上手并过渡到PHP7--轻量级“集合”迭代器-Generator
轻量级“集合”迭代器-Generator
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Generator是PHP 5.5加入的新语言特性。但是,它似乎并没有被很多PHP开发者广泛采用。因此,在我们了解PHP 7对Generator的改进之前,我们先通过一个简单却显而易见的例子来了解下Generator究竟为我们解决什么问题。
一千万个整数的数组
如果我们要“创建一个指定元素个数的数组”,怎么做呢?我们可能会不加思索的写下下面的代码:
function makeRange($range) { $data = []; for ($i = 0; $i < $range; $i++) { $data[] = $i; } return $data;}
然后,在我们的开发环境里,分别创建1万个,10万个,100万个元素的数组,一切看起来都如我们想象。
makeRange(10000);makeRange(100000);makeRange(1000000);
但是,当我们创建一个包涵1千万个整数的数组时,情况就不一样了,系统没有那么多内存分配给我们:
makeRange(10000000);
在资源有限的情况下,解决大文件或大批量数据的处理问题,就是PHP generator的一个典型应用。
把PHP Generator请出来帮忙
定义PHP generator很简单,看上去就像定义一个函数一样:
function makeRangeByGenerator($range) { for ($i = 0; $i < $range; $i++) { yield $i; }}
只是,makeRangeByGenerator没有返回值,也没有把整个数组创建在内存里,而只是通过关键字yield,标记了每次循环应该生成的值。接下来,我们可以像访问一个普通集合一样使用generator:
foreach ($makeRangeByGenerator(100) as $i) { echo $i.'<br>';}
现在重新执行我们的PHP文件,就不会再报错了。并且,我们可以在页面上,看到generator生成的值。这就是generator的典型应用场景,简单来说:Generator就是一个轻量级迭代器,它可以自动的记住每一次被调用时的状态,并返回给我们正确的值。
PHP 7做了哪些改进?
了解了generator的基本用法之后,我们就可以看一下PHP 7对它做了哪些改进了。
PHP 7允许我们给generator加入返回值,就像我们定义函数的返回值一样:
function makeRangeByGenerator($range) {
for ($i = 0; $i < $range; $i++) { yield $i; } return "Finish yielding";
Copy after login}
迭代完generator的所有元素时,我们就可以通过getReturn()方法,来读取generator的返回值:
$gen = makeRangeByGenerator(100);foreach ($gen as $i) { echo $i.'<br>';}echo $gen->getReturn();
*“我们必须在所有迭代完generator所有值之后,才能读取generator的返回值,否则PHP会报错。”
——最佳实践*
PHP 7对generator的第二个改进就是允许嵌套。例如:
function outer() { yield "PHP 7 "; yield "is one of "; yield "the best "; yield from inner();}function inner() { yield "programming languages in the world";}
我们使用关键字from引入一个新的generator,当我们遍历outer generator时,我们就会发现,它自动生成了inner generator的值:
foreach(outer() as $str) { echo $str;}
这就是关于PHP generator的全部内容。简单来说,generator就是一个轻量级的,可以记住自身状态的“集合”迭代器。如果你之前还不了解它,现在是时候考虑下它在哪些地方可以帮助到你了。
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