PHP generator syntax details analysis

A generator function looks like a normal function, the difference is that the generator will produce multiple values ​​on demand instead of returning a single value.

When the generator function is called, it will return an iterable object When you iterate over that object (for example, through a foreach loop), PHP will call the generator function each time a value is needed, and then when the generator produces a value, it will save the generator's state for the next time. A value can be restored when needed.

If no more values ​​are produced, the generator function can simply exit, and the calling code will continue as if an array has used up all values.

Note:

Generators cannot return values : Doing so will cause a compilation error. An empty return statement within a generator is valid syntax and will break that generator.

yield Keyword

At the heart of generator functions is the yield keyword. In its simplest form, A yield statement looks very much like a return statement, except that instead of returning a value and then stopping execution, yield provides a value to the code that looks at the generator and then pauses the execution of the generator.

Example #1 Yielding a value Simple example

<?php
    function gen_one_to_three() {
        for ($i = 1; $i <= 3; $i++) {
            // Note that $i is preserved between yields.
            yield $i;
        }
    }
    $generator = gen_one_to_three();
    foreach ($generator as $value) {
         echo "$value\n";
    }
?>

The above routine will output:

1
2
3

Note:

Internally, the serialized integer key will be paired with the output value, just like a non-associative array.

Caution If you use context expressions When using yield (for example, in an assignment statement to the right of the equal sign), be sure to use parentheses. For example, the following code is correct:

$data = (yield $value);

But the following code will A parsing error occurred:

$data = yield $value;

This syntax may be used with the send() method in the generator object.

Use key to get the value

PHP supports associative arrays, and the same goes for generators. In addition to generating simple values, as shown above, you can also generate keys at the same time.

The syntax for generating key/value pairs is very simple and can be used to define associative arrays, as shown below.

Example #2 Yielding a key/ value pair

<?php
    /* The input is semi-colon separated fields, with the first
     * field being an ID to use as a key. */
    $input = <<<&#39;EOF&#39;
1;PHP;Likes dollar signs
2;Python;Likes whitespace
3;Ruby;Likes blocks
EOF;
    function input_parser($input) {
        foreach (explode("\n", $input) as $line) {
            $fields = explode(&#39;;&#39;, $line);
            $id = array_shift($fields);
            yield $id => $fields;
        }
    }
    foreach (input_parser($input) as $id => $fields) {
        echo "$id:\n";
        echo " $fields[0]\n";
        echo " $fields[1]\n";
    }
?>

The above routine will output:

1:
    PHP
    Likes dollar signs
2:
    Python
    Likes whitespace
3:
    Ruby
    Likes blocks

Caution is the same as the earlier generation of simple values. Generating key/value pairs in a context expression requires the yield statement to be enclosed in parentheses:

$data = (yield $key => $value);

Produces null value

Yield can be called without arguments to generate NULL values ​​using automatically generated keys.

Example #3 Yielding NULLs

<?php
    function gen_three_nulls() {
        foreach (range(1, 3) as $i) {
            yield;
        }
    }
    var_dump(iterator_to_array(gen_three_nulls()));
?>

The above routine will output:

array(3) {
    [0]=>NULL
    [1]=>NULL
    [2]=>NULL
}

Producing values ​​by reference

Generator functions can generate values ​​by reference. This is the same as returning references from functions: by adding & in front of the function name.

Example #4 Yielding values ​​by reference

<?php
    function &gen_reference() {
        $value = 3;
        while ($value > 0) {
            yield $value;
        }
    }
    /* 注意我们可以在循环中改变$number
     * because the generator is yielding references, $value
     * within gen_reference() changes. */
    foreach (gen_reference() as &$number) {
        echo (--$number).&#39;... &#39;;
    }
?>

The above routine will output:

2... 1... 0...