[PHP] yield
http://php.net/manual/en/language.generators.syntax.php
Generator syntax ¶
A generator function looks just like a normal function, except that instead of returning a value, a generator yields as many values as it needs to.
When a generator function is called, it returns an object that can be iterated over. When you iterate over that object (for instance, via aforeach loop), PHP will call the generator function each time it needs a value, then saves the state of the generator when the generator yields a value so that it can be resumed when the next value is required.
Once there are no more values to be yielded, then the generator function can simply exit, and the calling code continues just as if an array has run out of values.
yield keyword ¶
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.
Example #1 A simple example of yielding values
<?phpfunction 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 example will output:
1 2 3
Yielding values with keys ¶
PHP also supports associative arrays, and generators are no different. In addition to yielding simple values, as shown above, you can also yield a key at the same time.
The syntax for yielding a key/value pair is very similar to that used to define an associative array, 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.
*/
$input = <<<'EOF'1;PHP;Likes dollar signs
2;Python;Likes whitespace
3;Ruby;Likes blocksEOF;
function input_parser($input) {
foreach (explode("\n", $input) as $line) {
$fields = explode(';', $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 example will output:
1: PHP Likes dollar signs 2: Python Likes whitespace 3: Ruby Likes blocks
Yielding null values ¶
Yield can be called without an argument to yield a
NULL
value with an automatic key.
Example #3 Yielding
NULL
s<?phpfunction gen_three_nulls() {
foreach (range(1, 3) as $i) {
yield;
}
}
var_dump(iterator_to_array(gen_three_nulls()));?>
The above example will output:
array(3) { [0]=> NULL [1]=> NULL [2]=> NULL }
Yielding by reference ¶
Generator functions are able to yield values by reference as well as by value. This is done in the same way as returning references from functions: by prepending an ampersand to the function name.
Example #4 Yielding values by reference
<?phpfunction &gen_reference() {
$value = 3;
while ($value > 0) {
yield $value;
}
}
/*
* Note that we can change $number within the loop, and
* because the generator is yielding references, $value
* within gen_reference() changes.
*/foreach (gen_reference() as &$number) {
echo (--$number).'... ';
}?>
The above example will output:
2... 1... 0...
留言
張貼留言