鸿蒙Harmony角落里的知识：从ECMA规范到ArkTS接口（一）

前言：

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, thena. 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, theni. 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.lengthIntconst relStart = normalizeIndex(begin, len)const relEnd = normalizeIndex(end, len)let count = relEnd - relStartif (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 ArrayBufferreturn 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 SharedArrayBufferreturn 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。

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