path: root/modules/PropertyListUtils.jsm

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this file,
 * You can obtain one at http://mozilla.org/MPL/2.0/. */

/**
 * Module for reading Property Lists (.plist) files
 * ------------------------------------------------
 * This module functions as a reader for Apple Property Lists (.plist files).
 * It supports both binary and xml formatted property lists.  It does not
 * support the legacy ASCII format.  Reading of Cocoa's Keyed Archives serialized
 * to binary property lists isn't supported either.
 *
 * Property Lists objects are represented by standard JS and Mozilla types,
 * namely:
 *
 * XML type            Cocoa Class    Returned type(s)
 * --------------------------------------------------------------------------
 * <true/> / <false/>  NSNumber       TYPE_PRIMITIVE    boolean
 * <integer> / <real>  NSNumber       TYPE_PRIMITIVE    number
 *                                    TYPE_INT64        String [1]
 * Not Available       NSNull         TYPE_PRIMITIVE    null   [2]
 *                                    TYPE_PRIMITIVE    undefined [3]
 * <date/>             NSDate         TYPE_DATE         Date
 * <data/>             NSData         TYPE_UINT8_ARRAY  Uint8Array
 * <array/>            NSArray        TYPE_ARRAY        Array
 * Not Available       NSSet          TYPE_ARRAY        Array  [2][4]
 * <dict/>             NSDictionary   TYPE_DICTIONARY   Map
 *
 * Use PropertyListUtils.getObjectType to detect the type of a Property list
 * object.
 *
 * -------------
 * 1) Property lists supports storing U/Int64 numbers, while JS can only handle
 *    numbers that are in this limits of float-64 (±2^53).  For numbers that
 *    do not outbound this limits, simple primitive number are always used.
 *    Otherwise, a String object.
 * 2) About NSNull and NSSet values: While the xml format has no support for
 *    representing null and set values, the documentation for the binary format
 *    states that it supports storing both types.  However, the Cocoa APIs for
 *    serializing property lists do not seem to support either types (test with
 *    NSPropertyListSerialization::propertyList:isValidForFormat). Furthermore,
 *    if an array or a dictionary (Map) contains a NSNull or a NSSet value, they cannot
 *    be serialized to a property list.
 *    As for usage within OS X, not surprisingly there's no known usage of
 *    storing either of these types in a property list.  It seems that, for now,
 *    Apple is keeping the features of binary and xml formats in sync, probably as
 *    long as the XML format is not officially deprecated.
 * 3) Not used anywhere.
 * 4) About NSSet representation: For the time being, we represent those
 *    theoretical NSSet objects the same way NSArray is represented.
 *    While this would most certainly work, it is not the right way to handle
 *    it.  A more correct representation for a set is a js generator, which would
 *    read the set lazily and has no indices semantics.
 */

"use strict";

this.EXPORTED_SYMBOLS = ["PropertyListUtils"];

const Cc = Components.classes;
const Ci = Components.interfaces;
const Cu = Components.utils;

Cu.importGlobalProperties(['File', 'FileReader']);
Cu.import("resource://gre/modules/XPCOMUtils.jsm");

XPCOMUtils.defineLazyModuleGetter(this, "ctypes",
                                  "resource://gre/modules/ctypes.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "Services",
                                  "resource://gre/modules/Services.jsm");

this.PropertyListUtils = Object.freeze({
  /**
   * Asynchronously reads a file as a property list.
   *
   * @param aFile (nsIDOMBlob/nsILocalFile)
   *        the file to be read as a property list.
   * @param aCallback
   *        If the property list is read successfully, aPropertyListRoot is set
   *        to the root object of the property list.
   *        Use getPropertyListObjectType to detect its type.
   *        If it's not read successfully, aPropertyListRoot is set to null.
   *        The reaon for failure is reported to the Error Console.
   */
  read: function PLU_read(aFile, aCallback) {
    if (!(aFile instanceof Ci.nsILocalFile || aFile instanceof File))
      throw new Error("aFile is not a file object");
    if (typeof(aCallback) != "function")
      throw new Error("Invalid value for aCallback");

    // We guarantee not to throw directly for any other exceptions, and always
    // call aCallback.
    Services.tm.mainThread.dispatch(function() {
      let file = aFile;
      try {
        if (file instanceof Ci.nsILocalFile) {
          if (!file.exists())
            throw new Error("The file pointed by aFile does not exist");

          file = File.createFromNsIFile(file);
        }

        let fileReader = new FileReader();
        let onLoadEnd = function() {
          let root = null;
          try {
            fileReader.removeEventListener("loadend", onLoadEnd, false);
            if (fileReader.readyState != fileReader.DONE)
              throw new Error("Could not read file contents: " + fileReader.error);

            root = this._readFromArrayBufferSync(fileReader.result);
          }
          finally {
            aCallback(root);
          }
        }.bind(this);
        fileReader.addEventListener("loadend", onLoadEnd, false);
        fileReader.readAsArrayBuffer(file);
      }
      catch (ex) {
        aCallback(null);
        throw ex;
      }
    }.bind(this), Ci.nsIThread.DISPATCH_NORMAL);
  },

  /**
   * DO NOT USE ME.  Once Bug 718189 is fixed, this method won't be public.
   *
   * Synchronously read an ArrayBuffer contents as a property list.
   */
  _readFromArrayBufferSync: function PLU__readFromArrayBufferSync(aBuffer) {
    if (BinaryPropertyListReader.prototype.canProcess(aBuffer))
      return new BinaryPropertyListReader(aBuffer).root;

    // Convert the buffer into an XML tree.
    let domParser = Cc["@mozilla.org/xmlextras/domparser;1"].
                    createInstance(Ci.nsIDOMParser);
    let bytesView = new Uint8Array(aBuffer);
    try {
      let doc = domParser.parseFromBuffer(bytesView, bytesView.length,
                                          "application/xml");
      return new XMLPropertyListReader(doc).root;
    }
    catch (ex) {
      throw new Error("aBuffer cannot be parsed as a DOM document: " + ex);
    }
  },

  TYPE_PRIMITIVE:    0,
  TYPE_DATE:         1,
  TYPE_UINT8_ARRAY:  2,
  TYPE_ARRAY:        3,
  TYPE_DICTIONARY:   4,
  TYPE_INT64:        5,

  /**
   * Get the type in which the given property list object is represented.
   * Check the header for the mapping between the TYPE* constants to js types
   * and objects.
   *
   * @return one of the TYPE_* constants listed above.
   * @note this method is merely for convenience.  It has no magic to detect
   * that aObject is indeed a property list object created by this module.
   */
  getObjectType: function PLU_getObjectType(aObject) {
    if (aObject === null || typeof(aObject) != "object")
      return this.TYPE_PRIMITIVE;

    // Given current usage, we could assume that aObject was created in the
    // scope of this module, but in future, this util may be used as part of
    // serializing js objects to a property list - in which case the object
    // would most likely be created in the caller's scope.
    let global = Cu.getGlobalForObject(aObject);

    if (aObject instanceof global.Map)
      return this.TYPE_DICTIONARY;
    if (Array.isArray(aObject))
      return this.TYPE_ARRAY;
    if (aObject instanceof global.Date)
      return this.TYPE_DATE;
    if (aObject instanceof global.Uint8Array)
      return this.TYPE_UINT8_ARRAY;
    if (aObject instanceof global.String && "__INT_64_WRAPPER__" in aObject)
      return this.TYPE_INT64;

    throw new Error("aObject is not as a property list object.");
  },

  /**
   * Wraps a 64-bit stored in the form of a string primitive as a String object,
   * which we can later distiguish from regular string values.
   * @param aPrimitive
   *        a number in the form of either a primitive string or a primitive number.
   * @return a String wrapper around aNumberStr that can later be identified
   * as holding 64-bit number using getObjectType.
   */
  wrapInt64: function PLU_wrapInt64(aPrimitive) {
    if (typeof(aPrimitive) != "string" && typeof(aPrimitive) != "number")
      throw new Error("aPrimitive should be a string primitive");

    let wrapped = new String(aPrimitive);
    Object.defineProperty(wrapped, "__INT_64_WRAPPER__", { value: true });
    return wrapped;
  }
});

/**
 * Here's the base structure of binary-format property lists.
 * 1) Header - magic number
 *   - 6 bytes - "bplist"
 *   - 2 bytes - version number. This implementation only supports version 00.
 * 2) Objects Table
 *    Variable-sized objects, see _readObject for how various types of objects
 *    are constructed.
 * 3) Offsets Table
 *    The offset of each object in the objects table. The integer size is
 *    specified in the trailer.
 * 4) Trailer
 *    - 6 unused bytes
 *    - 1 byte:  the size of integers in the offsets table
 *    - 1 byte:  the size of object references for arrays, sets and
 *               dictionaries.
 *    - 8 bytes: the number of objects in the objects table
 *    - 8 bytes: the index of the root object's offset in the offsets table.
 *    - 8 bytes: the offset of the offsets table.
 *
 * Note: all integers are stored in big-endian form.
 */

/**
 * Reader for binary-format property lists.
 *
 * @param aBuffer
 *        ArrayBuffer object from which the binary plist should be read.
 */
function BinaryPropertyListReader(aBuffer) {
  this._dataView = new DataView(aBuffer);

  const JS_MAX_INT = Math.pow(2, 53);
  this._JS_MAX_INT_SIGNED = ctypes.Int64(JS_MAX_INT);
  this._JS_MAX_INT_UNSIGNED = ctypes.UInt64(JS_MAX_INT);
  this._JS_MIN_INT = ctypes.Int64(-JS_MAX_INT);

  try {
    this._readTrailerInfo();
    this._readObjectsOffsets();
  }
  catch (ex) {
    throw new Error("Could not read aBuffer as a binary property list");
  }
  this._objects = [];
}

BinaryPropertyListReader.prototype = {
  /**
   * Checks if the given ArrayBuffer can be read as a binary property list.
   * It can be called on the prototype.
   */
  canProcess: function BPLR_canProcess(aBuffer) {
    return Array.from(new Uint8Array(aBuffer, 0, 8)).map(c => String.fromCharCode(c)).
           join("") == "bplist00";
  },

  get root() {
    return this._readObject(this._rootObjectIndex);
  },

  _readTrailerInfo: function BPLR__readTrailer() {
    // The first 6 bytes of the 32-bytes trailer are unused
    let trailerOffset = this._dataView.byteLength - 26;
    [this._offsetTableIntegerSize, this._objectRefSize] =
      this._readUnsignedInts(trailerOffset, 1, 2);

    [this._numberOfObjects, this._rootObjectIndex, this._offsetTableOffset] =
      this._readUnsignedInts(trailerOffset + 2, 8, 3);
  },

  _readObjectsOffsets: function BPLR__readObjectsOffsets() {
    this._offsetTable = this._readUnsignedInts(this._offsetTableOffset,
                                               this._offsetTableIntegerSize,
                                               this._numberOfObjects);
  },

  _readSignedInt64: function BPLR__readSignedInt64(aByteOffset) {
    let lo = this._dataView.getUint32(aByteOffset + 4);
    let hi = this._dataView.getInt32(aByteOffset);
    let int64 = ctypes.Int64.join(hi, lo);
    if (ctypes.Int64.compare(int64, this._JS_MAX_INT_SIGNED) == 1 ||
        ctypes.Int64.compare(int64, this._JS_MIN_INT) == -1)
      return PropertyListUtils.wrapInt64(int64.toString());

    return parseInt(int64.toString(), 10);
  },

  _readReal: function BPLR__readReal(aByteOffset, aRealSize) {
    if (aRealSize == 4)
      return this._dataView.getFloat32(aByteOffset);
    if (aRealSize == 8)
      return this._dataView.getFloat64(aByteOffset);

    throw new Error("Unsupported real size: " + aRealSize);
  },

  OBJECT_TYPE_BITS: {
    SIMPLE:                  parseInt("0000", 2),
    INTEGER:                 parseInt("0001", 2),
    REAL:                    parseInt("0010", 2),
    DATE:                    parseInt("0011", 2),
    DATA:                    parseInt("0100", 2),
    ASCII_STRING:            parseInt("0101", 2),
    UNICODE_STRING:          parseInt("0110", 2),
    UID:                     parseInt("1000", 2),
    ARRAY:                   parseInt("1010", 2),
    SET:                     parseInt("1100", 2),
    DICTIONARY:              parseInt("1101", 2)
  },

  ADDITIONAL_INFO_BITS: {
    // Applies to OBJECT_TYPE_BITS.SIMPLE
    NULL:                    parseInt("0000", 2),
    FALSE:                   parseInt("1000", 2),
    TRUE:                    parseInt("1001", 2),
    FILL_BYTE:               parseInt("1111", 2),
    // Applies to OBJECT_TYPE_BITS.DATE
    DATE:                    parseInt("0011", 2),
    // Applies to OBJECT_TYPE_BITS.DATA, ASCII_STRING, UNICODE_STRING, ARRAY,
    // SET and DICTIONARY.
    LENGTH_INT_SIZE_FOLLOWS: parseInt("1111", 2)
  },

  /**
   * Returns an object descriptor in the form of two integers: object type and
   * additional info.
   *
   * @param aByteOffset
   *        the descriptor's offset.
   * @return [objType, additionalInfo] - the object type and additional info.
   * @see OBJECT_TYPE_BITS and ADDITIONAL_INFO_BITS
   */
  _readObjectDescriptor: function BPLR__readObjectDescriptor(aByteOffset) {
    // The first four bits hold the object type.  For some types, additional
    // info is held in the other 4 bits.
    let byte = this._readUnsignedInts(aByteOffset, 1, 1)[0];
    return [(byte & 0xF0) >> 4, byte & 0x0F];
  },

  _readDate: function BPLR__readDate(aByteOffset) {
    // That's the reference date of NSDate.
    let date = new Date("1 January 2001, GMT");

    // NSDate values are float values, but setSeconds takes an integer.
    date.setMilliseconds(this._readReal(aByteOffset, 8) * 1000);
    return date;
  },

  /**
   * Reads a portion of the buffer as a string.
   *
   * @param aByteOffset
   *        The offset in the buffer at which the string starts
   * @param aNumberOfChars
   *        The length of the string to be read (that is the number of
   *        characters, not bytes).
   * @param aUnicode
   *        Whether or not it is a unicode string.
   * @return the string read.
   *
   * @note this is tested to work well with unicode surrogate pairs.  Because
   * all unicode characters are read as 2-byte integers, unicode surrogate
   * pairs are read from the buffer in the form of two integers, as required
   * by String.fromCharCode.
   */
  _readString:
  function BPLR__readString(aByteOffset, aNumberOfChars, aUnicode) {
    let codes = this._readUnsignedInts(aByteOffset, aUnicode ? 2 : 1,
                                       aNumberOfChars);
    return codes.map(c => String.fromCharCode(c)).join("");
  },

  /**
   * Reads an array of unsigned integers from the buffer.  Integers larger than
   * one byte are read in big endian form.
   *
   * @param aByteOffset
   *        The offset in the buffer at which the array starts.
   * @param aIntSize
   *        The size of each int in the array.
   * @param aLength
   *        The number of ints in the array.
   * @param [optional] aBigIntAllowed (default: false)
   *        Whether or not to accept integers which outbounds JS limits for
   *        numbers (±2^53) in the form of a String.
   * @return an array of integers (number primitive and/or Strings for large
   * numbers (see header)).
   * @throws if aBigIntAllowed is false and one of the integers in the array
   * cannot be represented by a primitive js number.
   */
  _readUnsignedInts:
  function BPLR__readUnsignedInts(aByteOffset, aIntSize, aLength, aBigIntAllowed) {
    let uints = [];
    for (let offset = aByteOffset;
         offset < aByteOffset + aIntSize * aLength;
         offset += aIntSize) {
      if (aIntSize == 1) {
        uints.push(this._dataView.getUint8(offset));
      }
      else if (aIntSize == 2) {
        uints.push(this._dataView.getUint16(offset));
      }
      else if (aIntSize == 3) {
        let int24 = Uint8Array(4);
        int24[3] = 0;
        int24[2] = this._dataView.getUint8(offset);
        int24[1] = this._dataView.getUint8(offset + 1);
        int24[0] = this._dataView.getUint8(offset + 2);
        uints.push(Uint32Array(int24.buffer)[0]);
      }
      else if (aIntSize == 4) {
        uints.push(this._dataView.getUint32(offset));
      }
      else if (aIntSize == 8) {
        let lo = this._dataView.getUint32(offset + 4);
        let hi = this._dataView.getUint32(offset);
        let uint64 = ctypes.UInt64.join(hi, lo);
        if (ctypes.UInt64.compare(uint64, this._JS_MAX_INT_UNSIGNED) == 1) {
          if (aBigIntAllowed === true)
            uints.push(PropertyListUtils.wrapInt64(uint64.toString()));
          else
            throw new Error("Integer too big to be read as float 64");
        }
        else {
          uints.push(parseInt(uint64, 10));
        }
      }
      else {
        throw new Error("Unsupported size: " + aIntSize);
      }
    }

    return uints;
  },

  /**
   * Reads from the buffer the data object-count and the offset at which the
   * first object starts.
   *
   * @param aObjectOffset
   *        the object's offset.
   * @return [offset, count] - the offset in the buffer at which the first
   * object in data starts, and the number of objects.
   */
  _readDataOffsetAndCount:
  function BPLR__readDataOffsetAndCount(aObjectOffset) {
    // The length of some objects in the data can be stored in two ways:
    // * If it is small enough, it is stored in the second four bits of the
    //   object descriptors.
    // * Otherwise, those bits are set to 1111, indicating that the next byte
    //   consists of the integer size of the data-length (also stored in the form
    //   of an object descriptor).  The length follows this byte.
    let [, maybeLength] = this._readObjectDescriptor(aObjectOffset);
    if (maybeLength != this.ADDITIONAL_INFO_BITS.LENGTH_INT_SIZE_FOLLOWS)
      return [aObjectOffset + 1, maybeLength];

    let [, intSizeInfo] = this._readObjectDescriptor(aObjectOffset + 1);

    // The int size is 2^intSizeInfo.
    let intSize = Math.pow(2, intSizeInfo);
    let dataLength = this._readUnsignedInts(aObjectOffset + 2, intSize, 1)[0];
    return [aObjectOffset + 2 + intSize, dataLength];
  },

  /**
   * Read array from the buffer and wrap it as a js array.
   * @param aObjectOffset
   *        the offset in the buffer at which the array starts.
   * @param aNumberOfObjects
   *        the number of objects in the array.
   * @return a js array.
   */
  _wrapArray: function BPLR__wrapArray(aObjectOffset, aNumberOfObjects) {
    let refs = this._readUnsignedInts(aObjectOffset,
                                      this._objectRefSize,
                                      aNumberOfObjects);

    let array = new Array(aNumberOfObjects);
    let readObjectBound = this._readObject.bind(this);

    // Each index in the returned array is a lazy getter for its object.
    Array.prototype.forEach.call(refs, function(ref, objIndex) {
      Object.defineProperty(array, objIndex, {
        get: function() {
          delete array[objIndex];
          return array[objIndex] = readObjectBound(ref);
        },
        configurable: true,
        enumerable: true
      });
    }, this);
    return array;
  },

  /**
   * Reads dictionary from the buffer and wraps it as a Map object.
   * @param aObjectOffset
   *        the offset in the buffer at which the dictionary starts
   * @param aNumberOfObjects
   *        the number of keys in the dictionary
   * @return Map-style dictionary.
   */
  _wrapDictionary: function(aObjectOffset, aNumberOfObjects) {
    // A dictionary in the binary format is stored as a list of references to
    // key-objects, followed by a list of references to the value-objects for
    // those keys. The size of each list is aNumberOfObjects * this._objectRefSize.
    let dict = new Proxy(new Map(), LazyMapProxyHandler());
    if (aNumberOfObjects == 0)
      return dict;

    let keyObjsRefs = this._readUnsignedInts(aObjectOffset, this._objectRefSize,
                                             aNumberOfObjects);
    let valObjsRefs =
      this._readUnsignedInts(aObjectOffset + aNumberOfObjects * this._objectRefSize,
                             this._objectRefSize, aNumberOfObjects);
    for (let i = 0; i < aNumberOfObjects; i++) {
      let key = this._readObject(keyObjsRefs[i]);
      let readBound = this._readObject.bind(this, valObjsRefs[i]);

      dict.setAsLazyGetter(key, readBound);
    }
    return dict;
  },

  /**
   * Reads an object at the spcified index in the object table
   * @param aObjectIndex
   *        index at the object table
   * @return the property list object at the given index.
   */
  _readObject: function BPLR__readObject(aObjectIndex) {
    // If the object was previously read, return the cached object.
    if (this._objects[aObjectIndex] !== undefined)
      return this._objects[aObjectIndex];

    let objOffset = this._offsetTable[aObjectIndex];
    let [objType, additionalInfo] = this._readObjectDescriptor(objOffset);
    let value;
    switch (objType) {
      case this.OBJECT_TYPE_BITS.SIMPLE: {
        switch (additionalInfo) {
          case this.ADDITIONAL_INFO_BITS.NULL:
            value = null;
            break;
          case this.ADDITIONAL_INFO_BITS.FILL_BYTE:
            value = undefined;
            break;
          case this.ADDITIONAL_INFO_BITS.FALSE:
            value = false;
            break;
          case this.ADDITIONAL_INFO_BITS.TRUE:
            value = true;
            break;
          default:
            throw new Error("Unexpected value!");
        }
        break;
      }

      case this.OBJECT_TYPE_BITS.INTEGER: {
        // The integer is sized 2^additionalInfo.
        let intSize = Math.pow(2, additionalInfo);

        // For objects, 64-bit integers are always signed.  Negative integers
        // are always represented by a 64-bit integer.
        if (intSize == 8)
          value = this._readSignedInt64(objOffset + 1);
        else
          value = this._readUnsignedInts(objOffset + 1, intSize, 1, true)[0];
        break;
      }

      case this.OBJECT_TYPE_BITS.REAL: {
        // The real is sized 2^additionalInfo.
        value = this._readReal(objOffset + 1, Math.pow(2, additionalInfo));
        break;
      }

      case this.OBJECT_TYPE_BITS.DATE: {
        if (additionalInfo != this.ADDITIONAL_INFO_BITS.DATE)
          throw new Error("Unexpected value");

        value = this._readDate(objOffset + 1);
        break;
      }

      case this.OBJECT_TYPE_BITS.DATA: {
        let [offset, bytesCount] = this._readDataOffsetAndCount(objOffset);
        value = new Uint8Array(this._readUnsignedInts(offset, 1, bytesCount));
        break;
      }

      case this.OBJECT_TYPE_BITS.ASCII_STRING: {
        let [offset, charsCount] = this._readDataOffsetAndCount(objOffset);
        value = this._readString(offset, charsCount, false);
        break;
      }

      case this.OBJECT_TYPE_BITS.UNICODE_STRING: {
        let [offset, unicharsCount] = this._readDataOffsetAndCount(objOffset);
        value = this._readString(offset, unicharsCount, true);
        break;
      }

      case this.OBJECT_TYPE_BITS.UID: {
        // UIDs are only used in Keyed Archives, which are not yet supported.
        throw new Error("Keyed Archives are not supported");
      }

      case this.OBJECT_TYPE_BITS.ARRAY:
      case this.OBJECT_TYPE_BITS.SET: {
        // Note: For now, we fallback to handle sets the same way we handle
        // arrays.  See comments in the header of this file.

        // The bytes following the count are references to objects (indices).
        // Each reference is an unsigned int with size=this._objectRefSize.
        let [offset, objectsCount] = this._readDataOffsetAndCount(objOffset);
        value = this._wrapArray(offset, objectsCount);
        break;
      }

      case this.OBJECT_TYPE_BITS.DICTIONARY: {
        let [offset, objectsCount] = this._readDataOffsetAndCount(objOffset);
        value = this._wrapDictionary(offset, objectsCount);
        break;
      }

      default: {
        throw new Error("Unknown object type: " + objType);
      }
    }

    return this._objects[aObjectIndex] = value;
  }
};

/**
 * Reader for XML property lists.
 *
 * @param aDOMDoc
 *        the DOM document to be read as a property list.
 */
function XMLPropertyListReader(aDOMDoc) {
  let docElt = aDOMDoc.documentElement;
  if (!docElt || docElt.localName != "plist" || !docElt.firstElementChild)
    throw new Error("aDoc is not a property list document");

  this._plistRootElement = docElt.firstElementChild;
}

XMLPropertyListReader.prototype = {
  get root() {
    return this._readObject(this._plistRootElement);
  },

  /**
   * Convert a dom element to a property list object.
   * @param aDOMElt
   *        a dom element in a xml tree of a property list.
   * @return a js object representing the property list object.
   */
  _readObject: function XPLR__readObject(aDOMElt) {
    switch (aDOMElt.localName) {
      case "true":
        return true;
      case "false":
        return false;
      case "string":
      case "key":
        return aDOMElt.textContent;
      case "integer":
        return this._readInteger(aDOMElt);
      case "real": {
        let number = parseFloat(aDOMElt.textContent.trim());
        if (isNaN(number))
          throw "Could not parse float value";
        return number;
      }
      case "date":
        return new Date(aDOMElt.textContent);
      case "data":
        // Strip spaces and new lines.
        let base64str = aDOMElt.textContent.replace(/\s*/g, "");
        let decoded = atob(base64str);
        return new Uint8Array(Array.from(decoded, c => c.charCodeAt(0)));
      case "dict":
        return this._wrapDictionary(aDOMElt);
      case "array":
        return this._wrapArray(aDOMElt);
      default:
        throw new Error("Unexpected tagname");
    }
  },

  _readInteger: function XPLR__readInteger(aDOMElt) {
    // The integer may outbound js's max/min integer value.  We recognize this
    // case by comparing the parsed number to the original string value.
    // In case of an outbound, we fallback to return the number as a string.
    let numberAsString = aDOMElt.textContent.toString();
    let parsedNumber = parseInt(numberAsString, 10);
    if (isNaN(parsedNumber))
      throw new Error("Could not parse integer value");

    if (parsedNumber.toString() == numberAsString)
      return parsedNumber;

    return PropertyListUtils.wrapInt64(numberAsString);
  },

  _wrapDictionary: function XPLR__wrapDictionary(aDOMElt) {
    // <dict>
    //   <key>my true bool</key>
    //   <true/>
    //   <key>my string key</key>
    //   <string>My String Key</string>
    // </dict>
    if (aDOMElt.children.length % 2 != 0)
      throw new Error("Invalid dictionary");
    let dict = new Proxy(new Map(), LazyMapProxyHandler());
    for (let i = 0; i < aDOMElt.children.length; i += 2) {
      let keyElem = aDOMElt.children[i];
      let valElem = aDOMElt.children[i + 1];

      if (keyElem.localName != "key")
        throw new Error("Invalid dictionary");

      let keyName = this._readObject(keyElem);
      let readBound = this._readObject.bind(this, valElem);

      dict.setAsLazyGetter(keyName, readBound);
    }
    return dict;
  },

  _wrapArray: function XPLR__wrapArray(aDOMElt) {
    // <array>
    //   <string>...</string>
    //   <integer></integer>
    //   <dict>
    //     ....
    //   </dict>
    // </array>

    // Each element in the array is a lazy getter for its property list object.
    let array = [];
    let readObjectBound = this._readObject.bind(this);
    Array.prototype.forEach.call(aDOMElt.children, function(elem, elemIndex) {
      Object.defineProperty(array, elemIndex, {
        get: function() {
          delete array[elemIndex];
          return array[elemIndex] = readObjectBound(elem);
        },
        configurable: true,
        enumerable: true
      });
    });
    return array;
  }
};

/**
   * Simple handler method to proxy calls to dict/Map objects to implement the
   * setAsLazyGetter API. With this, a value can be set as a function that will
   * evaluate its value and only be called when it's first retrieved.
   * @member _lazyGetters
   *         Set() object to hold keys invoking LazyGetter.
   * @method get
   *         Trap for getting property values. Ensures that if a lazyGetter is present
   *         as value for key, then the function is evaluated, the value is cached,
   *         and its value will be returned.
   * @param  target
   *         Target object. (dict/Map)
   * @param  name
   *         Name of operation to be invoked on target.
   * @param  key
   *         Key to be set, retrieved or deleted. Keys are checked for laziness.
   * @return Returns value of "name" property of target by default. Otherwise returns
   *         updated target.
   */
function LazyMapProxyHandler () {
  return {
    _lazyGetters: new Set(),
    get: function(target, name) {
      switch (name) {
        case "setAsLazyGetter":
          return (key, value) => {
            this._lazyGetters.add(key);
            target.set(key, value);
          };
        case "get":
          return key => {
            if (this._lazyGetters.has(key)) {
              target.set(key, target.get(key)());
              this._lazyGetters.delete(key);
            }
            return target.get(key);
          };
        case "delete":
          return key => {
            if (this._lazyGetters.has(key)) {
              this._lazyGetters.delete(key);
            }
            return target.delete(key);
          };
        case "has":
          return key => target.has(key);
        default:
          return target[name];
      }
    }
  }
}