/**
 * 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 ReactPropTransferer
 */

"use strict";

var emptyFunction = require('emptyFunction');
var invariant = require('invariant');
var joinClasses = require('joinClasses');
var merge = require('merge');

/**
 * Creates a transfer strategy that will merge prop values using the supplied
 * `mergeStrategy`. If a prop was previously unset, this just sets it.
 *
 * @param {function} mergeStrategy
 * @return {function}
 */
function createTransferStrategy(mergeStrategy) {
  return function(props, key, value) {
    if (!props.hasOwnProperty(key)) {
      props[key] = value;
    } else {
      props[key] = mergeStrategy(props[key], value);
    }
  };
}

var transferStrategyMerge = createTransferStrategy(function(a, b) {
  // `merge` overrides the first object's (`props[key]` above) keys using the
  // second object's (`value`) keys. An object's style's existing `propA` would
  // get overridden. Flip the order here.
  return merge(b, a);
});

/**
 * Transfer strategies dictate how props are transferred by `transferPropsTo`.
 * NOTE: if you add any more exceptions to this list you should be sure to
 * update `cloneWithProps()` accordingly.
 */
var TransferStrategies = {
  /**
   * Never transfer `children`.
   */
  children: emptyFunction,
  /**
   * Transfer the `className` prop by merging them.
   */
  className: createTransferStrategy(joinClasses),
  /**
   * Never transfer the `key` prop.
   */
  key: emptyFunction,
  /**
   * Never transfer the `ref` prop.
   */
  ref: emptyFunction,
  /**
   * Transfer the `style` prop (which is an object) by merging them.
   */
  style: transferStrategyMerge
};

/**
 * Mutates the first argument by transferring the properties from the second
 * argument.
 *
 * @param {object} props
 * @param {object} newProps
 * @return {object}
 */
function transferInto(props, newProps) {
  for (var thisKey in newProps) {
    if (!newProps.hasOwnProperty(thisKey)) {
      continue;
    }

    var transferStrategy = TransferStrategies[thisKey];

    if (transferStrategy && TransferStrategies.hasOwnProperty(thisKey)) {
      transferStrategy(props, thisKey, newProps[thisKey]);
    } else if (!props.hasOwnProperty(thisKey)) {
      props[thisKey] = newProps[thisKey];
    }
  }
  return props;
}

/**
 * ReactPropTransferer are capable of transferring props to another component
 * using a `transferPropsTo` method.
 *
 * @class ReactPropTransferer
 */
var ReactPropTransferer = {

  TransferStrategies: TransferStrategies,

  /**
   * Merge two props objects using TransferStrategies.
   *
   * @param {object} oldProps original props (they take precedence)
   * @param {object} newProps new props to merge in
   * @return {object} a new object containing both sets of props merged.
   */
  mergeProps: function(oldProps, newProps) {
    return transferInto(merge(oldProps), newProps);
  },

  /**
   * @lends {ReactPropTransferer.prototype}
   */
  Mixin: {

    /**
     * Transfer props from this component to a target component.
     *
     * Props that do not have an explicit transfer strategy will be transferred
     * only if the target component does not already have the prop set.
     *
     * This is usually used to pass down props to a returned root component.
     *
     * @param {ReactDescriptor} descriptor Component receiving the properties.
     * @return {ReactDescriptor} The supplied `component`.
     * @final
     * @protected
     */
    transferPropsTo: function(descriptor) {
      invariant(
        descriptor._owner === this,
        '%s: You can\'t call transferPropsTo() on a component that you ' +
        'don\'t own, %s. This usually means you are calling ' +
        'transferPropsTo() on a component passed in as props or children.',
        this.constructor.displayName,
        descriptor.type.displayName
      );

      // Because descriptors are immutable we have to merge into the existing
      // props object rather than clone it.
      transferInto(descriptor.props, this.props);

      return descriptor;
    }

  }
};

module.exports = ReactPropTransferer;
