生成器与 Iterator 对象的比较

生成器最主要的优点是简洁。和实现一个 Iterator 类相较而言, 同样的功能,用生成器可以编写更少的代码,可读性也更强。 举例,下面的类和函数是相等的:

<?php
function getLinesFromFile($fileName) {
    if (!
$fileHandle fopen($fileName'r')) {
        return;
    }
 
    while (
false !== $line fgets($fileHandle)) {
        yield 
$line;
    }
 
    
fclose($fileHandle);
}

// 比较下...

class LineIterator implements Iterator {
    protected 
$fileHandle;
 
    protected 
$line;
    protected 
$i;
 
    public function 
__construct($fileName) {
        if (!
$this->fileHandle fopen($fileName'r')) {
            throw new 
RuntimeException('Couldn\'t open file "' $fileName '"');
        }
    }
 
    public function 
rewind() {
        
fseek($this->fileHandle0);
        
$this->line fgets($this->fileHandle);
        
$this->0;
    }
 
    public function 
valid() {
        return 
false !== $this->line;
    }
 
    public function 
current() {
        return 
$this->line;
    }
 
    public function 
key() {
        return 
$this->i;
    }
 
    public function 
next() {
        if (
false !== $this->line) {
            
$this->line fgets($this->fileHandle);
            
$this->i++;
        }
    }
 
    public function 
__destruct() {
        
fclose($this->fileHandle);
    }
}
?>

不过,这也付出了灵活性的代价: 生成器是一个只能向前的迭代器,一旦开始遍历就无法后退。 意思也就是说,同样的生成器无法遍历多遍:要么再次调用生成器函数,重新生成后再遍历。

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User Contributed Notes 4 notes

up
83
mNOSPAMsenghaa at nospam dot gmail dot com
7 years ago
This hardly seems a fair comparison between the two examples, size-for-size. As noted, generators are forward-only, meaning that it should be compared to an iterator with a dummy rewind function defined. Also, to be fair, since the iterator throws an exception, shouldn't the generator example also throw the same exception? The code comparison would become more like this:

<?php
function getLinesFromFile($fileName) {
    if (!
$fileHandle = fopen($fileName, 'r')) {
        throw new
RuntimeException('Couldn\'t open file "' . $fileName . '"');
    }

    while (
false !== $line = fgets($fileHandle)) {
        yield
$line;
    }

   
fclose($fileHandle);
}

// versus...

class LineIterator implements Iterator {
    protected
$fileHandle;

    protected
$line;
    protected
$i;

    public function
__construct($fileName) {
        if (!
$this->fileHandle = fopen($fileName, 'r')) {
            throw new
RuntimeException('Couldn\'t open file "' . $fileName . '"');
        }
    }

    public function
rewind() { }

    public function
valid() {
        return
false !== $this->line;
    }

    public function
current() {
        return
$this->line;
    }

    public function
key() {
        return
$this->i;
    }

    public function
next() {
        if (
false !== $this->line) {
           
$this->line = fgets($this->fileHandle);
           
$this->i++;
        }
    }

    public function
__destruct() {
       
fclose($this->fileHandle);
    }
}
?>

The generator is still obviously much shorter, but this seems a more reasonable comparison.
up
17
sergeyzsg at yandex dot ru
6 years ago
I think that this is bad generator example.
If user will not consume all lines then file will not be closed.

<?php
function getLinesFromFile($fileHandle) {
    while (
false !== $line = fgets($fileHandle)) {
        yield
$line;
    }
}

if (
$fileHandle = fopen($fileName, 'r')) {
   
/*
    something with getLinesFromFile
    */
   
fclose($fileHandle);
}
?>
up
-2
sou at oand dot re
7 years ago
I think to be more similar the samples in the function, throw a new exception is better. But looking into "Generator syntax" session, you can see there this: "An empty return statement is valid syntax within a generator and it will terminate the generator.". By this point of view, we can imagine that this is just to exemplify an usage of the empty return.
up
-3
jorgeley at gmail dot com
1 year ago
I think the power of generators is underestimated here, look at my example:

<?php

/**
* simple example class just to have something to instantiate
*/
class obj {

    private
$i = 1;
    private
$a = [];

    function
__construct($i = 1) {
       
$this->i = $i;
       
$this->a = range(0, $i);
    }

    public function
getI() {
        return
$this->i;
    }
   
//more getters and setters...
}

/**
* this is a common way of returning objects in a bulk
* @param int $n
* @return \obj
*/
function returnObjects($n = 1000) {
   
$objs = [];
    for (
$i = 1; $i <= $n; $i++) {
       
$objs[] = new obj($i);
    }
    return
$objs;
}

/**
* this is a  better way using generator, rather than returning all objects,
* it returns one by one (it saves the state of the function in every call)
* @param int $n
*/
function generateObjects($n = 1000) {
    for (
$i = 1; $i <= $n; $i++) {
       
/**
         * 'yield' returns the object and save the status of the function, so
         * next call starts from next loop iteration and so on...
         */
       
yield (new obj($i));
    }
}

//main script: get current memory, run one of the functions and calculate memory usage after
$m = memory_get_peak_usage();
/**
* comment 'returnObjects()' call bellow and uncomment 'generateObjects()' call
* if you want to see the generator memory usage
*/
//$objs = returnObjects();

/**
* comment 'generateObjects()' and uncomment 'returnObjects()' call if you
* want to see the common function return memory usage
*/
$objs = generateObjects();
foreach (
$objs as $obj) {
    echo
get_class($obj) . ": {$obj->getI()}\n";
}
echo
"total memory comsuption: " . (memory_get_peak_usage() - $m) . " bytes\n";

?>

what is the outcome? Using the 'returnObjects()' we return an array within 1000 objects, but using the 'generateObjects()' we only instantiate one object since the yield returns (stop the loop) but also saves the state of the function, so next call the function resumes rather than restarting. In my environment I got a difference of 37K to 25M
Thanks to my dear friend Ivan Frezza who helped me to understand this better!
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