从ECMA规范到ArkTS接口（一）--typedArray.slice接口

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在深入理解和使用ArkTS中的数组操作之前，我们有必要先了解ECMAScript（ECMA）规范中的规定。ECMA规范是JavaScript语言的官方标准，而ArkTS作为JavaScript的超集，也遵守了这些规定。本系列文章旨在探讨ECMA规范如何定义接口，以及如何在ArkTS该接口如何使用。

ECMA介绍

ECMAScript是由Ecma国际（前身为欧洲计算机制造商协会ECMA）制定的语言标准，目标是标准化JavaScript语言。JavaScript通常用于网页和服务器端编程，而ECMAScript则为这些使用情境提供了一套规范。

ECMAScript（ES）的标准自2015年以来进入了年度发布周期，意味着每年都会发布一个新版本。这个变化始于ECMAScript 6（ES6）或ECMAScript 2015的发布，之后的每个版本通常被称为ECMAScript 2016（ES7）、ECMAScript 2017（ES8）、以此类推，直到最新的版本。

这种年度发布计划允许JavaScript语言以小的、可管理的步骤来引入新特性，使得开发者更容易跟上语言的变化，同时也使得浏览器和环境开发者更容易实现和支持新标准。每个版本的新特性通常在年初被确定，并在年中的ECMA General Assembly会议上正式批准。

截至本文发布，当前最新版本为ECMAScript 2023，也称为ES14。

ECMA对typedArray.slice接口的定义

在ECMA规范中，typedArray 是一个泛型对象，表示一个固定长度的二进制数据缓冲区，并且可以用来创建多种类型的视图，比如 Uint8Array、Int16Array、Float32Array 等。而其中的 .slice() 方法，用于浅复制 typedArray 的一部分到一个新的 typedArray 实例。

根据ECMAScript规范，typedArray.slice(begin, end) 方法接受两个参数：begin 表示起始位置，end 表示结束位置（不包含）。如果不提供 end 参数，slice 方法会复制从 begin 到原数组末尾的所有元素。如果 begin 或 end 是负数，则表示从数组末尾开始的偏移量。返回值是一个新的 typedArray 实例，包含指定范围的元素。

我们可以直接查看ECMA规范中对该函数的描述：

23.2.3.27 %TypedArray%.prototype.slice ( start, end )
The interpretation and use of the arguments of this method are the same as for Array.prototype.slice as defined in 23.1.3.28.

This method performs the following steps when called:
1. Let O be the this value.
2. Perform ? ValidateTypedArray(O).
3. Let len be O.[[ArrayLength]].
4. Let relativeStart be ? ToIntegerOrInfinity(start).
5. If relativeStart = -∞, let k be 0.
6. Else if relativeStart < 0, let k be max(len + relativeStart, 0).
7. Else, let k be min(relativeStart, len).
8. If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
9. If relativeEnd = -∞, let final be 0.
10. Else if relativeEnd < 0, let final be max(len + relativeEnd, 0).
11. Else, let final be min(relativeEnd, len).
12. Let count be max(final - k, 0).
13. Let A be ? TypedArraySpeciesCreate(O, « 𝔽(count) »).
14. If count > 0, then
    a. If IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is true, throw a TypeError exception.
    b. Let srcType be TypedArrayElementType(O).
    c. Let targetType be TypedArrayElementType(A).
    d. If srcType is targetType, then
        i. NOTE: The transfer must be performed in a manner that preserves the bit-level encoding of the source data.
        ii. Let srcBuffer be O.[[ViewedArrayBuffer]].
        iii. Let targetBuffer be A.[[ViewedArrayBuffer]].
        iv. Let elementSize be TypedArrayElementSize(O).
        v. Let srcByteOffset be O.[[ByteOffset]].
        vi. Let srcByteIndex be (k × elementSize) + srcByteOffset.
        vii. Let targetByteIndex be A.[[ByteOffset]].
        viii. Let limit be targetByteIndex + count × elementSize.
        ix. Repeat, while targetByteIndex < limit,
            1. Let value be GetValueFromBuffer(srcBuffer, srcByteIndex, Uint8, true, Unordered).
            2. Perform SetValueInBuffer(targetBuffer, targetByteIndex, Uint8, value, true, Unordered).
            3. Set srcByteIndex to srcByteIndex + 1.
            4. Set targetByteIndex to targetByteIndex + 1.
    e. Else,
        i. Let n be 0.
        ii. Repeat, while k < final,
            1.Let Pk be ! ToString(𝔽(k)).
            2.Let kValue be ! Get(O, Pk).
            3.Perform ! Set(A, ! ToString(𝔽(n)), kValue, true).
            4.Set k to k + 1.
            5.Set n to n + 1.
15. Return A.
This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.

大家可以看到，ECMA规范中对接口的描述非常详细，对各种参数的输入和输出都做非常细致的说明，无论是符合标准的语言接口的实现，还是

ArkTS对slice的接口描述

在ArkTS中，接口（Interfaces）是一个非常强大的特性，允许开发者定义对象的类型。为了实现一个符合ECMA规范的 slice 方法，ArkTS对该方法的定义声明如下：

以\static_core\plugins\ets\stdlib\escompat\TypedUArrays.ets的Uint8Array定义为例：

/**
     * Creates a slice of current Uint8Array using range [begin, end)
     *
     * @param begin start index to be taken into slice
     *
     * @param end last index to be taken into slice
     *
     * @returns a new Uint8Array with elements of current Uint8Array[begin;end) where end index is excluded
     *
     * @link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray/slice
     */
    public slice(begin?: Number, end?: Number): Uint8Array {
        return this.slice(asIntOrDefault(begin, 0 as int), asIntOrDefault(end, this.lengthInt))
    }

    /**
     * Creates a slice of current Uint8Array using range [begin, end)
     *
     * @param begin start index to be taken into slice
     *
     * @param end last index to be taken into slice
     *
     * @returns a new Uint8Array with elements of current Uint8Array[begin;end) where end index is excluded
     *
     * @link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray/slice
     */
    public slice(begin: number, end: number): Uint8Array {
        return this.slice(begin as int, end as int)
    }

    /**
     * Creates a slice of current Uint8Array using range [begin, end)
     *
     * @param begin start index to be taken into slice
     *
     * @param end last index to be taken into slice
     *
     * @returns a new Uint8Array with elements of current Uint8Array[begin;end) where end index is excluded
     *
     * @link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray/slice
     */
    public slice(begin: number, end: int): Uint8Array {
        return this.slice(begin as int, end as int)
    }

    /**
     * Creates a slice of current Uint8Array using range [begin, end)
     *
     * @param begin start index to be taken into slice
     *
     * @param end last index to be taken into slice
     *
     * @returns a new Uint8Array with elements of current Uint8Array[begin;end) where end index is excluded
     *
     * @link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray/slice
     */
    public slice(begin: int, end: number): Uint8Array {
        return this.slice(begin as int, end as int)
    }

    /**
     * Creates a slice of current Uint8Array using range [begin, end)
     *
     * @param begin start index to be taken into slice
     *
     * @param end last index to be taken into slice
     *
     * @returns a new Uint8Array with elements of current Uint8Array[begin;end) where end index is excluded
     *
     * @link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray/slice
     */
    public slice(begin: int, end: int): Uint8Array {
        const len: int = this.lengthInt
        const relStart = normalizeIndex(begin, len)
        const relEnd = normalizeIndex(end, len)
        let count = relEnd - relStart
        if (count < 0) {
            count = 0
        }
        if (this.buffer instanceof ArrayBuffer) {
            let buf = (this.buffer as ArrayBuffer).slice(relStart * Uint8Array.BYTES_PER_ELEMENT as int, relEnd * Uint8Array.BYTES_PER_ELEMENT as int) as ArrayBuffer
            return new Uint8Array(buf)
        } else if (this.buffer instanceof SharedArrayBuffer) {
            let buf = (this.buffer as SharedArrayBuffer).slice(relStart * Uint8Array.BYTES_PER_ELEMENT as int, relEnd * Uint8Array.BYTES_PER_ELEMENT as int) as SharedArrayBuffer
            return new Uint8Array(buf)
        } else {
            throw new Error("unexpected type of buffer")
        }
    }

    /**
     * Creates a slice of current Uint8Array using range [begin, this.lengthInt).
     *
     * @param begin start index to be taken into slice
     *
     * @returns a new Uint8Array with elements of current Uint8Array[begin, this.lengthInt)
     */
    public slice(begin: number): Uint8Array {
        return this.slice(begin as int)
    }

    /**
     * Creates a slice of current Uint8Array using range [begin, this.lengthInt).
     *
     * @param begin start index to be taken into slice
     *
     * @returns a new Uint8Array with elements of current Uint8Array[begin, this.lengthInt)
     */
    public slice(begin: int): Uint8Array {
        return this.slice(begin, this.lengthInt)
    }

在这个例子中，我们创建了一个 Int32ArrayWithSlice 类，它实现了 TypedArray 接口，包含了符合ECMA规范的 slice 方法。这个方法返回的是 Int32ArrayWithSlice 的新实例，包含了切片后的元素。

测试用例

要验证我们的 slice 方法实现是否正确，我们可以编写单元测试用例。这里我们使用了ArkTS和Jest测试框架，来确保我们的实现与ECMA规范一致。

const success = 0;
const fail = 1;
function testSliceWithOutParam(): int {
  let source: number[] = [10, 20, 30, 40, 50, 60];
  let ss = new ArrayBuffer(source.length as int * 1);

  let origin: Uint8Array;

  try {
    origin = new Uint8Array(ss);
    origin.set(source);
  } catch(e) {
    console.log(e);
    return fail;
  }


  let target: Uint8Array;

  try {
    target = origin.slice();
  } catch(e) {
    console.log(e);
    return fail;
  }

  if(target.length as int != origin.length as int) {
    console.log("Array length mismatch on slice");
    return fail;
  }

  //Check all the data copied;
  for(let i:int = 0; i< origin.length as int; i++) {
    let tv = target[i] as number;
    let ov = origin[i] as number;
    console.log(source[i] + "->" + tv + "->" + ov);
    if(tv != ov) {
      console.log("Array data mismatch");
      return fail;
    }
  }

  origin= new Uint8Array(0);
  if(origin.length as int != 0){
    return fail;
  }

  try {
    target = origin.slice();
  } catch(e) {
    console.log(e);
    return fail;
  }

  if(target.length as int != 0){
    return fail;
  }
  return success;
}

function testSliceOneParam(): int {
  let source: number[] = [10, 20, 30, 40, 50, 60];
  let ss = new ArrayBuffer(source.length as int * 1);

  let origin: Uint8Array;

  try {
    origin = new Uint8Array(ss);
    origin.set(source);
  } catch(e) {
    console.log(e);
    return fail;
  }

  let sliceStart: int = 1;
  let sliceEnd: int = origin.length as int;

  let target: Uint8Array;

  try {
    target = origin.slice(sliceStart);
  } catch(e) {
    console.log(e);
    return fail;
  }

  if(target.length as int != origin.length as int - sliceStart) {
    console.log("Array length mismatch on slice One Params" + target.length);
    return fail;
  }

  //Check all the data copied;
  for(let i:int = sliceStart; i< sliceEnd; i++) {
    let tv = target[i - sliceStart] as number;
    let ov = origin[i] as number;
    console.log(source[i] + "->" + tv + "->" + ov);
    if(tv != ov) {
      console.log("Array data mismatch");
      return fail;
    }
  }

  sliceStart = 0;
  try {
    target = origin.slice(undefined);
  } catch(e) {
    console.log(e);
    return fail;
  }

  if(target.length as int != origin.length as int) {
    console.log("Array length mismatch on slice One Params" + target.length);
    return fail;
  }

  //Check all the data copied;
  for(let i:int = sliceStart; i< sliceEnd; i++) {
    let tv = target[i - sliceStart] as number;
    let ov = origin[i] as number;
    console.log(source[i] + "->" + tv + "->" + ov);
    if(tv != ov) {
      console.log("Array data mismatch");
      return fail;
    }
  }

  return success;
}

function testSliceTwoParams(): int {
  let source: number[] = [10, 20, 30, 40, 50, 60, 70, 80];
  let ss = new ArrayBuffer(source.length as int * 1);

  let origin: Uint8Array;

  try {
    origin = new Uint8Array(ss);
    origin.set(source);
  } catch(e) {
    console.log(e);
    return fail;
  }

  let sliceStart: int = 2;
  let sliceEnd: int = 4;

  let target: Uint8Array;

  try {
    target = origin.slice(sliceStart, sliceEnd);
  } catch(e) {
    console.log(e);
    return fail;
  }

  if(target.length as int != sliceEnd - sliceStart) {
    console.log("Array length mismatch on slice2");
    return fail;
  }

  //Check all the data copied;
  for(let i:int = sliceStart; i< sliceEnd; i++) {
    let tv = target[i - sliceStart] as number;
    let ov = origin[i] as number;
    console.log(source[i] + "->" + tv + "->" + ov);
    if(tv != ov) {
      console.log("Array data mismatch");
      return fail;
    }
  }

  sliceStart = 0;
  sliceEnd = origin.length as int;
  try {
    target = origin.slice(sliceStart, sliceEnd);
  } catch(e) {
    console.log(e);
    return fail;
  }

  if(target.length as int != sliceEnd - sliceStart) {
    console.log("Array length mismatch on slice2");
    return fail;
  }

  //Check all the data copied;
  for(let i:int = sliceStart; i< sliceEnd; i++) {
    let tv = target[i - sliceStart] as number;
    let ov = origin[i] as number;
    console.log(source[i] + "->" + tv + "->" + ov);
    if(tv != ov) {
      console.log("Array data mismatch");
      return fail;
    }
  }

  try {
    target = origin.slice(new Number(sliceStart), undefined);
  } catch(e) {
    console.log(e);
    return fail;
  }

  if(target.length as int != sliceEnd - sliceStart) {
    console.log("Array length mismatch on slice2");
    return fail;
  }

  //Check all the data copied;
  for(let i:int = sliceStart; i< sliceEnd; i++) {
    let tv = target[i - sliceStart] as number;
    let ov = origin[i] as number;
    console.log(source[i] + "->" + tv + "->" + ov);
    if(tv != ov) {
      console.log("Array data mismatch");
      return fail;
    }
  }

  try {
    target = origin.slice(undefined, undefined);
  } catch(e) {
    console.log(e);
    return fail;
  }

  if(target.length as int != sliceEnd - sliceStart) {
    console.log("Array length mismatch on slice2");
    return fail;
  }

  //Check all the data copied;
  for(let i:int = sliceStart; i< sliceEnd; i++) {
    let tv = target[i - sliceStart] as number;
    let ov = origin[i] as number;
    console.log(source[i] + "->" + tv + "->" + ov);
    if(tv != ov) {
      console.log("Array data mismatch");
      return fail;
    }
  }

  try {
    target = origin.slice(undefined, new Number(sliceEnd));
  } catch(e) {
    console.log(e);
    return fail;
  }

  if(target.length as int != sliceEnd - sliceStart) {
    console.log("Array length mismatch on slice2");
    return fail;
  }

  //Check all the data copied;
  for(let i:int = sliceStart; i< sliceEnd; i++) {
    let tv = target[i - sliceStart] as number;
    let ov = origin[i] as number;
    console.log(source[i] + "->" + tv + "->" + ov);
    if(tv != ov) {
      console.log("Array data mismatch");
      return fail;
    }
  }

  try {
    target = origin.slice(0, 0);
  } catch(e) {
    console.log(e);
    return fail;
  }

  if(target.length as int != 0) {
    console.log("Array length mismatch on slice2");
    return fail;
  }

  return success;
}

function testSliceTwoParamsWithOtherNumber(): int {
  let source: number[] = [10, 20, 30, 40, 50, 60, 70, 80];
  let ss = new ArrayBuffer(source.length as int * 1);

  let origin: Uint8Array;

  try {
    origin = new Uint8Array(ss);
    origin.set(source);
  } catch(e) {
    console.log(e);
    return fail;
  }

  let sliceStart: int = 4;
  let sliceEnd: int = 2;

  let target: Uint8Array;

  try {
    target = origin.slice(sliceStart, sliceEnd);
  } catch(e) {
    return fail;
  }

  if(target.length as int != 0) {
    console.log("Array length mismatch on slice2");
    return fail;
  }

  sliceStart = -1;
  sliceEnd = origin.length as int;
  try {
    target = origin.slice(sliceStart, sliceEnd);
  } catch(e) {
    return fail;
  }

  if(target.length as int != sliceEnd - (origin.length + sliceStart)) {
    console.log("Array length mismatch on slice2");
    return fail;
  }

  //Check all the data copied;
  for(let i:int = (origin.length + sliceStart) as int; i< sliceEnd; i++) {
    let tv = target[i - (origin.length + sliceStart)] as number;
    let ov = origin[i] as number;
    console.log(source[i] + "->" + tv + "->" + ov);
    if(tv != ov) {
      console.log("Array data mismatch");
      return fail;
    }
  }

  sliceStart = 0;
  sliceEnd = -origin.length as int;
  try {
    target = origin.slice(sliceStart, sliceEnd);
  } catch(e) {
    console.log(e);
    return fail;
  }

  if(target.length as int != (origin.length + sliceEnd) - sliceStart) {
    console.log("Array length mismatch on slice2");
    return fail;
  }

  return success;
}

function testSliceOneLengthTwoParams(): int {
  let source: number[] = [10];
  let ss = new ArrayBuffer(source.length as int * 1);

  let origin: Uint8Array;

  try {
    origin = new Uint8Array(ss);
    origin.set(source);
  } catch(e) {
    console.log(e);
    return fail;
  }
  
  let sliceStart: int = 4;
  let sliceEnd: int = 2;

  let target: Uint8Array;

  try {
    target = origin.slice(sliceStart, sliceEnd);
  } catch(e) {
    return fail;
  }

  if(target.length as int != 0) {
    console.log("Array length mismatch on slice2");
    return fail;
  }

  sliceStart = 2;
  sliceEnd = 4;
  try {
    target = origin.slice(sliceStart, sliceEnd);
  } catch(e) {
    return fail;
  }

  if(target.length as int != 0) {
    console.log("Array length mismatch on slice2");
    return fail;
  }

  return success;
}

这些测试用例检查了 slice 方法在正常条件下和非正常条件下的行为。通过运行这些测试，我们可以确认我们的实现是否符合接口定义预期和ECMA规范。

总结来说，理解ECMA规范中的接口是任何ArkTS开发者的基本功，而且通过单元测试可以保证实现的准确性和可靠性。通过逐步实现这些接口并验证它们的行为，我们可以确保我们的ArkTS代码既强大又稳健。

[经验分享] 从ECMA规范到ArkTS接口（一）--typedArray.slice接口原创精华

ECMA介绍

ECMA对typedArray.slice接口的定义

ArkTS对slice的接口描述

测试用例

参考文献：

相关帖子

[经验分享] 从ECMA规范到ArkTS接口（一）--typedArray.slice接口 原创 精华

ECMA介绍

ECMA对typedArray.slice接口的定义

ArkTS对slice的接口描述

测试用例

参考文献：

相关帖子

[经验分享] 从ECMA规范到ArkTS接口（一）--typedArray.slice接口原创精华