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