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cursor.js
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cursor.js
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/** api: module = cursor */
/** api: synopsis
* Cursor related functionality.
*/
/** api: summary
* The :mod:`cursor` module provides a constructor for Cursor objects.
*
* .. code-block:: javascript
*
* js> var CURSOR = require("geoscript/cursor");
*/
var UTIL = require("./util");
var GeoObject = require("./object").GeoObject;
/** api: class = Cursor */
var Cursor = UTIL.extend(GeoObject, {
/** private: property[_cursor]
* ``Object``
* The underlying cursor object with ``hasNext``, `next``, and ``close``
* methods.
*/
_cursor: null,
/** private: property[cast]
* ``Function``
* The method called to cast an underlying result object to a GeoScript
* object.
*/
cast: null,
/** private: property[open]
* ``Function``
* Optional method to open the cursor. If not provided, the cursor must
* be provided an ``iterator`` and will be considered open upon
* construction.
*/
open: null,
/** api: property[index]
* The index of the most recently read result. This is intialized as
* ``-1`` and is incremented with each result read.
*/
index: -1,
/** api: property[current]
* ``Object``
* The most recently read object in the collection. This will be ``null``
* if no results have been read or if the cursor has been exhausted.
*/
current: null,
/** private: property[closed]
* ``Boolean``
* Close has been called on the cursor.
*/
closed: false,
/** private: property[opened]
* ``Boolean``
* The cursor has been opened.
*/
opened: false,
/** api: constructor
* .. class:: Cursor
*
* A cursor will not generally need to be created. Instead, cursors
* are returned when querying features from a layer.
*/
constructor: function Cursor(config) {
this._cursor = config._cursor;
this.cast = config.cast;
this.open = config.open;
if (!this.open) {
this.opened = true;
}
this.scope = config.scope;
},
/** api: method[hasNext]
* :returns: ``Boolean``
*
* Determine whether there are more results to retrieve.
*/
hasNext: function() {
var has = false;
if (!this.opened) {
this._cursor = this.open.call(this.scope);
this.opened = true;
}
if (this._cursor) {
try {
has = this._cursor.hasNext();
} catch (err) {
has = false;
}
}
if (!has) {
this.close();
}
return has;
},
/** api: method[next]
* :return: ``Object`` The next result.
*
* Retrieve the next result in the results set. If no more results are
* available, `undefined` will be returned.
*/
next: function() {
var results = this.read(1);
return results[0];
},
/** api: method[get]
* :arg num: ``Number`` The number of results to get. Default is ``1``.
* :return: ``Array || Object`` An array of results. In the case where
* ``num`` is ``1``, a single result will be returned instead of an
* array of results.
*
* Get the next number of results. The length of the resulting array
* will be shorter than the given number in cases where the cursor is
* exhausted of results before the given number is reached. This method
* differs from the :meth:`read` method in that the cursor is always closed
* when calling :meth:`get` before the results are returned. This serves
* as a convenient way to get a list of results and close a cursor in one
* call.
*
* Note that in the special case where ``num`` is ``1`` a single result
* will be returned instead of an array of results.
*/
get: function(num) {
num = num || 1;
var results = this.read(num);
this.close();
return num === 1 ? results[0] : results;
},
/** api: method[read]
* :arg len: ``Number`` Number of results to read.
* :returns: ``Array`` An array of results.
*
* Read the next number of results. The length of the resulting array
* will be shorter than the given number in cases where the cursor is
* exhausted of results before the given number is reached.
*/
read: function(len) {
var _result, results = [];
for (var i=0; i<len; ++i) {
if (this.hasNext()) {
_result = this._cursor.next();
} else {
break;
}
++this.index;
this.current = this.cast.call(this.scope, _result);
results.push(this.current);
}
return results;
},
/** api: method[skip]
* :arg skip: ``Number`` Number of results to skip.
* :return: :class:`cursor.Cursor` This cursor.
*
* Advance the cursor to skip the given number of results.
*/
skip: function(offset) {
for (var i=0; i<offset; ++i) {
if (this.hasNext()) {
this._cursor.next();
} else {
break;
}
++this.index;
}
return this;
},
/** api: method[forEach]
* :arg func: ``Function``
* :arg scope: ``Object`` If provided, the function will be called as if
* it were a method of this object.
*
* Call a function with each result in the results set. If the function
* explicitly returns `false`, the iteration will stop.
*
* .. note::
*
* Client code should call :meth:`close` if the cursor is not exhausted
* by calling :meth:`next` or :meth:`forEach`.
*/
forEach: function(func, scope) {
var i=0, ret;
while (this.hasNext()) {
ret = func.call(scope, this.next(), i);
++i;
if (ret === false) {
break;
}
}
},
/** api: method[close]
*
* Close access to this cursor. This method should be called if the cursor
* is not exhausted by calling :meth:`next` or :meth:`forEach`.
*/
close: function() {
this.current = null;
if (this._cursor) {
this._cursor.close();
delete this._cursor;
}
this.closed = true;
}
});
/**
* Allow cursors to be used as iterators.
*/
Cursor.prototype.__iterator__ = function() {
var feature;
while (feature = this.next()) {
yield feature;
}
return;
};
exports.Cursor = Cursor;