import { omit } from 'lodash';
import VueApollo from 'vue-apollo';

const VUE_APOLLO_QUERY_KEYWORDS = [
  'variables',
  'watch',
  'update',
  'result',
  'error',
  'loadingKey',
  'watchLoading',
  'skip',
  'throttle',
  'debounce',
  'subscribeToMore',
  'prefetch',
  'manual',
];

VueApollo.prototype.prefetchQuery = prefetchQuery;

function prefetchQuery(queryOptions, context, client) {
  let variables;

  // Client
  if (!client) {
    client = this.defaultClient;
  } else if (typeof client === 'string') {
    client = this.clients[client];
    if (!client) {
      throw new Error(`[vue-apollo] Missing client '${client}' in 'apolloProvider'`);
    }
  }

  // Simple query
  if (!queryOptions.query) {
    queryOptions = {
      query: queryOptions,
    }
  } else {
    const { prefetch } = queryOptions;
    const prefetchType = typeof prefetch;

    // Resolve variables
    if (prefetchType !== 'undefined') {
      let result;
      if (prefetchType === 'function') {
        result = prefetch(context);
      } else {
        result = prefetch;
      }

      if (!result) {
        return Promise.resolve();
      } else if (prefetchType === 'boolean') {
        const optVariables = queryOptions.variables;
        if (typeof optVariables !== 'undefined') {
          // Reuse `variables` option with `prefetch: true`
          if (typeof optVariables === 'function') {
            variables = optVariables.call(context);
          } else {
            variables = optVariables;
          }
        } else {
          variables = undefined;
        }
      } else {
        variables = result;
      }
    }
  }

  // Query
  if (typeof queryOptions.query === 'function') {
    queryOptions.query = queryOptions.query(context);
  }
  return new Promise((resolve, reject) => {
    const options = omit(queryOptions, VUE_APOLLO_QUERY_KEYWORDS);
    options.variables = variables;
    if (process.env.VUE_ENV === 'server') {
      options.fetchPolicy = 'network-only';
    }
    client.query(options).then(resolve, reject);
  });
}