window表示的是一个抽象窗口类，该类只是一个抽象窗口类，其具体的唯一实现类是PhoneWindow类。Window对象的获取通过在Activity中调用getWindow（）方法获取到Window对象；该类定义了一个CallBack接口，用于处理用户的消息数据，该接口的方法有：dispatchKeyEvent，dispatchTouchEvent等，
他设置了一组通用的窗口操作api。比如setContentView方法，实际上Activity中的setContentView方法是调用的Activity内部的Window中的setContentView方法来实现的，除了这个方法之外，还有findViewById（）方法也是调用的Window方法的findViewById（）方法。
在Window类中，定义了一组常量值，用于表示窗口的样式：常量值如下所示：
/** Flag for the "options panel" feature. This is enabled by default.表示有菜单项，默认有菜单项 */
public static final int FEATURE_OPTIONS_PANEL = 0;
/** Flag for the "no title" feature, turning off the title at the top
* of the screen.表示无标题栏，也就是应用程序上面没有标题栏 */
public static final int FEATURE_NO_TITLE = 1;
/** Flag for the progress indicator feature进度条 */
public static final int FEATURE_PROGRESS = 2;
/** Flag for having an icon on the left side of the title bar在标题栏的左侧有图标 */
public static final int FEATURE_LEFT_ICON = 3;
/** Flag for having an icon on the right side of the title bar在标题栏的右侧有图标 */
public static final int FEATURE_RIGHT_ICON = 4;
/** Flag for indeterminate progress 不确定的进度条，比如圆圈状的等待条*/
public static final int FEATURE_INDETERMINATE_PROGRESS = 5;
/** Flag for the context menu. This is enabled by default.上下文菜单 */
public static final int FEATURE_CONTEXT_MENU = 6;
/** Flag for custom title. You cannot combine this feature with other title features. 自定义的标题栏，该属性不能和其他属性合用*/
public static final int FEATURE_CUSTOM_TITLE = 7;
/**
* Flag for enabling the Action Bar.
* This is enabled by default for some devices. The Action Bar
* replaces the title bar and provides an alternate location
* for an on-screen menu button on some devices.
*/
public static final int FEATURE_ACTION_BAR = 8;
public static final int FEATURE_ACTION_BAR_OVERLAY = 9;
/**
* Flag for specifying the behavior of action modes when an Action Bar is not present.
* If overlay is enabled, the action mode UI will be allowed to cover existing window content.
*/
public static final int FEATURE_ACTION_MODE_OVERLAY = 10;
/**
* Max value used as a feature ID
* @hide
*/
public static final int FEATURE_MAX = FEATURE_ACTION_MODE_OVERLAY;
/** Flag for setting the progress bar‘s visibility to VISIBLE */
public static final int PROGRESS_VISIBILITY_ON = -1;
/** Flag for setting the progress bar‘s visibility to GONE */
public static final int PROGRESS_VISIBILITY_OFF = -2;
/** Flag for setting the progress bar‘s indeterminate mode on */
public static final int PROGRESS_INDETERMINATE_ON = -3;
/** Flag for setting the progress bar‘s indeterminate mode off */
public static final int PROGRESS_INDETERMINATE_OFF = -4;
/** Starting value for the (primary) progress */
public static final int PROGRESS_START = 0;
/** Ending value for the (primary) progress */
public static final int PROGRESS_END = 10000;
/** Lowest possible value for the secondary progress */
public static final int PROGRESS_SECONDARY_START = 20000;
/** Highest possible value for the secondary progress */实际上我们熟知的Activity与Widnow类联系很紧密，每个Activity中都有自己的Window类。在每个Activity类的内部都有一个私有变量private Window mWindow;该对象的赋值是在Activity的attach（）方法中赋值的，通过mWindow = PolicyManager.makeNewWindow(this);这个语句来为对象赋值，而attach方法的调用是在ActivityThread中创建Activity对象时调用的。PolicyManager.makeNewWindow(this);该方法归根结底只是创建出一个PhoneWindow对象赋值给mWindow对象。另外Window类可以接受用户的消息处理的，而Activity通过实现Window类的CallBack接口从而也能够处理用户的消息。
window对象的一些常用方法：
setContentView（），activity中setContentView就是调用该方法
findViewById（），activity中findViewById就是调用该方法
getLayoutInflater（），activity中getLayoutInflater就是调用该方法
getContext（），返回当前的上下文环境
getWindowManager（）返回窗口管理器