer Publisher

Source: https://heckj.github.io/swiftui-notes/index_zh-CN.html

Create a Timer publisher and manually connect it using .connect() to start receiving values. This allows for delayed activation of the timer.

```swift
let timerPublisher = Timer.publish(every: 1.0, on: RunLoop.main, in: .default)
let cancellableSink = timerPublisher
    .sink {
        print("passed through: ", $0)
    }
// no values until the following is invoked elsewhere/later:
let cancellablePublisher = timerPublisher.connect()
```

--------------------------------

### Swift Combine Pipeline Example

Source: https://heckj.github.io/swiftui-notes/index.html

Demonstrates a basic Combine pipeline starting with a `Just` publisher, transforming the value with a `map` operator, and terminating with a `sink` subscriber. The `Just` publisher emits a single value and never fails. The `map` operator transforms the emitted value's type, and `sink` acts as the final subscriber to process the result.

```swift
let _ = Just(5) \
    .map { value -> String in \
        // do something with the incoming value here
        // and return a string
        return "a string"
    }
    .sink { receivedValue in \
        // sink is the subscriber and terminates the pipeline
        print("The end result was \(receivedValue)")
    }
```

--------------------------------

### FormViewController Setup with Combine

Source: https://heckj.github.io/swiftui-notes/index.html

Sets up IBOutlets for text fields, buttons, and labels, along with IBAction methods to update @Published properties when text fields change. This forms the basis for reactive updates.

```swift
import UIKit
import Combine

class FormViewController: UIViewController {

    @IBOutlet weak var value1_input: UITextField!
    @IBOutlet weak var value2_input: UITextField!
    @IBOutlet weak var value2_repeat_input: UITextField!
    @IBOutlet weak var submission_button: UIButton!
    @IBOutlet weak var value1_message_label: UILabel!
    @IBOutlet weak var value2_message_label: UILabel!

    @IBAction func value1_updated(_ sender: UITextField) { __**(1)**
        value1 = sender.text ?? ""
    }
    @IBAction func value2_updated(_ sender: UITextField) {
        value2 = sender.text ?? ""
    }
    @IBAction func value2_repeat_updated(_ sender: UITextField) {
        value2_repeat = sender.text ?? ""
    }

    @Published var value1: String = ""
    @Published var value2: String = ""
    @Published var value2_repeat: String = ""

    var validatedValue1: AnyPublisher<String?, Never> { __**(2)**
        return $value1.map { value1 in
            guard value1.count > 2 else {
                DispatchQueue.main.async { __**(3)**
                    self.value1_message_label.text = "minimum of 3 characters required"
                }
                return nil
            }
            DispatchQueue.main.async {
                self.value1_message_label.text = ""
            }
            return value1
        }.eraseToAnyPublisher()
    }

    var validatedValue2: AnyPublisher<String?, Never> {
        return $value2.combineLatest($value2_repeat)
            .receive(on: RunLoop.main) __**(5)**
            .map { value2, value2_repeat in
                guard value2_repeat == value2, value2.count > 4 else {
                    self.value2_message_label.text = "values must match and have at least 5 characters"
                    return nil
                }
                self.value2_message_label.text = ""
                return value2
            }.eraseToAnyPublisher()
    }

    var readyToSubmit: AnyPublisher<(String, String)?, Never> {
        return Publishers.CombineLatest(validatedValue2, validatedValue1)
            .map { value2, value1 in
                guard let realValue2 = value2, let realValue1 = value1 else {
                    return nil
                }
                return (realValue2, realValue1)
            }
            .eraseToAnyPublisher()
    }

    private var cancellableSet: Set<AnyCancellable> = [] __**(7)**

    override func viewDidLoad() {
        super.viewDidLoad()

        self.readyToSubmit
            .map { $0 != nil } __**(8)**
            .receive(on: RunLoop.main)
            .assign(to: \.isEnabled, on: submission_button)
            .store(in: &cancellableSet) __**(9)**
    }
}
```

--------------------------------

### Create Publisher with Resolved Failure

Source: https://heckj.github.io/swiftui-notes/index.html

A `Future` can also be created to immediately resolve with a failure. This is useful for publishers that should indicate an error from the start.

```swift
enum ExampleFailure: Error {
    case oneCase
}

let resolvedFailureAsPublisher = Future<Bool, Error> { promise in
    promise(.failure(ExampleFailure.oneCase))
}.eraseToAnyPublisher()
```

--------------------------------

### Create a Repeating Timer Publisher

Source: https://heckj.github.io/swiftui-notes/index_zh-CN.html

Use Timer.publish to create a publisher that emits dates at a specified interval. autoconnect() starts the timer immediately upon subscription.

```swift
let cancellable = Timer.publish(every: 1.0, on: RunLoop.main, in: .common)
    .autoconnect()
    .sink {
        print("passed through: ", $0)
    }
```

--------------------------------

### Setup Coordinated Pipeline

Source: https://heckj.github.io/swiftui-notes/index.html

Sets up a complex Combine pipeline involving serial and parallel asynchronous operations. It uses `createFuturePublisher` and operators like `flatMap` and `Zip3` to orchestrate the workflow. This is typically called within `viewDidLoad`.

```swift
// MARK: - view setup

override func viewDidLoad() {
    super.viewDidLoad()
    self.activityIndicator.stopAnimating()

    // Do any additional setup after loading the view.

    coordinatedPipeline = createFuturePublisher(button: self.step1_button) 
        .flatMap { flatMapInValue -> AnyPublisher<Bool, Error> in
        let step2_1 = self.createFuturePublisher(button: self.step2_1_button)
        let step2_2 = self.createFuturePublisher(button: self.step2_2_button)
        let step2_3 = self.createFuturePublisher(button: self.step2_3_button)
        return Publishers.Zip3(step2_1, step2_2, step2_3)
            .map { _ -> Bool in
                return true
            }
            .eraseToAnyPublisher()
        }
    .flatMap { _ in
        return self.createFuturePublisher(button: self.step3_button)
    }
    .flatMap { _ in
        return self.createFuturePublisher(button: self.step4_button)
    }
    .eraseToAnyPublisher()
}
```

--------------------------------

### Observe Network Activity Publisher

Source: https://heckj.github.io/swiftui-notes/index.html

Subscribes to a network activity publisher to animate an activity indicator. Starts animating when network activity begins and stops when it ends.

```swift
let apiActivitySub = GithubAPI.networkActivityPublisher __**(1)**
.receive(on: RunLoop.main)
    .sink { doingSomethingNow in
        if (doingSomethingNow) {
            self.activityIndicator.startAnimating()
        } else {
            self.activityIndicator.stopAnimating()
        }
}
```

--------------------------------

### Test DataTaskPublisher with XCTestExpectation

Source: https://heckj.github.io/swiftui-notes/index.html

This example demonstrates testing a one-shot publisher, URLSession.dataTaskPublisher, using XCTestExpectation. It sets up an expectation to wait for the publisher to complete successfully, fulfilling the expectation in the `receiveCompletion` closure. Failures are explicitly handled with `XCTFail()`.

```swift
func testDataTaskPublisher() {
        // setup
        let expectation = XCTestExpectation(description: "Download from \(String(describing: testURL))") __**(1)**
        let remoteDataPublisher = URLSession.shared.dataTaskPublisher(for: self.testURL!)
            // validate
            .sink(receiveCompletion: { fini in
                print(".sink() received the completion", String(describing: fini))
                switch fini {
                case .finished: expectation.fulfill() __**(2)**
                case .failure: XCTFail() __**(3)**
                }
            }, receiveValue: { (data, response) in
                guard let httpResponse = response as? HTTPURLResponse else {
                    XCTFail("Unable to parse response an HTTPURLResponse")
                    return
                }
                XCTAssertNotNil(data)
                // print(".sink() data received \(data)")
                XCTAssertNotNil(httpResponse)
                XCTAssertEqual(httpResponse.statusCode, 200) __**(4)**
                // print(".sink() httpResponse received \(httpResponse)")
            })

        XCTAssertNotNil(remoteDataPublisher)
        wait(for: [expectation], timeout: 5.0) __**(5)**
    }
```

--------------------------------

### Create LocationHeadingProxy Publisher

Source: https://heckj.github.io/swiftui-notes/index.html

Wraps CLLocationManager to publish heading updates using a PassthroughSubject. Requires CoreLocation and Combine frameworks. The proxy starts and stops heading updates via its enable/disable methods.

```swift
import Foundation
import Combine
import CoreLocation

final class LocationHeadingProxy: NSObject, CLLocationManagerDelegate {

    let mgr: CLLocationManager __**(1)**
    private let headingPublisher: PassthroughSubject<CLHeading, Error> __**(2)**
    var publisher: AnyPublisher<CLHeading, Error> __**(3)**

    override init() {
        mgr = CLLocationManager()
        headingPublisher = PassthroughSubject<CLHeading, Error>()
        publisher = headingPublisher.eraseToAnyPublisher()

        super.init()
        mgr.delegate = self __**(4)**
    }

    func enable() {
        mgr.startUpdatingHeading() __**(5)**
    }

    func disable() {
        mgr.stopUpdatingHeading()
    }
    // MARK - delegate methods

    /*
     *  locationManager:didUpdateHeading:
     *
     *  Discussion:
     *    Invoked when a new heading is available.
     */
    func locationManager(_ manager: CLLocationManager, didUpdateHeading newHeading: CLHeading) {
        headingPublisher.send(newHeading) __**(6)**
    }

    /*
     *  locationManager:didFailWithError:
     *  Discussion:
     *    Invoked when an error has occurred. Error types are defined in "CLError.h".
     */
    func locationManager(_ manager: CLLocationManager, didFailWithError error: Error) {
        headingPublisher.send(completion: Subscribers.Completion.failure(error)) __**(7)**
    }
}
```

--------------------------------

### Subscribe to Custom Notifications

Source: https://heckj.github.io/swiftui-notes/index.html

Subscribe to a custom notification using its defined name. This example demonstrates how to receive and print the notification details, including its name, object, and user info.

```swift
let cancellable = NotificationCenter.default.publisher(for: .myExampleNotification, object: nil)
    .sink { receivedNotification in
        print("passed through: ", receivedNotification)
        // receivedNotification.name
        // receivedNotification.object - object sending the notification (sometimes nil)
        // receivedNotification.userInfo - often nil
    }
```

--------------------------------

### Run Asynchronous Sequence

Source: https://heckj.github.io/swiftui-notes/index.html

Initiates the entire asynchronous sequence. It cancels any ongoing sequence, resets UI elements, starts an activity indicator, and attaches a sink to the coordinated pipeline to begin execution.

```swift
func runItAll() {
        if self.cancellable != nil) {
            print("Cancelling existing run")
            cancellable?.cancel()
            self.activityIndicator.stopAnimating()
        }
        print("resetting all the steps")
        self.resetAllSteps()
        // driving it by attaching it to .sink
        self.activityIndicator.startAnimating()
        print("attaching a new sink to start things going")
        self.cancellable = coordinatedPipeline?
            .print()
            .sink(receiveCompletion: { completion in
                print(".sink() received the completion: ", String(describing: completion))
                self.activityIndicator.stopAnimating()
            }, receiveValue: { value in
                print(".sink() received value: ", value)
            })
    }
```

--------------------------------

### Create a Fail Publisher (inferred types)

Source: https://heckj.github.io/swiftui-notes/index.html

Initialize Fail by providing the output and failure types as parameters. This publisher immediately terminates with the specified error.

```swift
let cancellable = Fail(outputType: String.self, failure: TestFailureCondition.exampleFailure)
```

--------------------------------

### Create a Fail Publisher (with types)

Source: https://heckj.github.io/swiftui-notes/index.html

Initialize Fail by explicitly specifying the output and failure types. This publisher immediately terminates with the specified error.

```swift
let cancellable = Fail<String, Error>(error: TestFailureCondition.exampleFailure)
```

--------------------------------

### Create a Sequence Publisher

Source: https://heckj.github.io/swiftui-notes/index.html

Use the .publisher extension on any Sequence to create a Publishers.Sequence. This publisher emits elements from the sequence as demanded by the subscriber.

```swift
let initialSequence = ["one", "two", "red", "blue"]
_ = initialSequence.publisher
    .sink {
        print($0)
    }
}
```

--------------------------------

### Get Min Value of Publisher Stream

Source: https://heckj.github.io/swiftui-notes/index.html

Use `.min()` when the upstream publisher's output type conforms to `Comparable`. If not, provide a closure to define the ordering.

```swift
.min()
```

```swift
.min { (struct1, struct2) -> Bool in
    return struct1.property1 < struct2.property1
}
```

--------------------------------

### Create a Connectable Publisher

Source: https://heckj.github.io/swiftui-notes/index_zh-CN.html

Use `makeConnectable()` to wrap a publisher, making it explicitly connectable. The subscription will not activate until `connect()` is called.

```swift
var cancellables = Set<AnyCancellable>()
let publisher = Just("woot")
    .makeConnectable()

publisher.sink { value in
    print("Value received in sink: ", value)
}
.store(in: &cancellables)
```

--------------------------------

### Get Max Value of Publisher Stream

Source: https://heckj.github.io/swiftui-notes/index.html

Use `.max()` when the upstream publisher's output type conforms to `Comparable`. If not, provide a closure to define the ordering.

```swift
.max()
```

```swift
.max { (struct1, struct2) -> Bool in
    return struct1.property1 < struct2.property1
}
```

--------------------------------

### Pipeline with Subscribe, Receive, and Assign

Source: https://heckj.github.io/swiftui-notes/index.html

This pipeline demonstrates subscribing to a publisher on a background queue, receiving on the main run loop for UI updates, and assigning the result to a property.

```swift
networkDataPublisher
    .subscribe(on: backgroundQueue) __**(1)**
    .receive(on: RunLoop.main) __**(2)**
    .assign(to: \.text, on: yourLabel) __**(3)**
```

--------------------------------

### Create an Autoconnectable Publisher

Source: https://heckj.github.io/swiftui-notes/index_zh-CN.html

Use `autoconnect()` on a connectable publisher to automatically establish the connection upon the first subscription. This makes the publisher behave like a regular publisher.

```swift
var cancellables = Set<AnyCancellable>()
let publisher = Just("woot")
    .makeConnectable() __**(1)**
    .autoconnect() __**(2)**

publisher.sink { value in
    print("Value received in sink: ", value)
}
.store(in: &cancellables)
```

--------------------------------

### Explicit Type Mapping with map

Source: https://heckj.github.io/swiftui-notes/index_zh-CN.html

When the compiler cannot infer the return type, explicitly define the return type in the `map` closure. This example maps a struct to its boolean property.

```swift
struct MyStruct {
    isValid: bool = true
}
//
Just(MyStruct())
.map { inValue -> Bool in
  inValue.isValid
}
```

--------------------------------

### Use first operator to get the first element

Source: https://heckj.github.io/swiftui-notes/index.html

The first operator publishes only the first element it receives and then finishes. If the upstream publisher finishes before any values are published, no values are emitted.

```swift
.first()
```

--------------------------------

### Create a Record Publisher in Swift

Source: https://heckj.github.io/swiftui-notes/index.html

Use Record to create a publisher that replays a predefined sequence of values and a completion event. This is useful for testing or scenarios requiring deterministic output. The timing of value delivery is not controlled.

```swift
let recordedPublisher = Record<String, Never> { example in
    // example : type is Record<String, Never>.Recording
    example.receive("one")
    example.receive("two")
    example.receive("three")
    example.receive(completion: .finished)
}
```

--------------------------------

### Use last operator to get the last element

Source: https://heckj.github.io/swiftui-notes/index.html

The last operator waits for the upstream publisher to complete, then publishes the last element it received. If no elements were received before completion, no value is published.

```swift
.last()
```

--------------------------------

### Manually Connect a Publisher

Source: https://heckj.github.io/swiftui-notes/index_zh-CN.html

After creating a connectable publisher with `makeConnectable()`, explicitly call `connect()` to enable the subscription and allow data flow.

```swift
publisher
    .connect()
    .store(in: &cancellables)
```

--------------------------------

### 使用 handleEvents 调试 Combine 管道

Source: https://heckj.github.io/swiftui-notes/index_zh-CN.html

handleEvents 操作符允许在管道中的特定点插入自定义逻辑，例如打印日志或更新 UI。它可以接收多个闭包来处理不同的事件，如订阅、请求、取消、输出和完成。

```swift
.handleEvents(receiveSubscription: { aValue in
    print("receiveSubscription event called with \(String(describing: aValue))") __**(2)**
}, receiveOutput: { aValue in __**(3)**
    print("receiveOutput was invoked with \(String(describing: aValue))")
}, receiveCompletion: { aValue in __**(4)**
    print("receiveCompletion event called with \(String(describing: aValue))")
}, receiveCancel: { __**(5)**
    print("receiveCancel event invoked")
}, receiveRequest: { aValue in __**(1)**
    print("receiveRequest event called with \(String(describing: aValue))")
})
```

--------------------------------

### Reduce: Accumulate String Values

Source: https://heckj.github.io/swiftui-notes/index.html

The reduce operator collects values from an upstream publisher and applies a closure to accumulate them into a single result, which is only published upon successful completion. This example concatenates strings.

```swift
.reduce("", { prevVal, newValueFromPublisher -> String in
    return prevVal+newValueFromPublisher
})
```

--------------------------------

### Request Data from Alternate URL When Network is Constrained

Source: https://heckj.github.io/swiftui-notes/index_zh-CN.html

Implement an adaptive network loader that attempts a primary URL and falls back to a secondary URL if the network is constrained. This uses tryCatch to detect specific network errors and switch the request.

```swift
func adaptiveLoader(regularURL: URL, lowDataURL: URL) -> AnyPublisher<Data, Error> {
    var request = URLRequest(url: regularURL)
    request.allowsConstrainedNetworkAccess = false
    return URLSession.shared.dataTaskPublisher(for: request)
        .tryCatch {
            error -> URLSession.DataTaskPublisher in
            guard error.networkUnavailableReason == .constrained else {
               throw error
            }
            return URLSession.shared.dataTaskPublisher(for: lowDataURL)
        }
        .tryMap { data, response -> Data in
            guard let httpResponse = response as? HTTPUrlResponse,
                   httpResponse.statusCode == 200 else {
                       throw MyNetworkingError.invalidServerResponse
            }
            return data
        }
        .eraseToAnyPublisher()
```

--------------------------------

### Swift Combine Map Operator Example

Source: https://heckj.github.io/swiftui-notes/index.html

Demonstrates the use of the `map` operator in Combine to transform an Int to a String. The closure provided to `map` handles the transformation logic. The output is then printed using the `sink` operator.

```swift
let _ = Just(5)
    .map { value -> String in __**(1)**
        switch value {
        case _ where value < 1:
            return "none"
        case _ where value == 1:
            return "one"
        case _ where value == 2:
            return "couple"
        case _ where value == 3:
            return "few"
        case _ where value > 8:
            return "many"
        default:
            return "some"
        }
    }
    .sink { receivedValue in
        print("The end result was \(receivedValue)")
    }
```

--------------------------------

### Prepend Publisher to Another Publisher

Source: https://heckj.github.io/swiftui-notes/index.html

Use `prepend` to emit all elements from a first publisher before emitting elements from a second publisher. Both publishers must match on Output and Failure types.

```swift
secondPublisher
.prepend(firstPublisher)
```

--------------------------------

### Create Publisher from URLSession Data Task

Source: https://heckj.github.io/swiftui-notes/index_zh-CN.html

Utilize `URLSession.shared.dataTaskPublisher(for:)` to create a publisher that fetches data from a given URL or URLRequest. The publisher emits a tuple containing the fetched Data and the URLResponse. This is a convenient way to handle network requests within a Combine pipeline.

```swift
let request = URLRequest(url: regularURL)
return URLSession.shared.dataTaskPublisher(for: request)
```

--------------------------------

### Transform Int Publisher to String Subscriber with map

Source: https://heckj.github.io/swiftui-notes/index.html

Use the `map` operator to transform an Int publisher to a String subscriber. Explicitly define the return type of the closure if the compiler cannot infer it. This example demonstrates converting an integer value into a descriptive string.

```swift
let _ = Just(5) \
    .map { value -> String in \
        switch value {
        case _ where value < 1:
            return "none"
        case _ where value == 1:
            return "one"
        case _ where value == 2:
            return "couple"
        case _ where value == 3:
            return "few"
        case _ where value > 8:
            return "many"
        default:
            return "some"
        }
    }
    .sink { receivedValue in \
        print("The end result was \(receivedValue)")
    }
```

--------------------------------

### Select element by index using output

Source: https://heckj.github.io/swiftui-notes/index.html

Use the `output(at:)` operator to select a single element from a publisher by its 0-indexed position. The upstream publisher must publish at least to the specified index.

```swift
.output(at: 3)
```

--------------------------------

### Map Publisher Output to Optional Type for Assignment

Source: https://heckj.github.io/swiftui-notes/index.html

When a publisher's output type does not match the optionality of the target property's KeyPath, use a `map` operator to adjust the type. This example converts a non-optional `UIImage` to an optional `UIImage?` to match `UIImageView.image`.

```swift
.map { image -> UIImage? in
    image
}
```

--------------------------------

### Subscribe to AppKit Text Field Changes

Source: https://heckj.github.io/swiftui-notes/index.html

Subscribe to `textDidChangeNotification` from an AppKit `NSTextField` using a Combine publisher. This example filters notifications by the specific text field and maps the received notification to the text field's string value, assigning it to a view model property.

```swift
let sub = NotificationCenter.default.publisher(for: NSControl.textDidChangeNotification, 
                                               object: filterField) 
    .map { ($0.object as! NSTextField).stringValue } 
    .assign(to: \MyViewModel.filterString, on: myViewModel)
```

--------------------------------

### Adaptive URL Loading with tryCatch

Source: https://heckj.github.io/swiftui-notes/index.html

Use this function to create a publisher that attempts to load data from a primary URL and falls back to a secondary URL if the network is constrained. Setting `allowsConstrainedNetworkAccess` to false on the request triggers the fallback mechanism. The `tryCatch` operator intercepts specific network errors, and `tryMap` validates the server response.

```swift
func adaptiveLoader(regularURL: URL, lowDataURL: URL) -> AnyPublisher<Data, Error> {
    var request = URLRequest(url: regularURL)
    request.allowsConstrainedNetworkAccess = false
    return URLSession.shared.dataTaskPublisher(for: request)
        .tryCatch {
            guard error.networkUnavailableReason == .constrained else {
               throw error
            }
            return URLSession.shared.dataTaskPublisher(for: lowDataURL)
        }.tryMap {
            guard let httpResponse = response as? HTTPUrlResponse,
                   httpResponse.statusCode == 200 else {
                       throw MyNetworkingError.invalidServerResponse
            }
            return data
        }
        .eraseToAnyPublisher()
}
```

--------------------------------

### Create Publisher with Resolved Success

Source: https://heckj.github.io/swiftui-notes/index.html

A `Future` can be created to immediately resolve with a successful value. This is useful for publishers that have a known, immediate result.

```swift
let resolvedSuccessAsPublisher = Future<Bool, Error> { promise in
    promise(.success(true))
}.eraseToAnyPublisher()
```

--------------------------------

### Select elements by range using output

Source: https://heckj.github.io/swiftui-notes/index.html

Use the `output(at:)` operator with a Swift range to select a sequence of elements from a publisher. The upstream publisher must publish at least up to the end of the specified range.

```swift
.output(at: 2...3)
```

--------------------------------

### Multicast Publisher with Inline Subject

Source: https://heckj.github.io/swiftui-notes/index_zh-CN.html

Create a multicast publisher by providing a closure that returns a `PassthroughSubject`. This consolidates upstream requests for multiple subscribers. Remember to explicitly connect the publisher using `connect()` or `autoconnect()`.

```swift
let multicastPublisher = somepublisher
    .multicast {
        PassthroughSubject<Bool, Error>()
    }
```

--------------------------------

### Create Subscriber with Sink (Simple)

Source: https://heckj.github.io/swiftui-notes/index.html

Use the simple version of `sink` to receive values from a publisher. This closure is invoked each time a value is emitted.

```swift
let cancellablePipeline = publishingSource.sink { someValue in 
    // do what you want with the resulting value passed down
    // be aware that depending on the publisher, this closure
    // may be invoked multiple times.
    print(".sink() received \(someValue)")
})
```

--------------------------------

### Create an Empty Publisher

Source: https://heckj.github.io/swiftui-notes/index.html

Use Empty to create a publisher that never publishes values and optionally finishes immediately. Define output and failure types when they cannot be inferred.

```swift
let myEmptyPublisher = Empty<String, Never>()
```

--------------------------------

### Prepend Sequence to Publisher

Source: https://heckj.github.io/swiftui-notes/index.html

Use `prepend` with a sequence to immediately publish sequence values upon subscriber demand. Further demand is propagated to the second publisher.

```swift
secondPublisher
.prepend(["one", "two"])
```

--------------------------------

### Prefix Until Output from Another Publisher

Source: https://heckj.github.io/swiftui-notes/index.html

Use `prefix(untilOutputFrom:)` to republish elements until a second publisher emits an element. The stream terminates after the trigger publisher publishes.

```swift
.prefix(untilOutputFrom: secondPublisher)
```

--------------------------------

### Subscribe to a Record Publisher in Swift

Source: https://heckj.github.io/swiftui-notes/index.html

Sink into a Record publisher to receive its pre-recorded values and completion. Ensure the expectation is fulfilled upon completion for testing purposes.

```swift
let cancellable = recordedPublisher.sink(receiveCompletion: { err in
    print(".sink() received the completion: ", String(describing: err))
    expectation.fulfill()
}, receiveValue: { value in
    print(".sink() received value: ", value)
})
```

--------------------------------

### 使用 breakpoint 操作符触发调试器

Source: https://heckj.github.io/swiftui-notes/index_zh-CN.html

breakpoint 操作符允许在管道中的特定点触发调试器。可以通过返回 true 来激活调试器，这对于在满足特定条件时检查管道状态非常有用。

```swift
.breakpoint(receiveSubscription: { subscription in
    return false // return true to throw SIGTRAP and invoke the debugger
}, receiveOutput: { value in
    return false // return true to throw SIGTRAP and invoke the debugger
}, receiveCompletion: { completion in
    return false // return true to throw SIGTRAP and invoke the debugger
})
```

--------------------------------

### 使用 assign 创建一个订阅者

Source: https://heckj.github.io/swiftui-notes/index_zh-CN.html

使用 `assign` 将发布者的数据设置到对象的属性上。需要确保在主线程更新 UI 元素，并且可以随时取消管道。

```swift
let cancellablePipeline = publishingSource __**(1)**
    .receive(on: RunLoop.main) __**(2)**
    .assign(to: \.isEnabled, on: yourButton) __**(3)**

cancellablePipeline.cancel() __**(4)**
```

--------------------------------

### Create Publisher from KVO-Compliant Object

Source: https://heckj.github.io/swiftui-notes/index_zh-CN.html

Use the `publisher(for:)` method on any NSObject to create a Combine publisher that emits values when a specified KeyPath changes. This is useful for observing changes in Objective-C objects.

```swift
private final class KVOAbleNSObject: NSObject {
    @objc dynamic var intValue: Int = 0
    @objc dynamic var boolValue: Bool = false
}

let foo = KVOAbleNSObject()

let _ = foo.publisher(for: \.intValue)
    .sink { someValue in
        print("value updated to: >>\(someValue)<<")
    }
```

--------------------------------

### Handle Publisher Data with onReceive

Source: https://heckj.github.io/swiftui-notes/index.html

Use `onReceive` to link publishers to local views and trigger state changes. The publisher's failure type must be `<Never>`. This subscriber allows reacting to published data by updating local view properties or transforming the data.

--------------------------------

### Test Publisher with Scheduled Send using DispatchQueue

Source: https://heckj.github.io/swiftui-notes/index.html

Use XCTestExpectation and DispatchQueue to test a pipeline's timing by scheduling .send() invocations. Ensure sufficient timeout is set for queued calls.

```swift
func testKVOPublisher() {
    let expectation = XCTestExpectation(description: self.debugDescription)
    let foo = KVOAbleNSObject()
    let q = DispatchQueue(label: self.debugDescription) __**(1)**

    let _ = foo.publisher(for: \.intValue)
        .print()
        .sink { someValue in
            print("value of intValue updated to: >>\(someValue)<<")
        }

    q.asyncAfter(deadline: .now() + 0.5, execute: { __**(2)**
        print("Updating to foo.intValue on background queue")
        foo.intValue = 5
        expectation.fulfill() __**(3)**
    })
    wait(for: [expectation], timeout: 5.0) __**(4)**
}
```

--------------------------------

### Retrieve GitHub User Data with Combine

Source: https://heckj.github.io/swiftui-notes/index.html

Fetches a GitHub user's data, returning a publisher that emits a list containing a single user or an empty list on failure. Prevents requests for usernames shorter than 3 characters.

```swift
static func retrieveGithubUser(username: String) -> AnyPublisher<[GithubAPIUser], Never> {

        if username.count < 3 {
            return Just([]).eraseToAnyPublisher()
        }
        let assembledURL = String("https://api.github.com/users/\(username)")
        let publisher = URLSession.shared.dataTaskPublisher(for: URL(string: assembledURL)!)
            .handleEvents(receiveSubscription: { _ in 
                networkActivityPublisher.send(true)
            }, receiveCompletion: { _ in
                networkActivityPublisher.send(false)
            }, receiveCancel: {
                networkActivityPublisher.send(false)
            })
            .tryMap { data, response -> Data in
                guard let httpResponse = response as? HTTPURLResponse,
                    httpResponse.statusCode == 200 else {
                        throw APIFailureCondition.invalidServerResponse
                }
                return data
            }
            .decode(type: GithubAPIUser.self, decoder: JSONDecoder())
            .map {
                [$0]
            }
            .catch {
                // When I originally wrote this method, I was returning
                // a GithubAPIUser? optional.
                // I ended up converting this to return an empty
                // list as the "error output replacement" so that I could
                // represent that the current value requested didn't *have* a
                // correct github API response.
                return Just([])
            }
            .eraseToAnyPublisher()
            return publisher
    }
```

--------------------------------

### first

Source: https://heckj.github.io/swiftui-notes/index.html

Publishes the first element of a stream and then finishes. If no values are received before the stream finishes, no values are published.

```APIDOC
## first

### Description
Publishes the first element received from the upstream publisher and then completes.

### Details
This operator passes through the first value it receives and then sends a `.finish` completion. If no values are received before the upstream publisher completes, no values are published. 

```swift
.first()
```
```

--------------------------------

### Update UI from User Input with Combine

Source: https://heckj.github.io/swiftui-notes/index.html

This snippet shows how to connect a UITextField's changes to a Combine publisher. Use @Published to create a publisher from a variable, and then subscribe to it to trigger network requests and update UI elements declaratively. Ensure proper handling of background queues and main run loop updates.

```swift
import UIKit
import Combine

class ViewController: UIViewController {

    @IBOutlet weak var github_id_entry: UITextField! __**(1)**

    var usernameSubscriber: AnyCancellable?

    // username from the github_id_entry field, updated via IBAction
    // @Published is creating a publisher $username of type <String, Never>
    @Published var username: String = "" __**(2)**

    // github user retrieved from the API publisher. As it's updated, it
    // is "wired" to update UI elements
    @Published private var githubUserData: [GithubAPIUser] = []

    // MARK - Actions

    @IBAction func githubIdChanged(_ sender: UITextField) {
        username = sender.text ?? "" __**(3)**
        print("Set username to ", username)
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.

        usernameSubscriber = $username __**(4)**
            .throttle(for: 0.5, scheduler: myBackgroundQueue, latest: true) __**(5)**
            // ^^ scheduler myBackGroundQueue publishes resulting elements
            // into that queue, resulting on this processing moving off the
            // main runloop.
            .removeDuplicates() __**(6)**
            .print("username pipeline: ") // debugging output for pipeline
            .map { username -> AnyPublisher<[GithubAPIUser], Never> in __**(7)**
                return GithubAPI.retrieveGithubUser(username: username)
            }
            // ^^ type returned by retrieveGithubUser is a Publisher, so we use
            // switchToLatest to resolve the publisher to its value
            // to return down the chain, rather than returning a
            // publisher down the pipeline.
            .switchToLatest() __**(8)**
            // using a sink to get the results from the API search lets us
            // get not only the user, but also any errors attempting to get it.
            .receive(on: RunLoop.main)
            .assign(to: \.githubUserData, on: self) __**(9)**

```

--------------------------------

### Wrap Future with Deferred for Demand-Based Execution

Source: https://heckj.github.io/swiftui-notes/index.html

Demonstrates how to use `Deferred` to make a `Future` publisher execute its closure only when a subscription is made, enabling it to work with operators like `retry`. The `Deferred` publisher invokes its closure, which in turn creates and resolves the `Future`.

```swift
let deferredPublisher = Deferred { __**(1)**
    return Future<Bool, Error> { promise in __**(2)**
        self.asyncAPICall(sabotage: false) { (grantedAccess, err) in
            if let err = err {
                return promise(.failure(err))
            }
            return promise(.success(grantedAccess))
        }
    }
}.eraseToAnyPublisher()
```

--------------------------------

### Drop Elements Until Another Publisher Emits (dropUntilOutput)

Source: https://heckj.github.io/swiftui-notes/index.html

Use dropUntilOutput to ignore elements from an upstream publisher until a second publisher emits a value. Errors from either publisher will terminate the pipeline.

```swift
.drop(untilOutputFrom: triggerPublisher)
```

--------------------------------

### 使用 breakpointOnError 操作符触发调试器

Source: https://heckj.github.io/swiftui-notes/index_zh-CN.html

breakpointOnError 操作符是一个便捷的工具，可以在管道中发生任何错误时自动触发调试器，无需额外的配置。

```swift
.breakpointOnError()
```

--------------------------------

### Use min() operator

Source: https://heckj.github.io/swiftui-notes/index_zh-CN.html

Use the min() operator when the upstream publisher's output type conforms to Comparable to find the minimum value upon completion.

```swift
.min()
```

--------------------------------

### Create Publisher from Swift Result Type

Source: https://heckj.github.io/swiftui-notes/index_zh-CN.html

Combine extends Swift's standard `Result` type with a `.publisher` property. This allows you to convert a `Result` instance into a publisher that emits the success value or a failure error, followed by a .finished or .failure completion.

```swift
Result.publisher
```

--------------------------------

### Handle Network Activity with PassthroughSubject

Source: https://heckj.github.io/swiftui-notes/index.html

Exposes a publisher to indicate network request status. Closures within `handleEvents` trigger updates to this publisher on subscription, completion, or cancellation.

```swift
static let networkActivityPublisher = PassthroughSubject<Bool, Never>()
```

--------------------------------

### Wrap Contacts Access Request with Future

Source: https://heckj.github.io/swiftui-notes/index.html

Illustrates how to wrap a completion-handler-based asynchronous API, like requesting Contacts access, into a Combine Future publisher. The Future's closure is executed immediately upon subscription.

```swift
import Contacts
let futureAsyncPublisher = Future<Bool, Error> { promise in __**(1)**
    CNContactStore().requestAccess(for: .contacts) { grantedAccess, err in __**(2)**
        // err is an optional
        if let err = err { __**(3)**
            promise(.failure(err))
        }
        return promise(.success(grantedAccess)) __**(4)**
    }
}
```
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