Introduction to PHP namespaces, traits and generators

This article mainly introduces you to the relevant information about the namespace, traits and generators of the new features of PHP. It mainly involves the content of traits (traits) and generators (generators) in PHP. For traits in PHP (Character) and generator (generator) Interested students can refer to it.

This article mainly introduces to you the relevant content about the namespace, traits and generators of PHP's new features. It is shared for your reference and study. Let's take a look at the detailed introduction:

1. Namespace

What is a namespace?

1). Namespace was introduced in PHP 5.3, similar to the function of folders. For example, Request and Response in the Symfony framework are located under the Symfony namespace.

2). The namespace should always be on the line below the

3) The namespace of the PHP file is different from the physical file system of the operating system. This is a virtual concept and does not necessarily correspond to the directory structure of the file system. Nonetheless, most PHP components place subnamespaces into subdirectories of the file system in order to be compatible with the widely used PSR4 autoloading standard.

4). Namespace is just a symbol of the PHP language. The PHP interpreter will add this symbol as a prefix to the names of classes, interfaces, functions and constants.

Why do we need a namespace?

1). Namespace allows the program to run like a sandbox and can be used with code written by other developers. Ensure that your code and projects can be used with the project's third-party dependencies.

Declare namespace

1). The top-level namespace is often used to set the top-level manufacturer name. 2). The manufacturer's namespace must be globally unique. Subnamespaces are not that important, but they help organize the project's code.

Import and alias

1). Starting from PHP5.3, you can import PHP classes, interfaces and other namespaces and create aliases for them. Starting from PHP5.6, you can import PHP functions and constants and create aliases for them.

2). When using the use keyword to import code, there is no need to add a symbol at the beginning, because PHP assumes that the imported namespace is a fully qualified namespace. The use keyword must appear in the global scope, that is, it cannot appear in a class or function, because this keyword is used during compilation. However, the use keyword can be used after the namespace declaration statement to import other namespaces. code.

Starting from PHP5.6 we can import functions and constants.

<?php
use func Namespace\functionName;

functionName();

You can also import constants,

use constant Namespace\CONS_NAME;
echo CONS_NAME;

Aliases for functions and constants are created in the same way as class names.

Best Practice

1).PHP allows multiple namespaces to be defined in one PHP file. But doing so can be confusing and goes against the good practice of one file per class. 2). When referencing the code of the global namespace in a namespace, a prefix needs to be added to tell PHP that it needs to find the class globally, such as PHP's native exception class.

Automatic loading

1). The namespace lays a solid foundation for the PSR4 autoloader developed by PHP-FIG.

2. Use the interface

1). Just like I can choose to drive a different car. Because they all have steering wheels, accelerators and brakes, and the fuel is gasoline.

3. Traits

1). Shape is a partial implementation of a class (constants, properties and methods), and can be mixed with one or more In existing PHP classes, traits have two functions: indicating what the class can do (similar to interfaces) and providing modular practices (similar to classes).

2). Traits allow two unrelated classes to use the same attributes and methods.

3).The PHP interpreter will copy and paste the properties into the definition body of the class.

4. Create a generator

1) Use the yield keyword once or multiple times in an ordinary function, without returning a value, only generating value, this function is a generator. For example:

<?php
function myGenerator() {
 yield &#39;value1&#39;；
 yield &#39;value2&#39;;
}

When calling the generator function, PHP will return an object belonging to the Generator class. This object can be iterated using the foreach() function. Each iteration, PHP An instance of this object will be asked to calculate and provide the next value to iterate. The elegance of the generator is that after each value is produced, the internal state of the generator will continue to switch between pausing and resuming until the end of the definition body is reached. Or until an empty return; statement is encountered, for example:

<?php
foreach (myGenerator() as $yieldedValue) {
 echo $yieldedValue, PHP_EOL;
}

The above example will output

value1
value2

2) .How does the generator save memory? Generate a range of values ​​(wrong way)

function makeRange($length) {
 $dataset = [];
 for ($i=0; $i < $length; $i++) {
 $dataset[] = $i;
 }
 return $dataset;
}

$customRange = makeRange(1000000);
foreach ($customeRange as $i) {
 echo $i, PHP_EOL;
}

预先创建了一个包含很大整数组成的数组，再看使用生成器的例子。

function makeRange($length) {
 for ($i = 0; $i < $length; $i++) {
 yield $i;
 }
}

foreach(makeRange(1000000) as $i) {
 echo $i, PHP_EOL;
}

在实际的例如迭代一个4GB大小的文件中功能中，迭代器大展身手。

function getRows($file) {
 $handle = fopen($file, 'rb');
 if ($handle === false) {
 throw new Exception();
 }
 //feof()函数检测是否到达文件末尾
 while (feof($handle) === false) {
 //fgetcsv()一次读取csv文件的一行
 yield fgetcsv($handle);
 }
 fclose($handle)
}

foreach (getRows('data.csv') as $row) {
 print_r($row);
}

3).生成器没有为PHP添加新功能，需要实现在数据集中执行快进、快退和查找，最好自己编写类实现Iterator接口，或者使用PHP标准库中的某个原生迭代器。

原生迭代器链接

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