blocks|key|2798585|text|什么是yield|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2798586|yield关键字returns+data+from+a+generator+function:|2798587|2798588|生成器函数的核心是yield关键字。在其最简单的形式中，yield语句看起来很像return语句，不同之处在于，yield不是停止函数的执行并返回，而是为在生成器上循环的代码提供一个值，并暂停生成器函数的执行。|blockquote|2798589|2798590|2798591|什么是生成器函数？|2798592|生成器函数实际上是编写Iterator的一种更紧凑、更有效的方法。它允许您在looping+over+it时定义一个函数(您的xrange)来计算和返回值|2798593|function+xrange($min,+$max)+{
++++for+($i+=+$min;+$i+<=+$max;+$i%2B%2B)+{
++++++++yield+$i;
++++}
}

[…]

foreach+(xrange(1,+10)+as+$key+=>+$value)+{
++++echo+"$key+=>+$value",+PHP_EOL;
}|code-block|syntax|javascript|2798594|这将创建以下输出：|2798595|0+=>+1
1+=>+2
…
9+=>+10|2798596|您还可以使用以下命令控制foreach中的$key|2798597|yield+$someKey+=>+$someValue;|2798598|在生成器函数中，$someKey是您希望为$key显示的任何内容，而$someValue是$val中的值。在问题的示例中，它是$i。|2798599|与普通函数有什么不同？|2798600|现在你可能想知道为什么我们不简单地使用PHP的原生range+function来实现该输出。你说的没错。输出将是相同的。不同之处在于我们是如何做到的。|2798601|当我们使用range+PHP时，将执行它，在内存中创建整个数字数组，并将整个数组return到foreach循环，然后循环遍历该数组并输出值。换句话说，foreach将对数组本身进行操作。range函数和foreach只“对话”一次。把它想象成在邮件中收到一个包裹。快递员会把包裹递给你然后离开。然后你打开整个包裹，把里面的东西拿出来。|2798602|当我们使用生成器函数时，PHP将进入该函数并执行它，直到它遇到end或yield关键字。当它遇到yield时，它会将当时的值返回给外部循环。然后，它返回到生成器函数，并从它产生的地方继续。因为您的xrange包含一个for循环，所以它将执行并返回，直到到达$max为止。可以把它想象成foreach和生成器在打乒乓球。|2798603|为什么我需要它？|2798604|显然，生成器可以用来绕过内存限制。根据您的环境，执行range(1,+1000000)将致命您的脚本，而同样的生成器将正常工作。或者就像维基百科说的那样：|2798605|2798606|因为生成器只根据需要计算它们的产出值，所以它们对于表示一次计算代价高昂或不可能的序列很有用。例如，这些包括无限序列和实时数据流。|2798607|2798608|2798609|生成器也被认为是相当快的。但请记住，当我们谈论fast时，我们通常谈论的是非常小的数字。因此，在您现在离开并将所有代码更改为使用生成器之前，先做一个基准测试，看看它在哪里有意义。|2798610|生成器的另一个用例是异步协程。yield关键字不仅返回值，而且还接受它们。有关这方面的详细信息，请参阅下面链接的两篇优秀的博客文章。|2798611|从什么时候开始我可以使用yield了|2798612|在PHP5.5中引入了生成器。尝试在该版本之前使用yield将导致各种解析错误，具体取决于关键字后面的代码。因此，如果从代码中获得解析错误，请更新PHP。|BOLD|2798613|来源和进一步阅读：|2798614|2798615|Official+docs|unordered-list-item|2798616|The+original+RFC|2798617|kelunik's+blog:+An+introduction+to+generators|2798618|ircmaxell's+blog:+What+generators+can+do+for+you|2798619|NikiC's+blog:+Cooperative+multitasking+using+coroutines+in+PHP|2798620|Co-operative+PHP+Multitasking|2798621|What+is+the+difference+between+a+generator+and+an+array?|2798622|Wikipedia+on+Generators+in+general|2798623|2798624|entityMap|0|LINK|mutability|MUTABLE|url|https://secure.php.net/manual/en/language.generators.syntax.php|1|https://secure.php.net/manual/en/class.iterator.php|2|https://secure.php.net/manual/en/control-structures.foreach.php|3|https://secure.php.net/range|4|https://secure.php.net/generators|5|https://wiki.php.net/rfc/generators|6|http://blog.kelunik.com/2017/09/14/an-introduction-to-generators-in-php.html|7|http://blog.ircmaxell.com/2012/07/what-generators-can-do-for-you.html|8|http://nikic.github.io/2012/12/22/Cooperative-multitasking-using-coroutines-in-PHP.html|9|https://medium.com/@assertchris/co-operative-php-multitasking-ce4ef52858a0|10|https://stackoverflow.com/questions/17223802/what-is-the-difference-between-a-generator-and-an-array|11|https://en.wikipedia.org/wiki/Generator_%2528computer_programming%2529^0|3|5|0|0|5|8|13|0|0|0|0|0|0|0|1R|6|B|8|1|12|F|2|0|0|0|0|C|7|L|4|0|0|8|8|L|4|Y|A|19|4|1R|2|0|0|P|5|P|E|3|0|5|5|14|6|1B|7|24|7|2M|5|2U|7|0|Z|5|1C|5|2Q|6|30|3|3K|4|3Y|7|0|0|Q|H|0|0|0|0|0|0|F|5|0|C|5|0|1|6|P|5|0|0|0|0|D|4|0|0|G|5|0|0|19|6|0|0|1C|7|0|0|1Q|8|0|0|T|9|0|0|1K|A|0|0|Y|B|0|0^^$0|@$1|2|3|4|5|6|7|3B|8|@$9|3C|A|3D|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|3E|8|@$9|3F|A|3G|B|C]]|D|@$9|3H|A|3I|1|3J]]|E|$]]|$1|H|3|-4|5|6|7|3K|8|@]|D|@]|E|$]]|$1|I|3|J|5|K|7|3L|8|@]|D|@]|E|$]]|$1|L|3|-4|5|6|7|3M|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|3N|8|@]|D|@]|E|$]]|$1|N|3|O|5|6|7|3O|8|@]|D|@]|E|$]]|$1|P|3|Q|5|6|7|3P|8|@$9|3Q|A|3R|B|C]]|D|@$9|3S|A|3T|1|3U]|$9|3V|A|3W|1|3X]]|E|$]]|$1|R|3|S|5|T|7|3Y|8|@]|D|@]|E|$U|V]]|$1|W|3|X|5|6|7|3Z|8|@]|D|@]|E|$]]|$1|Y|3|Z|5|T|7|40|8|@]|D|@]|E|$U|V]]|$1|10|3|11|5|6|7|41|8|@$9|42|A|43|B|C]|$9|44|A|45|B|C]]|D|@]|E|$]]|$1|12|3|13|5|T|7|46|8|@]|D|@]|E|$U|V]]|$1|14|3|15|5|6|7|47|8|@$9|48|A|49|B|C]|$9|4A|A|4B|B|C]|$9|4C|A|4D|B|C]|$9|4E|A|4F|B|C]|$9|4G|A|4H|B|C]]|D|@]|E|$]]|$1|16|3|17|5|6|7|4I|8|@]|D|@]|E|$]]|$1|18|3|19|5|6|7|4J|8|@$9|4K|A|4L|B|C]]|D|@$9|4M|A|4N|1|4O]]|E|$]]|$1|1A|3|1B|5|6|7|4P|8|@$9|4Q|A|4R|B|C]|$9|4S|A|4T|B|C]|$9|4U|A|4V|B|C]|$9|4W|A|4X|B|C]|$9|4Y|A|4Z|B|C]|$9|50|A|51|B|C]]|D|@]|E|$]]|$1|1C|3|1D|5|6|7|52|8|@$9|53|A|54|B|C]|$9|55|A|56|B|C]|$9|57|A|58|B|C]|$9|59|A|5A|B|C]|$9|5B|A|5C|B|C]|$9|5D|A|5E|B|C]]|D|@]|E|$]]|$1|1E|3|1F|5|6|7|5F|8|@]|D|@]|E|$]]|$1|1G|3|1H|5|6|7|5G|8|@$9|5H|A|5I|B|C]]|D|@]|E|$]]|$1|1I|3|-4|5|6|7|5J|8|@]|D|@]|E|$]]|$1|1J|3|1K|5|K|7|5K|8|@]|D|@]|E|$]]|$1|1L|3|-4|5|6|7|5L|8|@]|D|@]|E|$]]|$1|1M|3|-4|5|6|7|5M|8|@]|D|@]|E|$]]|$1|1N|3|1O|5|6|7|5N|8|@]|D|@]|E|$]]|$1|1P|3|1Q|5|6|7|5O|8|@$9|5P|A|5Q|B|C]]|D|@]|E|$]]|$1|1R|3|1S|5|6|7|5R|8|@$9|5S|A|5T|B|C]]|D|@]|E|$]]|$1|1T|3|1U|5|6|7|5U|8|@$9|5V|A|5W|B|1V]|$9|5X|A|5Y|B|C]]|D|@]|E|$]]|$1|1W|3|1X|5|6|7|5Z|8|@]|D|@]|E|$]]|$1|1Y|3|-4|5|6|7|60|8|@]|D|@]|E|$]]|$1|1Z|3|20|5|21|7|61|8|@]|D|@$9|62|A|63|1|64]]|E|$]]|$1|22|3|23|5|21|7|65|8|@]|D|@$9|66|A|67|1|68]]|E|$]]|$1|24|3|25|5|21|7|69|8|@]|D|@$9|6A|A|6B|1|6C]]|E|$]]|$1|26|3|27|5|21|7|6D|8|@]|D|@$9|6E|A|6F|1|6G]]|E|$]]|$1|28|3|29|5|21|7|6H|8|@]|D|@$9|6I|A|6J|1|6K]]|E|$]]|$1|2A|3|2B|5|21|7|6L|8|@]|D|@$9|6M|A|6N|1|6O]]|E|$]]|$1|2C|3|2D|5|21|7|6P|8|@]|D|@$9|6Q|A|6R|1|6S]]|E|$]]|$1|2E|3|2F|5|21|7|6T|8|@]|D|@$9|6U|A|6V|1|6W]]|E|$]]|$1|2G|3|-4|5|6|7|6X|8|@]|D|@]|E|$]]|$1|2H|3|-4|5|6|7|6Y|8|@]|D|@]|E|$]]]|2I|$2J|$5|2K|2L|2M|E|$2N|2O]]|2P|$5|2K|2L|2M|E|$2N|2Q]]|2R|$5|2K|2L|2M|E|$2N|2S]]|2T|$5|2K|2L|2M|E|$2N|2U]]|2V|$5|2K|2L|2M|E|$2N|2W]]|2X|$5|2K|2L|2M|E|$2N|2Y]]|2Z|$5|2K|2L|2M|E|$2N|30]]|31|$5|2K|2L|2M|E|$2N|32]]|33|$5|2K|2L|2M|E|$2N|34]]|35|$5|2K|2L|2M|E|$2N|36]]|37|$5|2K|2L|2M|E|$2N|38]]|39|$5|2K|2L|2M|E|$2N|3A]]]]

<h2>What is <code>yield</code>?</h2>
The <code>yield</code> keyword <a href="https://secure.php.net/manual/en/language.generators.syntax.php" rel="noreferrer">returns data from a generator function:</a>
<blockquote>
The heart of a generator function is the yield keyword. In its simplest form, a yield statement looks much like a return statement, except that instead of stopping execution of the function and returning, yield instead provides a value to the code looping over the generator and pauses execution of the generator function.
</blockquote>
<h2>What is a generator function?</h2>
A generator function is effectively a more compact and efficient way to write an <a href="https://secure.php.net/manual/en/class.iterator.php" rel="noreferrer">Iterator</a>. It allows you to define a function (your <code>xrange</code>) that will calculate and return values while you are <a href="https://secure.php.net/manual/en/control-structures.foreach.php" rel="noreferrer">looping over it</a>:
<pre class="lang-php prettyprint-override"><code>function xrange($min, $max) {
 for ($i = $min; $i &lt;= $max; $i++) {
 yield $i;
 }
}

[…]

foreach (xrange(1, 10) as $key =&gt; $value) {
 echo &quot;$key =&gt; $value&quot;, PHP_EOL;
}
</code></pre>
This would create the following output:
<pre><code>0 =&gt; 1
1 =&gt; 2
…
9 =&gt; 10
</code></pre>
You can also control the <code>$key</code> in the <code>foreach</code> by using
<pre><code>yield $someKey =&gt; $someValue;
</code></pre>
In the generator function, <code>$someKey</code> is whatever you want appear for <code>$key</code> and <code>$someValue</code> being the value in <code>$val</code>. In the question's example that's <code>$i</code>.
<h2>What's the difference to normal functions?</h2>
Now you might wonder why we are not simply using PHP's native <a href="https://secure.php.net/range" rel="noreferrer" title="Create an array containing a range of elements"><code>range</code> function</a> to achieve that output. And right you are. The output would be the same. The difference is how we got there.
When we use <code>range</code> PHP, will execute it, create the entire array of numbers in memory and <code>return</code> that entire array to the <code>foreach</code> loop which will then go over it and output the values. In other words, the <code>foreach</code> will operate on the array itself. The <code>range</code> function and the <code>foreach</code> only &quot;talk&quot; once. Think of it like getting a package in the mail. The delivery guy will hand you the package and leave. And then you unwrap the entire package, taking out whatever is in there.
When we use the generator function, PHP will step into the function and execute it until it either meets the end or a <code>yield</code> keyword. When it meets a <code>yield</code>, it will then return whatever is the value at that time to the outer loop. Then it goes back into the generator function and continues from where it yielded. Since your <code>xrange</code> holds a <code>for</code> loop, it will execute and yield until <code>$max</code> was reached. Think of it like the <code>foreach</code> and the generator playing ping pong.
<h2>Why do I need that?</h2>
Obviously, generators can be used to work around memory limits. Depending on your environment, doing a <code>range(1, 1000000)</code> will fatal your script whereas the same with a generator will just work fine. Or as Wikipedia puts it:
<blockquote>
Because generators compute their yielded values only on demand, they are useful for representing sequences that would be expensive or impossible to compute at once. These include e.g. infinite sequences and live data streams.
</blockquote>
Generators are also supposed to be pretty fast. But keep in mind that when we are talking about fast, we are usually talking in very small numbers. So before you now run off and change all your code to use generators, do a benchmark to see where it makes sense.
Another Use Case for Generators is asynchronous coroutines. The <code>yield</code> keyword does not only return values but it also accepts them. For details on this, see the two excellent blog posts linked below.
<h2>Since when can I use <code>yield</code>?</h2>
Generators have been introduced in PHP 5.5. Trying to use <code>yield</code> before that version will result in various parse errors, depending on the code that follows the keyword. So if you get a parse error from that code, update your PHP.
<h2>Sources and further reading:</h2>
<ul>
<li><a href="https://secure.php.net/generators" rel="noreferrer">Official docs</a></li>
<li><a href="https://wiki.php.net/rfc/generators" rel="noreferrer">The original RFC</a></li>
<li><a href="http://blog.kelunik.com/2017/09/14/an-introduction-to-generators-in-php.html" rel="noreferrer">kelunik's blog: An introduction to generators</a></li>
<li><a href="http://blog.ircmaxell.com/2012/07/what-generators-can-do-for-you.html" rel="noreferrer" title="Pragmatic high level overview">ircmaxell's blog: What generators can do for you</a></li>
<li><a href="http://nikic.github.io/2012/12/22/Cooperative-multitasking-using-coroutines-in-PHP.html" rel="noreferrer" title="has a more in-depth explanation of the xrange generator">NikiC's blog: Cooperative multitasking using coroutines in PHP</a></li>
<li><a href="https://medium.com/@assertchris/co-operative-php-multitasking-ce4ef52858a0" rel="noreferrer">Co-operative PHP Multitasking</a></li>
<li><a href="https://stackoverflow.com/questions/17223802/what-is-the-difference-between-a-generator-and-an-array">What is the difference between a generator and an array?</a></li>
<li><a href="https://en.wikipedia.org/wiki/Generator_%28computer_programming%29" rel="noreferrer">Wikipedia on Generators in general</a></li>
</ul>

blocks|key|181976|text|yield关键字用于PHP5.5中“生成器”的定义。好吧，那么什么是generator呢？|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|181977|来自php.net：|181978|迭代器生成器提供了一种实现简单迭代器的简单方法，而无需实现实现|181979|接口的类的开销或复杂性。|blockquote|181980|181981|生成器允许您编写使用foreach迭代一组数据的代码，而无需在内存中构建数组，这可能会导致超出内存限制，或者需要相当多的处理时间才能生成。相反，您可以编写一个生成器函数，该函数与普通函数相同，不同之处在于，生成器可以根据需要多次返回，以便提供要迭代的值。|181982|181983|在这里:生成器=生成器，其他函数(只是一个简单的函数)=函数。|181984|因此，它们在以下情况下非常有用：|181985|181986|you需要做简单的事情(或简单的事情)；|unordered-list-item|BOLD|181987|181988|generator实际上比实现Iterator接口简单得多。另一方面，当然，生成器的功能较差。compare+them.|181989|181990|you需要生成大量的数据保存内存；|181991|181992|实际上，为了节省内存，我们可以通过函数为每次循环迭代生成所需的数据，并在迭代后利用垃圾。所以这里的要点是-清晰的代码和可能的性能。看看哪种方法更适合你的需求。|181993|181994|you需要生成序列，序列依赖于中间值；|181995|181996|这是先前思想的延伸。与函数相比，生成器可以使事情变得更容易。检查Fibonacci+example，并尝试在没有生成器的情况下生成序列。此外，生成器可以更快地工作在这种情况下，至少因为将中间值存储在局部变量中；|181997|181998|you需要提高性能。|181999|182000|在某些情况下，它们可以比函数更快地工作(参见前面的好处)；|182001|entityMap|0|LINK|mutability|MUTABLE|url|http://www.php.net/manual/en/language.generators.overview.php|1|2|http://php.net/manual/en/language.generators.comparison.php^0|0|5|Y|9|0|0|0|0|0|0|0|0|0|0|0|0|K|0|0|1B|C|1|0|0|0|H|0|0|0|0|0|J|0|0|W|H|2|0|0|0|A|0|0|0^^$0|@$1|2|3|4|5|6|7|1V|8|@$9|1W|A|1X|B|C]]|D|@$9|1Y|A|1Z|1|20]]|E|$]]|$1|F|3|G|5|6|7|21|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|22|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|23|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|24|8|@]|D|@]|E|$]]|$1|N|3|O|5|6|7|25|8|@]|D|@]|E|$]]|$1|P|3|-4|5|6|7|26|8|@]|D|@]|E|$]]|$1|Q|3|R|5|6|7|27|8|@]|D|@]|E|$]]|$1|S|3|T|5|6|7|28|8|@]|D|@]|E|$]]|$1|U|3|-4|5|6|7|29|8|@]|D|@]|E|$]]|$1|V|3|W|5|X|7|2A|8|@$9|2B|A|2C|B|Y]]|D|@]|E|$]]|$1|Z|3|-4|5|6|7|2D|8|@]|D|@]|E|$]]|$1|10|3|11|5|6|7|2E|8|@]|D|@$9|2F|A|2G|1|2H]]|E|$]]|$1|12|3|-4|5|6|7|2I|8|@]|D|@]|E|$]]|$1|13|3|14|5|X|7|2J|8|@$9|2K|A|2L|B|Y]]|D|@]|E|$]]|$1|15|3|-4|5|6|7|2M|8|@]|D|@]|E|$]]|$1|16|3|17|5|6|7|2N|8|@]|D|@]|E|$]]|$1|18|3|-4|5|6|7|2O|8|@]|D|@]|E|$]]|$1|19|3|1A|5|X|7|2P|8|@$9|2Q|A|2R|B|Y]]|D|@]|E|$]]|$1|1B|3|-4|5|6|7|2S|8|@]|D|@]|E|$]]|$1|1C|3|1D|5|6|7|2T|8|@]|D|@$9|2U|A|2V|1|2W]]|E|$]]|$1|1E|3|-4|5|6|7|2X|8|@]|D|@]|E|$]]|$1|1F|3|1G|5|X|7|2Y|8|@$9|2Z|A|30|B|Y]]|D|@]|E|$]]|$1|1H|3|-4|5|6|7|31|8|@]|D|@]|E|$]]|$1|1I|3|1J|5|6|7|32|8|@]|D|@]|E|$]]|$1|1K|3|-4|5|6|7|33|8|@]|D|@]|E|$]]]|1L|$1M|$5|1N|1O|1P|E|$1Q|1R]]|1S|$5|1N|1O|1P|E|$1Q|1R]]|1T|$5|1N|1O|1P|E|$1Q|1U]]]]

<code>yield</code> keyword serves for definition of &quot;generators&quot; in PHP 5.5.
Ok, then what is a <a href="http://www.php.net/manual/en/language.generators.overview.php" rel="noreferrer">generator</a>?
From php.net:
<blockquote>
Generators provide an easy way to implement simple iterators without the overhead or complexity of implementing a class that implements the Iterator interface.
A generator allows you to write code that uses foreach to iterate over a set of data without needing to build an array in memory, which may cause you to exceed a memory limit, or require a considerable amount of processing time to generate. Instead, you can write a generator function, which is the same as a normal function, except that instead of returning once, a generator can yield as many times as it needs to in order to provide the values to be iterated over.
</blockquote>
From this place: generators = generators, other functions (just a simple functions) = functions.
So, they are useful when:
<ul>
<li>you need to do things simple (or simple things);
generator is really much simplier then implementing the Iterator interface. other hand is, ofcource, that generators are less functional. <a href="http://www.php.net/manual/en/language.generators.overview.php" rel="noreferrer">compare them</a>.
</li>
<li>you need to generate BIG amounts of data - saving memory;
actually to save memory we can just generate needed data via functions for every loop iteration, and after iteration utilize garbage. so here main points is - clear code and probably performance. see what is better for your needs.
</li>
<li>you need to generate sequence, which depends on intermediate values;
this is extending of the previous thought. generators can make things easier in comparison with functions. check <a href="http://php.net/manual/en/language.generators.comparison.php" rel="noreferrer">Fibonacci example</a>, and try to make sequence without generator. Also generators can work faster is this case, at least because of storing intermediate values in local variables;
</li>
<li>you need to improve performance.
they can work faster then functions in some cases (see previous benefit);
</li>
</ul>

blocks|key|182014|text|使用yield，您可以很容易地在单个函数中描述多个任务之间的断点。仅此而已，没有什么特别之处。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|182015|$closure+=+function+($injected1,+$injected2,+...){
++++$returned+=+array();
++++//task1+on+$injected1
++++$returned[]+=+$returned1;
//I+need+a+breakpoint+here!!!!!!!!!!!!!!!!!!!!!!!!!
++++//task2+on+$injected2
++++$returned[]+=+$returned2;
++++//...
++++return+$returned;
};
$returned+=+$closure($injected1,+$injected2,+...);|code-block|syntax|javascript|182016|如果task1和task2高度相关，但您需要在它们之间设置一个断点来做其他事情：|182017|182018|在处理数据库行之间释放内存|unordered-list-item|182019|运行其他任务，这些任务为下一个任务提供依赖性，但通过了解当前代码|182020|执行异步调用并等待结果|182021|等，这些任务是无关的...|182022|182023|那么生成器就是最好的解决方案，因为您不必将代码拆分成许多闭包，也不必将其与其他代码混合，也不必使用回调等。您只需使用yield添加一个断点，如果准备好了，您可以从该断点继续。|182024|添加不带生成器的断点：|182025|$closure1+=+function+($injected1){
++++//task1+on+$injected1
++++return+$returned1;
};
$closure2+=+function+($injected2){
++++//task2+on+$injected2
++++return+$returned1;
};
//...
$returned1+=+$closure1($injected1);
//breakpoint+between+task1+and+task2
$returned2+=+$closure2($injected2);
//...|182026|使用生成器添加断点|182027|$closure+=+function+(){
++++$injected1+=+yield;
++++//task1+on+$injected1
++++$injected2+=+(yield($returned1));
++++//task2+on+$injected2
++++$injected3+=+(yield($returned2));
++++//...
++++yield($returnedN);
};
$generator+=+$closure();
$returned1+=+$generator->send($injected1);
//breakpoint+between+task1+and+task2
$returned2+=+$generator->send($injected2);
//...
$returnedN+=+$generator->send($injectedN);|182028|注意:生成器很容易出错，所以在实现它们之前一定要编写单元测试！note2:在无限循环中使用生成器就像编写一个具有无限长度的闭包……|182029|entityMap^0|2|5|0|0|0|0|0|0|0|0|0|1M|5|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1B|8|@$9|1C|A|1D|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|1E|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|1F|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|1G|8|@]|D|@]|E|$]]|$1|N|3|O|5|P|7|1H|8|@]|D|@]|E|$]]|$1|Q|3|R|5|P|7|1I|8|@]|D|@]|E|$]]|$1|S|3|T|5|P|7|1J|8|@]|D|@]|E|$]]|$1|U|3|V|5|P|7|1K|8|@]|D|@]|E|$]]|$1|W|3|-4|5|6|7|1L|8|@]|D|@]|E|$]]|$1|X|3|Y|5|6|7|1M|8|@$9|1N|A|1O|B|C]]|D|@]|E|$]]|$1|Z|3|10|5|6|7|1P|8|@]|D|@]|E|$]]|$1|11|3|12|5|H|7|1Q|8|@]|D|@]|E|$I|J]]|$1|13|3|14|5|6|7|1R|8|@]|D|@]|E|$]]|$1|15|3|16|5|H|7|1S|8|@]|D|@]|E|$I|J]]|$1|17|3|18|5|6|7|1T|8|@]|D|@]|E|$]]|$1|19|3|-4|5|6|7|1U|8|@]|D|@]|E|$]]]|1A|$]]

With <code>yield</code> you can easily describe the breakpoints between multiple tasks in a single function. That's all, there is nothing special about it.

<pre><code>$closure = function ($injected1, $injected2, ...){
 $returned = array();
 //task1 on $injected1
 $returned[] = $returned1;
//I need a breakpoint here!!!!!!!!!!!!!!!!!!!!!!!!!
 //task2 on $injected2
 $returned[] = $returned2;
 //...
 return $returned;
};
$returned = $closure($injected1, $injected2, ...);
</code></pre>

If task1 and task2 are highly related, but you need a breakpoint between them to do something else:

<ul>
<li>free memory between processing database rows</li>
<li>run other tasks which provide dependency to the next task, but which are unrelated by understanding the current code</li>
<li>doing async calls and wait for the results</li>
<li>and so on ...</li>
</ul>

then generators are the best solution, because you don't have to split up your code into many closures or mix it with other code, or use callbacks, etc... You just use <code>yield</code> to add a breakpoint, and you can continue from that breakpoint if you are ready.

Add breakpoint without generators:

<pre><code>$closure1 = function ($injected1){
 //task1 on $injected1
 return $returned1;
};
$closure2 = function ($injected2){
 //task2 on $injected2
 return $returned1;
};
//...
$returned1 = $closure1($injected1);
//breakpoint between task1 and task2
$returned2 = $closure2($injected2);
//...
</code></pre>

Add breakpoint with generators

<pre><code>$closure = function (){
 $injected1 = yield;
 //task1 on $injected1
 $injected2 = (yield($returned1));
 //task2 on $injected2
 $injected3 = (yield($returned2));
 //...
 yield($returnedN);
};
$generator = $closure();
$returned1 = $generator-&gt;send($injected1);
//breakpoint between task1 and task2
$returned2 = $generator-&gt;send($injected2);
//...
$returnedN = $generator-&gt;send($injectedN);
</code></pre>

note: It is easy to make mistake with generators, so always write unit tests before you implement them!
note2: Using generators in an infinite loop is like writing a closure which has infinite length...

blocks|key|2798688|text|简单示例|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|2798689|<?php
echo+'#start+main#+';
function+a(){
++++echo+'{start[';
++++for($i=1;+$i<=9;+$i%2B%2B)
++++++++yield+$i;
++++echo+']end}+';
}
foreach(a()+as+$v)
++++echo+$v.',';
echo+'#end+main#';
?>|code-block|syntax|javascript|2798690|输出|2798691|#start+main#+{start[1,2,3,4,5,6,7,8,9,]end}+#end+main#|2798692|高级示例|2798693|<?php
echo+'#start+main#+';
function+a(){
++++echo+'{start[';
++++for($i=1;+$i<=9;+$i%2B%2B)
++++++++yield+$i;
++++echo+']end}+';
}
foreach(a()+as+$k+=>+$v){
++++if($k+===+5)
++++++++break;
++++echo+$k.'=>'.$v.',';
}
echo+'#end+main#';
?>|2798694|2798695|#start+main#+{start[0=>1,1=>2,2=>3,3=>4,4=>5,#end+main#|2798696|entityMap^0|0|4|0|0|0|2|0|0|0|4|0|0|0|2|0|0^^$0|@$1|2|3|4|5|6|7|X|8|@$9|Y|A|Z|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|10|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|11|8|@$9|12|A|13|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|14|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|15|8|@$9|16|A|17|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|H|7|18|8|@]|D|@]|E|$I|J]]|$1|S|3|L|5|6|7|19|8|@$9|1A|A|1B|B|C]]|D|@]|E|$]]|$1|T|3|U|5|H|7|1C|8|@]|D|@]|E|$I|J]]|$1|V|3|-4|5|6|7|1D|8|@]|D|@]|E|$]]]|W|$]]

simple example

<pre><code>&lt;?php
echo '#start main# ';
function a(){
 echo '{start[';
 for($i=1; $i&lt;=9; $i++)
 yield $i;
 echo ']end} ';
}
foreach(a() as $v)
 echo $v.',';
echo '#end main#';
?&gt;
</code></pre>

output

<pre><code>#start main# {start[1,2,3,4,5,6,7,8,9,]end} #end main#
</code></pre>

advanced example

<pre><code>&lt;?php
echo '#start main# ';
function a(){
 echo '{start[';
 for($i=1; $i&lt;=9; $i++)
 yield $i;
 echo ']end} ';
}
foreach(a() as $k =&gt; $v){
 if($k === 5)
 break;
 echo $k.'=&gt;'.$v.',';
}
echo '#end main#';
?&gt;
</code></pre>

output

<pre><code>#start main# {start[0=&gt;1,1=&gt;2,2=&gt;3,3=&gt;4,4=&gt;5,#end main#
</code></pre>

blocks|key|3011100|text|此函数使用yield：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3011101|function+a($items)+{
++++foreach+($items+as+$item)+{
++++++++yield+$item+%2B+1;
++++}
}|code-block|syntax|javascript|3011102|它几乎与这个相同，没有：|3011103|function+b($items)+{
++++$result+=+[];
++++foreach+($items+as+$item)+{
++++++++$result[]+=+$item+%2B+1;
++++}
++++return+$result;
}|3011104|唯一的区别是a()返回一个generator，而b()只返回一个简单的数组。你可以在这两个上面都迭代。|offset|length|style|CODE|3011105|而且，第一个不会分配一个完整的数组，因此对内存的要求较低。|3011106|entityMap|0|LINK|mutability|MUTABLE|url|http://php.net/manual/en/language.generators.overview.php^0|0|0|0|0|6|3|O|3|D|9|0|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|11|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|12|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|13|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|14|8|@$M|15|N|16|O|P]|$M|17|N|18|O|P]]|9|@$M|19|N|1A|1|1B]]|A|$]]|$1|Q|3|R|5|6|7|1C|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|1D|8|@]|9|@]|A|$]]]|T|$U|$5|V|W|X|A|$Y|Z]]]]

This function is using yield:
<pre><code>function a($items) {
 foreach ($items as $item) {
 yield $item + 1;
 }
}
</code></pre>
It is almost the same as this one without:
<pre><code>function b($items) {
 $result = [];
 foreach ($items as $item) {
 $result[] = $item + 1;
 }
 return $result;
}
</code></pre>
The only one difference is that <code>a()</code> returns a <a href="http://php.net/manual/en/language.generators.overview.php" rel="noreferrer">generator</a> and <code>b()</code> just a simple array. You can iterate on both.
Also, the first one does not allocate a full array and is therefore less memory-demanding.

blocks|key|182063|text|值得在这里讨论的一个有趣的方面是，通过引用进行让步。每次我们需要更改一个参数以使其反映在函数之外时，我们必须通过引用传递此参数。要将其应用于生成器，我们只需在生成器的名称和迭代中使用的变量前面加上一个与号&：|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|CODE|entityRanges|data|182064|+<?php+
+/**
+*+Yields+by+reference.
+*+@param+int+$from
+*/
function+&counter($from)+{
++++while+($from+>+0)+{
++++++++yield+$from;
++++}
}

foreach+(counter(100)+as+&$value)+{
++++$value--;
++++echo+$value+.+'...';
}

//+Output:+99...98...97...96...95...|code-block|syntax|javascript|182065|上面的示例显示了更改foreach循环中的迭代值如何更改生成器中的$from变量。这是因为$from是通过引用生成的，这是由于生成器名称之前的“与”符号。因此，foreach循环中的$value变量是对生成器函数中的$from变量的引用。|182066|entityMap^0|N|29|2U|1|0|0|0|1J|1M|1P|A|7|X|5|19|5|28|7|2J|6|30|5|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]|$9|S|A|T|B|D]]|E|@]|F|$]]|$1|G|3|H|5|I|7|U|8|@]|E|@]|F|$J|K]]|$1|L|3|M|5|6|7|V|8|@$9|W|A|X|B|C]|$9|Y|A|Z|B|C]|$9|10|A|11|B|D]|$9|12|A|13|B|D]|$9|14|A|15|B|D]|$9|16|A|17|B|D]|$9|18|A|19|B|D]|$9|1A|A|1B|B|D]]|E|@]|F|$]]|$1|N|3|-4|5|6|7|1C|8|@]|E|@]|F|$]]]|O|$]]

An interesting aspect, which worth to be discussed here, is yielding by reference. Every time we need to change a parameter such that it is reflected outside of the function, we have to pass this parameter by reference. To apply this to generators, we simply prepend an ampersand <code>&amp;</code> to the name of the generator and to the variable used in the iteration:

<pre><code> &lt;?php 
 /**
 * Yields by reference.
 * @param int $from
 */
function &amp;counter($from) {
 while ($from &gt; 0) {
 yield $from;
 }
}

foreach (counter(100) as &amp;$value) {
 $value--;
 echo $value . '...';
}

// Output: 99...98...97...96...95...
</code></pre>

The above example shows how changing the iterated values within the <code>foreach</code> loop changes the <code>$from</code> variable within the generator. This is because <code>$from</code> is yielded by reference due to the ampersand before the generator name. Because of that, the <code>$value</code> variable within the <code>foreach</code> loop is a reference to the <code>$from</code> variable within the generator function.

blocks|key|183493|text|下面的代码演示了如何使用生成器在完成之前返回结果，这与传统的非生成器方法不同，非生成器方法在完整迭代后返回完整的数组。使用下面的生成器，值在准备好时返回，不需要等待数组完全填充：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|183494|<?php+

function+sleepiterate($length)+{
++++for+($i=0;+$i+<+$length;+$i%2B%2B)+{
++++++++sleep(2);
++++++++yield+$i;
++++}
}

foreach+(sleepiterate(5)+as+$i)+{
++++echo+$i,+PHP_EOL;
}|code-block|syntax|javascript|183495|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

The below code illustrates how using a generator returns a result before completion, unlike the traditional non generator approach that returns a complete array after full iteration. With the generator below, the values are returned when ready, no need to wait for an array to be completely filled:

<pre><code>&lt;?php 

function sleepiterate($length) {
 for ($i=0; $i &lt; $length; $i++) {
 sleep(2);
 yield $i;
 }
}

foreach (sleepiterate(5) as $i) {
 echo $i, PHP_EOL;
}
</code></pre>

blocks|key|182082|text|没有一个答案显示了使用由非数字成员填充的大规模数组的具体示例。以下是在一个大型.txt文件(在我的用例中为262MB)上使用explode()生成的数组的示例：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|182083|<?php

ini_set('memory_limit','1000M');

echo+"Starting+memory+usage:+"+.+memory_get_usage()+.+" ";

$path+=+'./file.txt';
$content+=+file_get_contents($path);

foreach(explode("\n",+$content)+as+$ex)+{
++++$ex+=+trim($ex);
}

echo+"Final+memory+usage:+"+.+memory_get_usage();|code-block|syntax|javascript|182084|输出结果为：|182085|Starting+memory+usage:+415160
Final+memory+usage:+270948256|182086|现在，使用yield关键字将其与类似的脚本进行比较：|182087|<?php

ini_set('memory_limit','1000M');

echo+"Starting+memory+usage:+"+.+memory_get_usage()+.+" ";

function+x()+{
++++$path+=+'./file.txt';
++++$content+=+file_get_contents($path);
++++foreach(explode("\n",+$content)+as+$x)+{
++++++++yield+$x;
++++}
}

foreach(x()+as+$ex)+{
++++$ex+=+trim($ex);
}

echo+"Final+memory+usage:+"+.+memory_get_usage();|182088|此脚本的输出为：|182089|Starting+memory+usage:+415152
Final+memory+usage:+415616|182090|显然，内存使用量的节省是相当可观的(在第一个示例中为~270.5+MB，在第二个示例中为~450B+)。Δ内存使用量->|BOLD|182091|entityMap^0|1Q|9|0|0|0|0|5|5|0|0|0|0|Q|9|18|5|0^^$0|@$1|2|3|4|5|6|7|11|8|@$9|12|A|13|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|14|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|15|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|16|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|17|8|@$9|18|A|19|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|H|7|1A|8|@]|D|@]|E|$I|J]]|$1|S|3|T|5|6|7|1B|8|@]|D|@]|E|$]]|$1|U|3|V|5|H|7|1C|8|@]|D|@]|E|$I|J]]|$1|W|3|X|5|6|7|1D|8|@$9|1E|A|1F|B|Y]|$9|1G|A|1H|B|Y]]|D|@]|E|$]]|$1|Z|3|-4|5|6|7|1I|8|@]|D|@]|E|$]]]|10|$]]

None of the answers show a concrete example using massive arrays populated by non-numeric members. Here is an example using an array generated by <code>explode()</code> on a large .txt file (262MB in my use case):
<pre><code>&lt;?php

ini_set('memory_limit','1000M');

echo &quot;Starting memory usage: &quot; . memory_get_usage() . &quot;&lt;br&gt;&quot;;

$path = './file.txt';
$content = file_get_contents($path);

foreach(explode(&quot;\n&quot;, $content) as $ex) {
 $ex = trim($ex);
}

echo &quot;Final memory usage: &quot; . memory_get_usage();
</code></pre>
The output was:
<pre><code>Starting memory usage: 415160
Final memory usage: 270948256
</code></pre>
Now compare that to a similar script, using the <code>yield</code> keyword:
<pre><code>&lt;?php

ini_set('memory_limit','1000M');

echo &quot;Starting memory usage: &quot; . memory_get_usage() . &quot;&lt;br&gt;&quot;;

function x() {
 $path = './file.txt';
 $content = file_get_contents($path);
 foreach(explode(&quot;\n&quot;, $content) as $x) {
 yield $x;
 }
}

foreach(x() as $ex) {
 $ex = trim($ex);
}

echo &quot;Final memory usage: &quot; . memory_get_usage();
</code></pre>
The output for this script was:
<pre><code>Starting memory usage: 415152
Final memory usage: 415616
</code></pre>
Clearly memory usage savings were considerable (ΔMemoryUsage -----&gt; ~270.5 MB in first example, ~450B in second example).

blocks|key|2798838|text|在实现PHP+IteratorAggregate接口时，关键字yield将非常有用。查看文档，有几个使用ArrayIterator或yield的示例。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2798839|在php-ds/polyfill存储库中可以找到另一个示例：https://github.com/php-ds/polyfill/blob/e52796c50aac6e6cfa6a0e8182943027bacbe187/src/Traits/GenericSequence.php#L359|2798840|这个想法类似于下面的快速示例：|2798841|class+Collection+implements+\IteratorAggregate
{
++++private+$array+=+[];

++++public+function+push(...$values)
++++{
++++++++array_push($this->array,+...$values);
++++}

++++public+function+getIterator()
++++{
++++++++foreach+($this->array+as+$value)+{
++++++++++++yield+$value;
++++++++}
++++}
}

$collection+=+new+Collection();
$collection->push('apple',+'orange',+'banana');

foreach+($collection+as+$key+=>+$value)+{
++++echo+sprintf("[%25s]+=>+%25s\n",+$key,+$value);
}|code-block|syntax|javascript|2798842|输出：|2798843|[0]+=>+apple
[1]+=>+orange
[2]+=>+banana|2798844|entityMap|0|LINK|mutability|MUTABLE|url|https://www.php.net/manual/en/class.iteratoraggregate.php|1|https://github.com/php-ds/polyfill/blob/e52796c50aac6e6cfa6a0e8182943027bacbe187/src/Traits/GenericSequence.php#L359^0|V|5|1G|D|1U|5|7|H|0|0|1|F|U|38|1|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@$9|13|A|14|B|C]|$9|15|A|16|B|C]|$9|17|A|18|B|C]]|D|@$9|19|A|1A|1|1B]]|E|$]]|$1|F|3|G|5|6|7|1C|8|@$9|1D|A|1E|B|C]]|D|@$9|1F|A|1G|1|1H]]|E|$]]|$1|H|3|I|5|6|7|1I|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|1J|8|@]|D|@]|E|$M|N]]|$1|O|3|P|5|6|7|1K|8|@]|D|@]|E|$]]|$1|Q|3|R|5|L|7|1L|8|@]|D|@]|E|$M|N]]|$1|S|3|-4|5|6|7|1M|8|@]|D|@]|E|$]]]|T|$U|$5|V|W|X|E|$Y|Z]]|10|$5|V|W|X|E|$Y|11]]]]

When implement PHP <a href="https://www.php.net/manual/en/class.iteratoraggregate.php" rel="nofollow noreferrer">IteratorAggregate</a> interface, the <code>yield</code> keyword will be useful. Check out the documentation, there are couple examples either using <code>ArrayIterator</code> or <code>yield</code>.
Another example can be found is in the <code>php-ds/polyfill</code> repo: <a href="https://github.com/php-ds/polyfill/blob/e52796c50aac6e6cfa6a0e8182943027bacbe187/src/Traits/GenericSequence.php#L359" rel="nofollow noreferrer">https://github.com/php-ds/polyfill/blob/e52796c50aac6e6cfa6a0e8182943027bacbe187/src/Traits/GenericSequence.php#L359</a>
The idea is similar to the quick example below:
<pre><code>class Collection implements \IteratorAggregate
{
 private $array = [];

 public function push(...$values)
 {
 array_push($this-&gt;array, ...$values);
 }

 public function getIterator()
 {
 foreach ($this-&gt;array as $value) {
 yield $value;
 }
 }
}

$collection = new Collection();
$collection-&gt;push('apple', 'orange', 'banana');

foreach ($collection as $key =&gt; $value) {
 echo sprintf(&quot;[%s] =&gt; %s\n&quot;, $key, $value);
}
</code></pre>
Output:
<pre><code>[0] =&gt; apple
[1] =&gt; orange
[2] =&gt; banana
</code></pre>

I've recently stumbled over this code:

<pre><code>function xrange($min, $max) 
{
 for ($i = $min; $i &lt;= $max; $i++) {
 yield $i;
 }
}
</code></pre>

I've never seen this <code>yield</code> keyword before. Trying to run the code I get 

<blockquote>
 Parse error: syntax error, unexpected T_VARIABLE on line x
</blockquote>

So what is this <code>yield</code> keyword? Is it even valid PHP? And if it is, how do I use it?

What does yield mean in PHP?

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我最近偶然发现了这段代码：function xrange($min, $max) { for ($i = $min; $i <= $max; $i++) { yield $i; }}我以前从未见过这个yield关键字。尝试运行我得到的代码分析错误:语法错误，第%x行出现意外的T_VARIABL...

问在PHP中，yield是什么意思？
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问在PHP中，yield是什么意思？
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