/**
 * Copyright 2013-2014 Facebook, 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.
 *
 * @providesModule Transaction
 */

"use strict";

var invariant = require('invariant');

/**
 * `Transaction` creates a black box that is able to wrap any method such that
 * certain invariants are maintained before and after the method is invoked
 * (Even if an exception is thrown while invoking the wrapped method). Whoever
 * instantiates a transaction can provide enforcers of the invariants at
 * creation time. The `Transaction` class itself will supply one additional
 * automatic invariant for you - the invariant that any transaction instance
 * should not be run while it is already being run. You would typically create a
 * single instance of a `Transaction` for reuse multiple times, that potentially
 * is used to wrap several different methods. Wrappers are extremely simple -
 * they only require implementing two methods.
 *
 * <pre>
 *                       wrappers (injected at creation time)
 *                                      +        +
 *                                      |        |
 *                    +-----------------|--------|--------------+
 *                    |                 v        |              |
 *                    |      +---------------+   |              |
 *                    |   +--|    wrapper1   |---|----+         |
 *                    |   |  +---------------+   v    |         |
 *                    |   |          +-------------+  |         |
 *                    |   |     +----|   wrapper2  |--------+   |
 *                    |   |     |    +-------------+  |     |   |
 *                    |   |     |                     |     |   |
 *                    |   v     v                     v     v   | wrapper
 *                    | +---+ +---+   +---------+   +---+ +---+ | invariants
 * perform(anyMethod) | |   | |   |   |         |   |   | |   | | maintained
 * +----------------->|-|---|-|---|-->|anyMethod|---|---|-|---|-|-------->
 *                    | |   | |   |   |         |   |   | |   | |
 *                    | |   | |   |   |         |   |   | |   | |
 *                    | |   | |   |   |         |   |   | |   | |
 *                    | +---+ +---+   +---------+   +---+ +---+ |
 *                    |  initialize                    close    |
 *                    +-----------------------------------------+
 * </pre>
 *
 * Use cases:
 * - Preserving the input selection ranges before/after reconciliation.
 *   Restoring selection even in the event of an unexpected error.
 * - Deactivating events while rearranging the DOM, preventing blurs/focuses,
 *   while guaranteeing that afterwards, the event system is reactivated.
 * - Flushing a queue of collected DOM mutations to the main UI thread after a
 *   reconciliation takes place in a worker thread.
 * - Invoking any collected `componentDidUpdate` callbacks after rendering new
 *   content.
 * - (Future use case): Wrapping particular flushes of the `ReactWorker` queue
 *   to preserve the `scrollTop` (an automatic scroll aware DOM).
 * - (Future use case): Layout calculations before and after DOM upates.
 *
 * Transactional plugin API:
 * - A module that has an `initialize` method that returns any precomputation.
 * - and a `close` method that accepts the precomputation. `close` is invoked
 *   when the wrapped process is completed, or has failed.
 *
 * @param {Array<TransactionalWrapper>} transactionWrapper Wrapper modules
 * that implement `initialize` and `close`.
 * @return {Transaction} Single transaction for reuse in thread.
 *
 * @class Transaction
 */
var Mixin = {
  /**
   * Sets up this instance so that it is prepared for collecting metrics. Does
   * so such that this setup method may be used on an instance that is already
   * initialized, in a way that does not consume additional memory upon reuse.
   * That can be useful if you decide to make your subclass of this mixin a
   * "PooledClass".
   */
  reinitializeTransaction: function() {
    this.transactionWrappers = this.getTransactionWrappers();
    if (!this.wrapperInitData) {
      this.wrapperInitData = [];
    } else {
      this.wrapperInitData.length = 0;
    }
    this._isInTransaction = false;
  },

  _isInTransaction: false,

  /**
   * @abstract
   * @return {Array<TransactionWrapper>} Array of transaction wrappers.
   */
  getTransactionWrappers: null,

  isInTransaction: function() {
    return !!this._isInTransaction;
  },

  /**
   * Executes the function within a safety window. Use this for the top level
   * methods that result in large amounts of computation/mutations that would
   * need to be safety checked.
   *
   * @param {function} method Member of scope to call.
   * @param {Object} scope Scope to invoke from.
   * @param {Object?=} args... Arguments to pass to the method (optional).
   *                           Helps prevent need to bind in many cases.
   * @return Return value from `method`.
   */
  perform: function(method, scope, a, b, c, d, e, f) {
    invariant(
      !this.isInTransaction(),
      'Transaction.perform(...): Cannot initialize a transaction when there ' +
      'is already an outstanding transaction.'
    );
    var errorThrown;
    var ret;
    try {
      this._isInTransaction = true;
      // Catching errors makes debugging more difficult, so we start with
      // errorThrown set to true before setting it to false after calling
      // close -- if it's still set to true in the finally block, it means
      // one of these calls threw.
      errorThrown = true;
      this.initializeAll(0);
      ret = method.call(scope, a, b, c, d, e, f);
      errorThrown = false;
    } finally {
      try {
        if (errorThrown) {
          // If `method` throws, prefer to show that stack trace over any thrown
          // by invoking `closeAll`.
          try {
            this.closeAll(0);
          } catch (err) {
          }
        } else {
          // Since `method` didn't throw, we don't want to silence the exception
          // here.
          this.closeAll(0);
        }
      } finally {
        this._isInTransaction = false;
      }
    }
    return ret;
  },

  initializeAll: function(startIndex) {
    var transactionWrappers = this.transactionWrappers;
    for (var i = startIndex; i < transactionWrappers.length; i++) {
      var wrapper = transactionWrappers[i];
      try {
        // Catching errors makes debugging more difficult, so we start with the
        // OBSERVED_ERROR state before overwriting it with the real return value
        // of initialize -- if it's still set to OBSERVED_ERROR in the finally
        // block, it means wrapper.initialize threw.
        this.wrapperInitData[i] = Transaction.OBSERVED_ERROR;
        this.wrapperInitData[i] = wrapper.initialize ?
          wrapper.initialize.call(this) :
          null;
      } finally {
        if (this.wrapperInitData[i] === Transaction.OBSERVED_ERROR) {
          // The initializer for wrapper i threw an error; initialize the
          // remaining wrappers but silence any exceptions from them to ensure
          // that the first error is the one to bubble up.
          try {
            this.initializeAll(i + 1);
          } catch (err) {
          }
        }
      }
    }
  },

  /**
   * Invokes each of `this.transactionWrappers.close[i]` functions, passing into
   * them the respective return values of `this.transactionWrappers.init[i]`
   * (`close`rs that correspond to initializers that failed will not be
   * invoked).
   */
  closeAll: function(startIndex) {
    invariant(
      this.isInTransaction(),
      'Transaction.closeAll(): Cannot close transaction when none are open.'
    );
    var transactionWrappers = this.transactionWrappers;
    for (var i = startIndex; i < transactionWrappers.length; i++) {
      var wrapper = transactionWrappers[i];
      var initData = this.wrapperInitData[i];
      var errorThrown;
      try {
        // Catching errors makes debugging more difficult, so we start with
        // errorThrown set to true before setting it to false after calling
        // close -- if it's still set to true in the finally block, it means
        // wrapper.close threw.
        errorThrown = true;
        if (initData !== Transaction.OBSERVED_ERROR) {
          wrapper.close && wrapper.close.call(this, initData);
        }
        errorThrown = false;
      } finally {
        if (errorThrown) {
          // The closer for wrapper i threw an error; close the remaining
          // wrappers but silence any exceptions from them to ensure that the
          // first error is the one to bubble up.
          try {
            this.closeAll(i + 1);
          } catch (e) {
          }
        }
      }
    }
    this.wrapperInitData.length = 0;
  }
};

var Transaction = {

  Mixin: Mixin,

  /**
   * Token to look for to determine if an error occured.
   */
  OBSERVED_ERROR: {}

};

module.exports = Transaction;
