Activity启动

每个Activity的创建，都会执行到ActivityThread类中的
Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) 此类负责创建Activity实例和一些information。并回调一些Activity 的回调函数 onXXX().

其中调用了 activity.attach（*******）。

    final void attach(Context context, ActivityThread aThread,
            Instrumentation instr, IBinder token, int ident,
            Application application, Intent intent, ActivityInfo info,
            CharSequence title, Activity parent, String id,
            Object lastNonConfigurationInstance,
            HashMap<String,Object> lastNonConfigurationChildInstances,
            Configuration config) {
        attachBaseContext(context);

        mWindow = PolicyManager.makeNewWindow(this);
        mWindow.setCallback(this);
        if (info.softInputMode != WindowManager.LayoutParams.SOFT_INPUT_STATE_UNSPECIFIED) {
            mWindow.setSoftInputMode(info.softInputMode);
        }
        mUiThread = Thread.currentThread();

        mMainThread = aThread;
        mInstrumentation = instr;
        mToken = token;
        mIdent = ident;
        mApplication = application;
        mIntent = intent;
        mComponent = intent.getComponent();
        mActivityInfo = info;
        mTitle = title;
        mParent = parent;
        mEmbeddedID = id;
        mLastNonConfigurationInstance = lastNonConfigurationInstance;
        mLastNonConfigurationChildInstances = lastNonConfigurationChildInstances;

        mWindow.setWindowManager(null, mToken, mComponent.flattenToString());
        if (mParent != null) {
            mWindow.setContainer(mParent.getWindow());
        }
        mWindowManager = mWindow.getWindowManager();
        mCurrentConfig = config;
    }

创建Window ，创建的是PhoneWindow

private static final String POLICY_IMPL_CLASS_NAME =
        "com.android.internal.policy.impl.Policy";

    static {
        // Pull in the actual implementation of the policy at run-time
        try {
            Class policyClass = Class.forName(POLICY_IMPL_CLASS_NAME);
            sPolicy = (IPolicy)policyClass.newInstance();
        } catch (ClassNotFoundException ex) {
            throw new RuntimeException(
                    POLICY_IMPL_CLASS_NAME + " could not be loaded", ex);
        } catch (InstantiationException ex) {
            throw new RuntimeException(
                    POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);
        } catch (IllegalAccessException ex) {
            throw new RuntimeException(
                    POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);
        }
    }
上面代码是静态模块，类被加载时就被执行。
    public static Window makeNewWindow(Context context) {
        return sPolicy.makeNewWindow(context);
    }
    public PhoneWindow makeNewWindow(Context context) {
        return new PhoneWindow(context);
    }

下面是创建WindowManger

mWindow.setWindowManager(null, mToken, mComponent.flattenToString());
    public void setWindowManager(WindowManager wm,
            IBinder appToken, String appName) {
        mAppToken = appToken;
        mAppName = appName;
        if (wm == null) {
            wm = WindowManagerImpl.getDefault();
        }
        mWindowManager = new LocalWindowManager(wm);
    }

wm = WindowManagerImpl.getDefault();是返回一个默认的WindowManagerImpl 的静态实例，在android系统中的所有应用程序，都共用一个WindowManagerImpl实例。

Window 被创建的时候，会执行如下代码来生成Window  Attrivute

    private final WindowManager.LayoutParams mWindowAttributes =
        new WindowManager.LayoutParams();

下面是更改此属性：

    /**
     * Specify custom window attributes.  <strong>PLEASE NOTE:</strong> the
     * layout params you give here should generally be from values previously
     * retrieved with {@link #getAttributes()}; you probably do not want to
     * blindly create and apply your own, since this will blow away any values
     * set by the framework that you are not interested in.
     *
     * @param a The new window attributes, which will completely override any
     *          current values.
     */
    public void setAttributes(WindowManager.LayoutParams a) {
        mWindowAttributes.copyFrom(a);
        if (mCallback != null) {
            mCallback.onWindowAttributesChanged(mWindowAttributes);
        }
    }

要更改mWindowAttributes此属性，只能用此方法，并回调。