Are the following codes same? <div class="highlight highlight-text-html-php notran

How exactly this code runs? <div class="highlight highlight-text-html-php notransl

Question about promise HOT 4 CLOSED

sm2017 commented on May 18, 2024

Question

from promise.

Comments (4)

jsor commented on May 18, 2024

No, it is not the same. Chaining then() calls allows for transformation of the resolution value. See https://github.com/reactphp/promise/blob/master/README.md#resolution-forwarding.

$promise = $deferred->promise()
    ->then(function ($x) {
          // Here we get the initial resolution value: $x === 1;
          // Now add 1
          return $x + 1;
    })
    ->then(function ($x) {
          // Here we get the transformed value from the previous then() callback: $x === 2;
          // Now add 1 again
          return $x + 1;
    })
    ->then(function ($x) {
          // Here we get the transformed value from the previous then() callback: $x === 3;
          echo $x;
    });

$deferred->resolve(1);

from promise.

sm2017 commented on May 18, 2024

How exactly this code runs?

$promise = $deferred->promise();
$promise->then(function ($x) {
          // Here we get the initial resolution value: $x === 1;
          // Now add 1
          return $x + 1;
    });
$promise->then(function ($x) {
          // Here we get the transformed value from the previous then() callback: $x === 2;
          // Now add 1 again
          return $x + 1;
    });
$promise->then(function ($x) {
          // Here we get the transformed value from the previous then() callback: $x === 3;
          echo $x;
    });

$deferred->resolve(1);

How can I have a conditional then ?

Get a promise
Check redis , if found cached data , use it 
Else Fetch a row from database and then If(0<expireTime) then cache it to redis
then ...

from promise.

jsor commented on May 18, 2024

How exactly this code runs?

This code doesn't chain promises but creates 3 new independent promises. In this case $x === 1 in all 3 callbacks.

How can I have a conditional then ?

Get a promise
Check redis , if found cached data , use it 
Else Fetch a row from database and then If(0<expireTime) then cache it to redis
then ...

Something like this maybe:

function fetchFromDbAndStoreToRedis($key)
{
    return queryDb($key)
        ->then(function($row) use ($key) {
            $value = $row['value'];

            if ($row['expireTime'] <= 0) {
                return $value;
            }

            return storeToRedis($key, $value, $row['expireTime'])
                ->then(function() use ($value) {
                    return $value;
                });
        })
    ;
}

fetchFromRedis($key)
    ->then(function($value) use ($key) {
        if (null !== $value) {
            return $value;
        }

        return fetchFromDbAndStoreToRedis($key);
    })
;