What is Generator in PHP

Generator in PHP is a new feature added since PHP5.5, translated into Chinese as "generator"; Generator provides a convenient way to implement a simple Iterator. Using Generator to implement Iterator does not require creation A class to inherit the Iterator interface.

What is Generator

Starting from PHP 5.5, PHP has been added A new feature is Generator, which is translated into Chinese as Generator.

Generator provides a convenient way to implement a simple Iterator (iterator). Using Generator to implement Iterator does not require creating a class to inherit the Iterator interface.

Generators can be simply used to implement object iteration. Let’s start with a small official example.

xrange

In PHP, we all know that there is a function called range, which is used to generate an array of arithmetic sequences, and then We can use this array to iterate foreach. Specifically, this is what I want to do.

##

foreach (range(1, 100, 2) as $num) {    echo "{$num}\n";
}

This code will output the first item as 1 , an arithmetic sequence with the last term being 100 and a tolerance of 2. Its execution sequence is as follows. First,

range(1, 100, 2) will generate an array, which stores an arithmetic sequence as above, and then iterate this array in foreach.

So, a question arises, what if I want to generate 1 million numbers? Then we will occupy hundreds of megabytes of memory. Although memory is very cheap now, we can't waste memory like this. Then at this time, our generator can come in handy. Consider the following code.

##

function xrange($start, $limit, $step = 1) {    yield $start;
    $start++;
}foreach (xrange(1, 100, 2) as $num) {    echo "{$num}\n";
}

The result of this code is, and The previous piece of code is exactly the same, but its internal principles are completely different.

We just said that in the previous code,

range

will generate an array, and then foreach will iterate the array to take out a certain value. But in this code, we redefined a xrange function. In the function, we used the keyword yield. We all know that when defining a function and hoping it will return a value, use return to return it. So this yield can also return a value, but it is completely different from return. Using the

yield

keyword can interrupt the function while it is running, save the context of the entire function, and return an object of type Generator. When executing the object's next method, the context at the time of interruption will be reloaded and continue running until the next yield appears. If no yield appears later, , then the entire generator is considered finished. In this way, our function call above can be written equivalently like this.

$nums = xrange(1, 100, 2);while ($nums->valid()) {    echo $nums->current() . "\n";
    $nums->next();
}

Here, $num is an object of

Generator. We see three methods here, valid, current, and next. When our function is executed and there is no yield interrupt later, then our function in xrange is finished, and the valid method will become false . As for current, it will return the value behind the current yield. This means that the generator function will be interrupted. Then after calling the next method, the function will continue to execute until the next yield appears, or the function ends. Okay, so far, we have seen that yield

is used to "generate" a value and return it. In fact,

yield can also be written like this $ret = yield;. Like the return value, here a value is passed into the function when continuing to execute the function. It can be used through Generator::send($value). For example.

#

function sum(){
    $ret = yield;    echo "{$ret}\n";
}

$sum = sum();
$sum->send('I am from outside.');

In this way, the program will print out The string passed in by the send method. There can be calls on both sides of

yield

at the same time.

function xrange($start, $limit, $step = 1) {
    $ret = yield $start;
    $start++;    echo "{$ret}\n";
}

而像这样的使用，send() 可以返回下一个 yield 的返回。

其它的 Generator 方法

Generator::key()

对于 yield，我们可以这样使用 yield $id => $value，这是，我们可以通过 key 方法来获取 $id，而 current 方法返回的是 $value。

Generator::rewind()

这个方法，可以帮我们让生成器重新开始执行并保存上下文，同时呢，会返回第一个 yield 返回的内容。在第一次执行 send 方法的时候，rewind 会被隐式调用。

Generator::throw()

这个方法，向生成器中，抛送一个异常。

后记

yield 作为 PHP 5.5 的新特性，让我们用了新的方法来高效地迭代数据。同时，我们还可以使用 yield 来实现协程。

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