Combine 过滤操作符：解锁删除值的强大功能

2023-10-02 02:49:11

Combine 中的过滤操作符

在数据处理中，删除不必要或无关的值至关重要。Combine 提供了一组强大的过滤操作符，可让您从发布者流中排除特定值，从而实现更精细的数据处理。这些操作符包括：

dropFirst

dropFirst 操作符丢弃发布者流中的第一个值。这在您想从流中排除初始值时很有用，例如当发布者在启动后发布预热值时。

let publisher = PassthroughSubject<Int, Error>()

publisher
    .dropFirst()
    .sink(receiveValue: { value in
        print("Received value: \(value)")
    })

publisher.send(1)
publisher.send(2)
publisher.send(3)

// 输出：
// Received value: 2
// Received value: 3

dropUntilOutputFrom

dropUntilOutputFrom 操作符丢弃发布者流中的值，直到另一个发布者开始发布值。这在您想在第二个发布者开始发布之前忽略来自一个发布者的值时很有用。

let firstPublisher = PassthroughSubject<Int, Error>()
let secondPublisher = PassthroughSubject<Int, Error>()

firstPublisher
    .dropUntilOutputFrom(secondPublisher)
    .sink(receiveValue: { value in
        print("Received value from first publisher: \(value)")
    })

secondPublisher.send(1)
firstPublisher.send(2)
firstPublisher.send(3)

// 输出：
// Received value from first publisher: 2
// Received value from first publisher: 3

dropWhile

dropWhile 操作符丢弃发布者流中的值，只要给定的谓词为真。当您想在流中忽略特定条件的值时，这很有用。

let publisher = PassthroughSubject<Int, Error>()

publisher
    .drop(while: { $0 < 5 })
    .sink(receiveValue: { value in
        print("Received value: \(value)")
    })

publisher.send(1)
publisher.send(2)
publisher.send(3)
publisher.send(5)
publisher.send(6)

// 输出：
// Received value: 5
// Received value: 6

prefix

prefix 操作符从发布者流中获取指定数量的值。这在您只想处理流中的有限数量的值时很有用。

let publisher = PassthroughSubject<Int, Error>()

publisher
    .prefix(2)
    .sink(receiveValue: { value in
        print("Received value: \(value)")
    })

publisher.send(1)
publisher.send(2)
publisher.send(3)

// 输出：
// Received value: 1
// Received value: 2

prefix(while:)

prefix(while:) 操作符从发布者流中获取值，只要给定的谓词为真。这在您想处理流中满足特定条件的值时很有用。

let publisher = PassthroughSubject<Int, Error>()

publisher
    .prefix(while: { $0 < 5 })
    .sink(receiveValue: { value in
        print("Received value: \(value)")
    })

publisher.send(1)
publisher.send(2)
publisher.send(3)
publisher.send(5)
publisher.send(6)

// 输出：
// Received value: 1
// Received value: 2
// Received value: 3