arrays - 快速限制数组大小

Question

有没有一种方法可以在 swift 中限制数组大小，以便在数组已满时向其追加元素时，它不会追加？

我知道这可以通过编程方式完成。想知道 swift 是否对此有内置处理。

例如:

数组 -> 大小为 10 的数组

array.append(1)

.

.

.

array.append(10)

array.append(11) // Doesn't do anything?

array.insert(0, pos: 0) 

用例:将最后一个元素推出数组以为新元素腾出空间？

编辑 - 最后一行是主要用例。

Answer 1

不，Swift 没有提供这种数组——类似于数据库中的 View ——允许您查看前 N 个元素。虽然为此， View 和它的目标都应该是引用类型，但在 Swift 中数组不是这种情况。

但是废话不多说了，您可以快速在 Array 上编写一个包装器来满足您的需求:

/// an array-like struct that has a fixed maximum capacity
/// any element over the maximum allowed size gets discarded
struct LimitedArray<T> {
    private(set) var storage: [T] = []
    public let maxSize: Int

    /// creates an empty array
    public init(maxSize: Int) {
        self.maxSize = maxSize
    }

    /// takes the max N elements from the given collection
    public init<S: Sequence>(from other: S, maxSize: Int) where S.Element == T {
        self.maxSize = maxSize
        storage = Array(other.prefix(maxSize))
    }

    /// adds a new item to the array, does nothing if the array has reached its maximum capacity
    /// returns a bool indicated the operation success
    @discardableResult public mutating func append(_ item: T) -> Bool {
        if storage.count < maxSize {
            storage.append(item)
            return true
        } else {
            return false
        }
    }

    /// inserts an item at the specified position. if this would result in
    /// the array exceeding its maxSize, the extra element are dropped
    public mutating func insert(_ item: T, at index: Int) {
        storage.insert(item, at: index)
        if storage.count > maxSize {
            storage.remove(at: maxSize)
        }
    }

    // add here other methods you might need
}

// let's benefit all the awesome operations like map, flatMap, reduce, filter, etc
extension LimitedArray: MutableCollection {
    public var startIndex: Int { return storage.startIndex }
    public var endIndex: Int { return storage.endIndex }

    public subscript(_ index: Int) -> T {
        get { return storage[index] }
        set { storage[index] = newValue }
    }

    public func index(after i: Int) -> Int {
        return storage.index(after: i)
    }
}

由于该结构符合 Collection，您可以轻松地将其传递给只知道使用数组的代码，方法是将其内容转换为数组:Array(myLimitedArray) .