Allow post caching

Library Affected: workbox-strategies

Browser & Platform: all browsers

Issue or Feature Request Description: Allow post caching

Developers should be able to cache post requests as get requests using cacheKeyWillBeUsed. Currently there is a check that prevents post caching, which checks the initial request instead of the request returned by cacheKeyWillBeUsed, which should be respected as a custom cache key.

A related issue #2574 was filed but closed by the author.

[jeffposnick]

When using runtimeCaching in generateSW, you can specify custom plugins via the options.plugins array, along the lines of:

runtimeCaching: [{
  urlPattern: "https://pwa.test/post",
  method: "POST",
  handler: 'NetworkFirst',
  options: {
    cacheName: 'post',
    backgroundSync: {
      name: "myQueueName",
      options: {
        maxRetentionTime: 24 * 60
      },
    },
    plugins: [{
      cacheKeyWillBeUsed: () => {
        // You code to return a custom Request goes here.
      }
    }]
  },
}]

[jeffposnick]

I’ve merged your PR, and it will be in the next pre-release of v6.

I think it’s a good idea to perform the method check in the order in which your PR does it, but again, I don’t want to advertise this as “Workbox now supports caching POST requests.”