// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

library observe.src.path_observer;

import 'dart:async';
@MirrorsUsed(metaTargets: const [Reflectable, ObservableProperty],
    override: 'observe.src.path_observer')
import 'dart:mirrors';
import 'package:logging/logging.dart' show Logger, Level;
import 'package:observe/observe.dart';
import 'package:observe/src/observable.dart' show objectType;

// This code is inspired by ChangeSummary:
// https://github.com/rafaelw/ChangeSummary/blob/master/change_summary.js
// ...which underlies MDV. Since we don't need the functionality of
// ChangeSummary, we just implement what we need for data bindings.
// This allows our implementation to be much simpler.

/**
 * A data-bound path starting from a view-model or model object, for example
 * `foo.bar.baz`.
 *
 * When the [values] stream is being listened to, this will observe changes to
 * the object and any intermediate object along the path, and send [values]
 * accordingly. When all listeners are unregistered it will stop observing
 * the objects.
 *
 * This class is used to implement [Node.bind] and similar functionality.
 */
// TODO(jmesserly): consider specialized subclasses for:
// * empty path
// * "value"
// * single token in path, e.g. "foo"
class PathObserver extends ChangeNotifier {
  /** The path string. */
  final String path;

  /** True if the path is valid, otherwise false. */
  final bool _isValid;

  final List<Object> _segments;
  List<Object> _values;
  List<StreamSubscription> _subs;

  final Function _computeValue;

  /**
   * Observes [path] on [object] for changes. This returns an object that can be
   * used to get the changes and get/set the value at this path.
   *
   * You can optionally use [computeValue] to apply a function to the result of
   * evaluating the path. The function should be pure, as PathObserver will not
   * know to observe any of its dependencies. If you need to observe mutliple
   * values, use [CompoundPathObserver] instead.
   *
   * See [PathObserver.bindSync] and [PathObserver.value].
   */
  PathObserver(Object object, String path, {computeValue(newValue)})
      : path = path,
        _computeValue = computeValue,
        _isValid = _isPathValid(path),
        _segments = <Object>[] {

    if (_isValid) {
      for (var segment in path.trim().split('.')) {
        if (segment == '') continue;
        var index = int.parse(segment, radix: 10, onError: (_) => null);
        _segments.add(index != null ? index : new Symbol(segment));
      }
    }

    // Initialize arrays.
    // Note that the path itself can't change after it is initially
    // constructed, even though the objects along the path can change.
    _values = new List<Object>(_segments.length + 1);

    // If we have an empty path, we need to apply the transformation function
    // to the value. The "value" property should always show the transformed
    // value.
    if (_segments.isEmpty && computeValue != null) {
      object = computeValue(object);
    }

    _values[0] = object;
    _subs = new List<StreamSubscription>(_segments.length);
  }

  /** The object being observed. If the path is empty this will be [value]. */
  get object => _values[0];

  /** Gets the last reported value at this path. */
  @reflectable get value {
    if (!_isValid) return null;
    if (!hasObservers) _updateValues();
    return _values.last;
  }

  /** Sets the value at this path. */
  @reflectable void set value(Object newValue) {
    int len = _segments.length;

    // TODO(jmesserly): throw if property cannot be set?
    // MDV seems tolerant of these errors.
    if (len == 0) return;
    if (!hasObservers) _updateValues(end: len - 1);

    if (_setObjectProperty(_values[len - 1], _segments[len - 1], newValue)) {
      // Technically, this would get updated asynchronously via a change record.
      // However, it is nice if calling the getter will yield the same value
      // that was just set. So we use this opportunity to update our cache.
      _values[len] = newValue;
    }
  }

  /**
   * Invokes the [callback] immediately with the current [value], and every time
   * the value changes. This is useful for bindings, which want to be up-to-date
   * immediately and stay bound to the value of the path.
   */
  StreamSubscription bindSync(void callback(value)) {
    var result = changes.listen((records) { callback(value); });
    callback(value);
    return result;
  }

  void observed() {
    super.observed();
    _updateValues();
    _observePath();
  }

  void unobserved() {
    for (int i = 0; i < _subs.length; i++) {
      if (_subs[i] != null) {
        _subs[i].cancel();
        _subs[i] = null;
      }
    }
    super.unobserved();
  }

  // TODO(jmesserly): should we be caching these values if not observing?
  void _updateValues({int end}) {
    if (end == null) end = _segments.length;
    int last = _segments.length - 1;
    for (int i = 0; i < end; i++) {
      var newValue = _getObjectProperty(_values[i], _segments[i]);
      if (i == last && _computeValue != null) {
        newValue = _computeValue(newValue);
      }
      _values[i + 1] = newValue;
    }
  }

  void _updateObservedValues({int start: 0}) {
    var oldValue, newValue;
    for (int i = start, last = _segments.length - 1; i <= last; i++) {
      oldValue = _values[i + 1];
      newValue = _getObjectProperty(_values[i], _segments[i]);
      if (i == last && _computeValue != null) {
        newValue = _computeValue(newValue);
      }
      if (identical(oldValue, newValue)) {
        _observePath(start, i);
        return;
      }
      _values[i + 1] = newValue;
    }

    _observePath(start);
    notifyPropertyChange(#value, oldValue, newValue);
  }

  void _observePath([int start = 0, int end]) {
    if (end == null) end = _segments.length;

    for (int i = start; i < end; i++) {
      if (_subs[i] != null) _subs[i].cancel();
      _observeIndex(i);
    }
  }

  void _observeIndex(int i) {
    final object = _values[i];
    if (object is Observable) {
      // TODO(jmesserly): rather than allocating a new closure for each
      // property, we could try and have one for the entire path. However we'd
      // need to do a linear scan to find the index as soon as we got a change.
      // Also we need to fix ListChangeRecord and MapChangeRecord to contain
      // the target. Not sure if it's worth it.
      _subs[i] = object.changes.listen((List<ChangeRecord> records) {
        for (var record in records) {
          if (_changeRecordMatches(record, _segments[i])) {
            _updateObservedValues(start: i);
            return;
          }
        }
      });
    }
  }
}

bool _changeRecordMatches(record, key) {
  if (record is ListChangeRecord) {
    return key is int && (record as ListChangeRecord).indexChanged(key);
  }
  if (record is PropertyChangeRecord) {
    return (record as PropertyChangeRecord).name == key;
  }
  if (record is MapChangeRecord) {
    if (key is Symbol) key = MirrorSystem.getName(key);
    return (record as MapChangeRecord).key == key;
  }
  return false;
}

_getObjectProperty(object, property) {
  if (object == null) return null;

  if (property is int) {
    if (object is List && property >= 0 && property < object.length) {
      return object[property];
    }
  } else if (property is Symbol) {
    var mirror = reflect(object);
    final type = mirror.type;
    try {
      if (_maybeHasGetter(type, property)) {
        return mirror.getField(property).reflectee;
      }
      // Support indexer if available, e.g. Maps or polymer_expressions Scope.
      // This is the default syntax used by polymer/nodebind and
      // polymer/observe-js PathObserver.
      if (_hasMethod(type, const Symbol('[]'))) {
        return object[MirrorSystem.getName(property)];
      }
    } on NoSuchMethodError catch (e) {
      // Rethrow, unless the type implements noSuchMethod, in which case we
      // interpret the exception as a signal that the method was not found.
      if (!_hasMethod(type, #noSuchMethod)) rethrow;
    }
  }

  if (_logger.isLoggable(Level.FINER)) {
    _logger.log("can't get $property in $object");
  }
  return null;
}

bool _setObjectProperty(object, property, value) {
  if (object == null) return false;

  if (property is int) {
    if (object is List && property >= 0 && property < object.length) {
      object[property] = value;
      return true;
    }
  } else if (property is Symbol) {
    var mirror = reflect(object);
    final type = mirror.type;
    try {
      if (_maybeHasSetter(type, property)) {
        mirror.setField(property, value);
        return true;
      }
      // Support indexer if available, e.g. Maps or polymer_expressions Scope.
      if (_hasMethod(type, const Symbol('[]='))) {
        object[MirrorSystem.getName(property)] = value;
        return true;
      }
    } on NoSuchMethodError catch (e) {
      if (!_hasMethod(type, #noSuchMethod)) rethrow;
    }
  }

  if (_logger.isLoggable(Level.FINER)) {
    _logger.log("can't set $property in $object");
  }
  return false;
}

bool _maybeHasGetter(ClassMirror type, Symbol name) {
  while (type != objectType) {
    final members = type.members;
    if (members.containsKey(name)) return true;
    if (members.containsKey(#noSuchMethod)) return true;
    type = _safeSuperclass(type);
  }
  return false;
}

// TODO(jmesserly): workaround for:
// https://code.google.com/p/dart/issues/detail?id=10029
Symbol _setterName(Symbol getter) =>
    new Symbol('${MirrorSystem.getName(getter)}=');

bool _maybeHasSetter(ClassMirror type, Symbol name) {
  var setterName = _setterName(name);
  while (type != objectType) {
    final members = type.members;
    if (members[name] is VariableMirror) return true;
    if (members.containsKey(setterName)) return true;
    if (members.containsKey(#noSuchMethod)) return true;
    type = _safeSuperclass(type);
  }
  return false;
}

/**
 * True if the type has a method, other than on Object.
 * Doesn't consider noSuchMethod, unless [name] is `#noSuchMethod`.
 */
bool _hasMethod(ClassMirror type, Symbol name) {
  while (type != objectType) {
    final member = type.members[name];
    if (member is MethodMirror && member.isRegularMethod) return true;
    type = _safeSuperclass(type);
  }
  return false;
}

ClassMirror _safeSuperclass(ClassMirror type) {
  try {
    return type.superclass;
  } on UnsupportedError catch (e) {
    // TODO(jmesserly): dart2js throws this error when the type is not
    // reflectable.
    return objectType;
  }
}

// From: https://github.com/rafaelw/ChangeSummary/blob/master/change_summary.js

final _pathRegExp = () {
  const identStart = '[\$_a-zA-Z]';
  const identPart = '[\$_a-zA-Z0-9]';
  const ident = '$identStart+$identPart*';
  const elementIndex = '(?:[0-9]|[1-9]+[0-9]+)';
  const identOrElementIndex = '(?:$ident|$elementIndex)';
  const path = '(?:$identOrElementIndex)(?:\\.$identOrElementIndex)*';
  return new RegExp('^$path\$');
}();

final _spacesRegExp = new RegExp(r'\s');

bool _isPathValid(String s) {
  s = s.replaceAll(_spacesRegExp, '');

  if (s == '') return true;
  if (s[0] == '.') return false;
  return _pathRegExp.hasMatch(s);
}

final _logger = new Logger('observe.PathObserver');