View view = getWindow().getDecorView();
WindowManager.LayoutParams lp = (WindowManager.LayoutParams)view.getLayoutParams();
lp.gravity = Gravity.CENTER;
lp.width = (dm.widthPixels * 4) / 5;
lp.height = (dm.widthPixels * 4) / 5;
getWindowManager().updateViewLayout(view,lp);
放到onCreate中进行测试，结果在lp.gravity = Gravity.CENTER;这行报了空指针异常，所以view.getLayoutParams()获取的LayoutParams是空。
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public final class ActivityThread {
......
final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward) {
......
ActivityClientRecord r = performResumeActivity(token, clearHide);
if (r != null) {
final Activity a = r.activity;
......
// If the window hasn't yet been added to the window manager,
// and this guy didn't finish itself or start another activity,
// then go ahead and add the window.
boolean willBeVisible = !a.mStartedActivity;
if (!willBeVisible) {
try {
willBeVisible = ActivityManagerNative.getDefault().willActivityBeVisible(
a.getActivityToken());
} catch (RemoteException e) {
}
}
if (r.window == null && !a.mFinished && willBeVisible) {
r.window = r.activity.getWindow();
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
ViewManager wm = a.getWindowManager();
WindowManager.LayoutParams l = r.window.getAttributes();
a.mDecor = decor;
l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
......
if (a.mVisibleFromClient) {
a.mWindowAdded = true;
wm.addView(decor, l);
}
}
......
}
......
}
......
}
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