# All files / local persistence_promise.ts

84.81% Statements 67/79
83.33% Branches 15/18
85.19% Functions 23/27
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     /**
     * Copyright 2017 Google Inc.
     *
     * Licensed under the Apache License, Version 2.0 (the "License");
     * you may not use this file except in compliance with the License.
     * You may obtain a copy of the License at
     *
     *   http://www.apache.org/licenses/LICENSE-2.0
     *
     * Unless required by applicable law or agreed to in writing, software
     * distributed under the License is distributed on an "AS IS" BASIS,
     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     * See the License for the specific language governing permissions and
     * limitations under the License.
     */

    import { fail } from '../util/assert';

    export type FulfilledHandler<T, R> =
      | ((result: T) => R | PersistencePromise<R>)
      | null;
    export type RejectedHandler<R> =
      | ((reason: Error) => R | PersistencePromise<R>)
      | null;
    export type Resolver<T> = (value?: T) => void;
    export type Rejector = (error: Error) => void;

    /**
     * PersistencePromise<> is essentially a re-implementation of Promise<> except
     * it has a .next() method instead of .then() and .next() and .catch() callbacks
     * are executed synchronously when a PersistencePromise resolves rather than
     * asynchronously (Promise<> implementations use setImmediate() or similar).
     *
     * This is necessary to interoperate with IndexedDB which will automatically
     * commit transactions if control is returned to the event loop without
     * synchronously initiating another operation on the transaction.
     *
     * NOTE: .then() and .catch() only allow a single consumer, unlike normal
     * Promises.
     */
    export class PersistencePromise<T> {
      // NOTE: next/catchCallback will always point to our own wrapper functions,
      // not the user's raw next() or catch() callbacks.
      // tslint:disable-next-line:no-any Accept any result type for the next call in the Promise chain.
      private nextCallback: FulfilledHandler<T, any> = null;
      // tslint:disable-next-line:no-any Accept any result type for the error handler.
      private catchCallback: RejectedHandler<any> = null;

      // When the operation resolves, we'll set result or error and mark isDone.
      private result: T | undefined = undefined;
      private error: Error | undefined = undefined;
      private isDone = false;

      // Set to true when .then() or .catch() are called and prevents additional
      // chaining.
      private callbackAttached = false;

      constructor(callback: (resolve: Resolver<T>, reject: Rejector) => void) {
        callback(
          value => {
            this.isDone = true;
            this.result = value;
            if (this.nextCallback) {
              // value should be defined unless T is Void, but we can't express
              // that in the type system.
              this.nextCallback(value!);
            }
          },
          error => {
            this.isDone = true;
            this.error = error;
            if (this.catchCallback) {
              this.catchCallback(error);
            }
          }
        );
      }

      catch<R>(
        fn: (error: Error) => R | PersistencePromise<R>
      ): PersistencePromise<R> {
        return this.next(undefined, fn);
      }

      next<R>(
        nextFn?: FulfilledHandler<T, R>,
        catchFn?: RejectedHandler<R>
      ): PersistencePromise<R> {
        Iif (this.callbackAttached) {
          fail('Called next() or catch() twice for PersistencePromise');
        }
        this.callbackAttached = true;
        if (this.isDone) {
          if (!this.error) {
            return this.wrapSuccess(nextFn, this.result!);
          } else {
            return this.wrapFailure(catchFn, this.error);
          }
        } else {
          return new PersistencePromise<R>((resolve, reject) => {
            this.nextCallback = (value: T) => {
              this.wrapSuccess(nextFn, value).next(resolve, reject);
            };
            this.catchCallback = (error: Error) => {
              this.wrapFailure(catchFn, error).next(resolve, reject);
            };
          });
        }
      }

      toPromise(): Promise<T> {
        return new Promise((resolve, reject) => {
          this.next(resolve, reject);
        });
      }

      private wrapUserFunction<R>(
        fn: () => R | PersistencePromise<R>
      ): PersistencePromise<R> {
        try {
          const result = fn();
          if (result instanceof PersistencePromise) {
            return result;
          } else {
            return PersistencePromise.resolve(result);
          }
        } catch (e) {
          return PersistencePromise.reject<R>(e);
        }
      }

      private wrapSuccess<R>(
        nextFn: FulfilledHandler<T, R> | undefined,
        value: T
      ): PersistencePromise<R> {
        if (nextFn) {
          return this.wrapUserFunction(() => nextFn(value));
        } else {
          // If there's no nextFn, then R must be the same as T but we
          // can't express that in the type system.
          // tslint:disable-next-line:no-any
          return PersistencePromise.resolve<R>(value as any);
        }
      }

      private wrapFailure<R>(
        catchFn: RejectedHandler<R> | undefined,
        error: Error
      ): PersistencePromise<R> {
        if (catchFn) {
          return this.wrapUserFunction(() => catchFn(error));
        } else {
          return PersistencePromise.reject<R>(error);
        }
      }

      static resolve(): PersistencePromise<void>;
      static resolve<R>(result: R): PersistencePromise<R>;
      static resolve<R>(result?: R): PersistencePromise<R | void> {
        return new PersistencePromise<R>((resolve, reject) => {
          resolve(result);
        });
      }

      static reject<R>(error: Error): PersistencePromise<R> {
        return new PersistencePromise<R>((resolve, reject) => {
          reject(error);
        });
      }

      static waitFor(
        // tslint:disable-next-line:no-any Accept all Promise types in waitFor().
        all: Array<PersistencePromise<any>>
      ): PersistencePromise<void> {
        return all.reduce((promise, nextPromise, idx) => {
          return promise.next(() => {
            return nextPromise;
          });
        }, PersistencePromise.resolve());
      }

      static map<R>(all: Array<PersistencePromise<R>>): PersistencePromise<R[]> {
        const results: R[] = [];
        let first = true;
        // initial is ignored, so we can cheat on the type.
        // tslint:disable-next-line:no-any
        const initial = PersistencePromise.resolve<R>(null as any);
        return all
          .reduce((promise, nextPromise) => {
            return promise.next(result => {
              if (!first) {
                results.push(result);
              }
              first = false;
              return nextPromise;
            });
          }, initial)
          .next(result => {
            results.push(result);
            return results;
          });
      }
    }