gears旅程

gears是google推出的新一代web技术，用于离线应用，本地存储，提升js性能等，使用时网页会要求安装一个东西，就像flash要安装一个player，silverlight要安装一个runtime一样，地址为：http://gears.google.com/

Gears 模块包括：

	LocalServer 在本地缓存和提供应用程序资源（HTML、JavaScript、图片等）
	Database 将数据存储在本地可完全搜索的关系数据库中
	WorkerPool 通过异步执行资源密集型的操作使网络应用程序的响应速度更快

附一个官网的demo：http://code.google.com/intl/zh-CN/apis/gears/samples/hello_world_database.html

gears的本地存储原理比较简单，通过一个sqlite数据库将数据保存在本地，通过js进行数据库操纵。

gears的数据在各个浏览器中是不互通的，chrome中存储的数据只能在chrome中被读取。

通过一段js去调用gears：

<script src="gears_init.js"></script>

<script>

  if (!window.google || !google.gears) {

    location.href = "http://gears.google.com/?action=install&message=<your welcome message>" +

                    "&return=<your website url>";

  }

</script>

gears_init.js的路径为：http://code.google.com/intl/zh-CN/apis/gears/gears_init.js

看了源码之后觉得初始化工作还是比较简单的：

(function() {

  // We are already defined. Hooray!

  if (window.google && google.gears) {

    return;

  }



  var factory = null;



  // Firefox

  if (typeof GearsFactory != 'undefined') {

    factory = new GearsFactory();

  } else {

    // IE

    try {

      factory = new ActiveXObject('Gears.Factory');

      // privateSetGlobalObject is only required and supported on IE Mobile on

      // WinCE.

      if (factory.getBuildInfo().indexOf('ie_mobile') != -1) {

        factory.privateSetGlobalObject(this);

      }

    } catch (e) {

      // Safari

      if ((typeof navigator.mimeTypes != 'undefined')

           && navigator.mimeTypes["application/x-googlegears"]) {

        factory = document.createElement("object");

        factory.style.display = "none";

        factory.width = 0;

        factory.height = 0;

        factory.type = "application/x-googlegears";

        document.documentElement.appendChild(factory);

        if(factory && (typeof factory.create == 'undefined')) {

          // If NP_Initialize() returns an error, factory will still be created.

          // We need to make sure this case doesn't cause Gears to appear to

          // have been initialized.

          factory = null;

        }

      }

    }

  }



  // *Do not* define any objects if Gears is not installed. This mimics the

  // behavior of Gears defining the objects in the future.

  if (!factory) {

    return;

  }



  // Now set up the objects, being careful not to overwrite anything.

  //

  // Note: In Internet Explorer for Windows Mobile, you can't add properties to

  // the window object. However, global objects are automatically added as

  // properties of the window object in all browsers.

  if (!window.google) {

    google = {};

  }



  if (!google.gears) {

    google.gears = {factory: factory};

  }

})();

核心代码就这些，ie是通过ActiveXObject实现的。

而具体的操纵数据库的代码：

var recentPhrases = ['', '', ''];

 

  // Get the 3 most recent entries. Delete any others.

  var rs = db.execute('select * from Demo order by Timestamp desc');

  var index = 0;

  while (rs.isValidRow()) {

    if (index < 3) {

      recentPhrases[index] = rs.field(0);

    } else {

      db.execute('delete from Demo where Timestamp=?', [rs.field(1)]);

    }

    ++index;

    rs.next();

  }

  rs.close();

很像java的代码。。。

读取后将数据写入dom元素，这即是本地存储的应用。

此外，gears还有其他一系列api，例如地理定位，离线应用等，更多的例子，在http://code.google.com/intl/zh-CN/apis/gears/sample.html