Introduction to the new array TypedArray introduced in HTML5_html5 tutorial skills-H5 Tutorial-php.cn

Arrays in Javascript are powerful guys:

You can not specify the length when creating it, but change the length dynamically. You can read it as an ordinary array, or use it as a stack. You can change the value and even the type of each element in an array.

Well, actually it is an object. For example, we can create an array like this:

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Code As follows:

var array = new Array(10);

The power and omnipotence of Javascript’s arrays bring us convenience. But in general:

Almighty things can be used in various environments, but they may not be suitable for all environments.

TypedArray appeared precisely to solve the problem of "too many things done" by arrays in Javascript.

Origin

TypedArray is a general fixed-length buffer type that allows reading binary data in the buffer.

It was introduced in the WEBGL specification to solve the problem of Javascript processing binary data.

TypedArray has been supported by most modern browsers. For example, you can create TypedArray using the following method:

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The code is as follows:

// Create an 8-byte ArrayBuffer
var b = new ArrayBuffer(8);
// Create a reference to b, the type is Int32, starting The position is 0, the end position is the end of the buffer
var v1 = new Int32Array(b);
// Create a reference to b, the type is Uint8, the starting position is 2, the end position is the end of the buffer
var v2 = new Uint8Array(b, 2);
// Create a reference to b, type is Int16, starting position is 2, total length is 2
var v3 = new Int16Array(b, 2 , 2);

then the buffered and created reference layout is:

变量	索引
	字节数
b =	0	1	2	3	4	5	6	7
	索引数
v1 =	0				1
v2 =			0	1	2	3	4	5
v3 =			0		1

This means that the 0th element of the v1 array of type Int32 is the 0-3 bytes of b of type ArrayBuffer, and so on. Constructor

Above we created TypedArray through ArrayBuffer, but in fact, TypedArray provides 3 constructors to create his instances.

Constructor

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The code is as follows:

TypedArray(unsigned long length)
Create A new TypedArray, length is its fixed length.

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The code is as follows:

TypedArray(TypedArray array)
TypedArray(type[] array)
Creates a new TypedArray, each element of which is initialized according to the array, and the elements are type-converted accordingly.

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The code is as follows:

TypedArray(ArrayBuffer buffer, optional unsigned long byteOffset, optional unsigned long length)
Create a new TypedArray as a reference to the buffer, byteOffset is its starting offset, and length is its length.

So usually we create TypedArray in the following way:

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The code is as follows:

var array = new Uint8Array(10);

or:

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The code is as follows:

var array = new Uint8Array([1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ]);

Data Operation

TypedArray provides four methods: setter, getter, set and subarray for data operations.

Method getter type get(unsigned long index)

Returns the element at the specified index.

setter void set(unsigned long index, type value)

Set the element at the specified index to the specified value.

void set(TypedArray array, optional unsigned long offset) void set(type[] array, optional unsigned long offset)

Set the value according to the array, and offset is the offset position.

TypedArray subarray(long begin, optional long end)

Returns a new TypedArray, with the start bit being begin and the end bit being end.

For example, to read elements you can use :

var array = new Uint8Array([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]); alert(array[4]); //5

Setting elements can be used :

var array = new Uint8Array([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]); alert(array[4]); //5array[4] = 12;alert(array[ 4]); //12

Get a copy using :

var array = new Uint8Array([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]); var array2 = array.subarray(0); Array type

类型	大小	描述	Web IDL类型	C 类型
`Int8Array`	1	8位有符号整数	`byte`	`signed char`
`Uint8Array`	1	8位无符号整数	`octet`	`unsigned char`
`Uint8ClampedArray`	1	8位无符号整数 (clamped)	`octet`	`unsigned char`
`Int16Array`	2	16位有符号整数	`short`	`short`
`Uint16Array`	2	16位无符号整数	`unsigned short`	`unsigned short`
`Int32Array`	4	32位有符号整数	`long`	`int`
`Uint32Array`	4	32位无符号整数	`unsigned long`	`unsigned int`
`Float32Array`	4	32位IEEE浮点数	`unrestricted float`	`float`
`Float64Array`	8	64位IEEE浮点数	`unrestricted double`	`double`

Type Size Description Web IDL Type C type Int8Array 1 8-bit signed integer byte signed char Uint8Array 1 8-bit unsigned integer octet unsigned char Uint8ClampedArray 1 8-bit unsigned integer (clamped) octet unsigned char Int16Array 2 16-bit signed integer short short Uint16Array 2 16-bit unsigned integer unsigned short unsigned short Int32Array 4 32-bit signed integer long int Uint32Array 4 32-bit unsigned integer unsigned long unsigned int Float32Array 4 32-bit IEEE floating point number unrestricted float float Float64Array 8 64-bit IEEE floating point number unrestricted double double
Those who have played with canvas may find it familiar.

Because the array used to store image data in ImageData is of type Uint8ClampedArray.

For example:

var context = document.createElement("canvas").getContext("2d");var imageData = context.createImageData(100, 100);console.log(imageData.data);

which appears as in FireBug:

Uint8ClampedArray { 0=0, 1=0, 2=0, more...}

Why use TypedArray

We know that numbers in Javascript are 64-bit floating point numbers. For a binary image (each pixel of the image is stored as an 8-bit unsigned integer), if you want to use its data in a Javascript array, it is equivalent to using 8 times the memory of the image to store the data of an image. This is It's obviously unscientific. TypedArray can help us use only 1/8 of the original memory to store image data.

Or for WebSocket, using base64 for transmission is also a more expensive method, and switching to binary transmission may be a better method.

Of course, TypedArray has more benefits, such as better performance. Below we conduct some small tests to verify this.

The browsers participating in the test are :

FireFox 17.0.1 and Chrome 23.0.1271.97m

Test1: Sequential reading speed reading

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The code is as follows:

It can be seen that the reading speed of Uint8ClampedArray is obviously faster than Array (the longer the bar, the more time it takes).

Test2: Random reading

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The code is as follows:

//……
function check1(){
var array = new Uint8ClampedArray(5000000);
for(var i = array.length; i--;){
array[i] = Math.floor(Math.random() * 100);
}
var temp;
var time1 = (new Date()).getTime();
for(var i = array.length; i--;){
temp = array[Math.floor(Math.random() * 5000000)];
}
var time2 = (new Date()).getTime();
console.log(time2 - time1);
timeArray1.push(time2 - time1);
}
function check2(){
var array2 = new Array(5000000);
for(var i = array2.length; i--;){
array2[i] = Math.floor(Math.random() * 100);
}
var temp;
var time3 = (new Date()).getTime();
for(var i = array2.length; i--;){
temp = array2[Math.floor(Math.random() * 5000000)];
}
var time4 = (new Date()).getTime();
console.log(time4 - time3);
timeArray2.push(time4 - time3);
}
//……

随即读取中Uint8ClampedArray的读取速度也是比Array要快的。

Test3：顺序写入

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代码如下:

//……
function check1(){
var array = new Uint8ClampedArray(5000000);
var time1 = (new Date()).getTime();
for(var i = array.length; i--;){
array[i] = Math.floor(Math.random() * 100);
}
var time2 = (new Date()).getTime();
console.log(time2 - time1);
timeArray1.push(time2 - time1);
}
function check2(){
var array2 = new Array(5000000);
var time3 = (new Date()).getTime();
for(var i = array2.length; i--;){
array2[i] = Math.floor(Math.random() * 100);
}
var time4 = (new Date()).getTime();
console.log(time4 - time3);
timeArray2.push(time4 - time3);
}
//……

Test4：复制操作（U8C to U8C 和 Array to U8C）

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代码如下:

//……
function check1(){
var array = new Uint8ClampedArray(5000000);
for(var i = array.length; i--;){
array[i] = Math.floor(Math.random() * 100);
}
var temp;
var array2 = new Uint8ClampedArray(5000000);
var time1 = (new Date()).getTime();
array2.set(array);
var time2 = (new Date()).getTime();
console.log(time2 - time1);
timeArray2.push(time2 - time1);
}
function check2(){
var array = new Array(5000000);
for(var i = array.length; i--;){
array[i] = Math.floor(Math.random() * 100);
}
var temp;
var array2 = new Uint8ClampedArray(5000000);
var time1 = (new Date()).getTime();
array2.set(array);
var time2 = (new Date()).getTime();
console.log(time2 - time1);
timeArray2.push(time2 - time1);
}
//……

可见U8C复制到U8C，比Array复制到U8C快得多。