javascript - 为什么我在缓冲区上的 Javascript View 全为零？

Question

我创建了一个缓冲区，然后在其上创建了一个 Uint8Array，但该数组没有任何值。我希望它具有缓冲区的值。这是一个易于重现的示例:

var buf = new ArrayBuffer(32);
for (var index = 0; index < 32; index++) buf[index] = index;
console.log(buf);
var arr = new Uint8Array(buf);
console.log(arr);

我在现实中尝试的是这样的日期格式转换器:

//Buffers and views
function convertDateTimeToFormat(date, initialFormat, endFormat) {
    var buf = new ArrayBuffer(14);
    var result = new Uint8Array(buf);
    console.log(date);
    var initialPositions = {};
    var endPositions = {};
    var sizeSoFar = 0;
    for (var c of initialFormat) {
        if (c === 'y') {
            initialPositions.y = new Uint32Array(date, sizeSoFar, 1);
        } else {
            initialPositions[c] = new Uint8Array(date, sizeSoFar, 2);
        }
        sizeSoFar += ((c === 'y') ? 4 : 2);
    }
    sizeSoFar = 0;
    for (var c of endFormat) {
        if (c === 'y') {
            endPositions.y = new Uint32Array(buf, sizeSoFar, 1);
        } else {
            endPositions[c] = new Uint8Array(buf, sizeSoFar, 2);
        }
        sizeSoFar += ((c === 'y') ? 4 : 2);
    }
    for (var key in initialPositions) {
        var limit = (key === 'y') ? 4 : 2;
        for (var index = 0; index < limit; index++) endPositions[c][index] = initialPositions[c][index];
    }
    return result;
}
//2019-03-01 13:03:50
var buf = new ArrayBuffer( 14 );
buf[0] = 2;
buf[1] = 0;
buf[2] = 1;
buf[3] = 9;
buf[4] = 0;
buf[5] = 3;
buf[6] = 0;
buf[7] = 1;
buf[8] = 1;
buf[9] = 3;
buf[10] = 0;
buf[11] = 3
buf[12] = 5;
buf[13] = 0;
console.log(convertDateTimeToFormat(buf, "yMdHms", "yMdHms"));
console.log(convertDateTimeToFormat(buf, "yMdHms", "MdyHms"));

但是由于我在这个问题开头描述的行为，结果全为零。

这行得通，但不够优雅，因为它需要日期格式，如果我要确保输入与日期格式无关，那么代码将变得非常复杂:

//Buffers and views
var results = {};
var buf = new ArrayBuffer( 4 );
results.uint32 = new Uint32Array(buf);
results.int8 = new Uint8Array(buf);
results.uint8 = new Int8Array(buf);
results.int8[2] = -1;
console.log(results);
results.int8[2] = 0;
results.int8[1] = -1;
console.log(results);
results.int8[1] = 0;
results.int8[0] = -1;
console.log(results);

//Buffers and views
function convertDateTimeToFormat(date, format) {
    var buf = new ArrayBuffer(14);
    var result = new Uint8Array(buf);
    var positions = {
        y: 0,
        M: 4,
        d: 6,
        H: 8,
        m: 10,
        s: 12
    };
    for (var index = 0; index < 14; index++) {
        result[index] = date[positions[format[index]]++];
    }
    return result.join("");
}
var results = {};
//2019-03-01 13:03:50
var buf = new ArrayBuffer( 14 );
buf[0] = 2;
buf[1] = 0;
buf[2] = 1;
buf[3] = 9;
buf[4] = 0;
buf[5] = 3;
buf[6] = 0;
buf[7] = 1;
buf[8] = 1;
buf[9] = 3;
buf[10] = 0;
buf[11] = 3
buf[12] = 5;
buf[13] = 0;
console.log(convertDateTimeToFormat(buf, "yyyyMMddHHmmss"));
console.log(convertDateTimeToFormat(buf, "MMddyyyyHHmmss"));

Answer 1

安ArrayBuffer只是一个代表内存片段的对象。它有一个固定的大小，就是这样。它不具有表示内容的属性，因为您需要一个类型化数组作为缓冲区 View 。您的代码只是将属性分配给作为对象工作的缓冲区，但它实际上并不操作保持为零的字节内容。

如果您不需要缓冲区，则完全不要显式实例化它。随便写

var arr = new Uint8Array(32);
for (var index = 0; index < 32; index++) arr[index] = index;
console.log(arr.buffer); 
console.log(arr);

---

在您的实际代码中，您似乎想要使用 Uint8Array 将数字存储在单独的字节中。也许您应该只将该数组而不是底层缓冲区传递给函数。

你可以这样创建它:

const arr = Uint8Array.of(2,0,1,9,0,3,0,1,1,3,0,3,5,0);
// or       Uint8Array.from([2,0,1,9,0,3,0,1,1,3,0,3,5,0])
// or       new Uint8Array(2,0,1,9,0,3,0,1,1,3,0,3,5,0]);
const buf = arr.buffer;