DisplayMetrics dm = new DisplayMetrics();
context.getWindowManager().getDefaultDisplay().getMetrics(dm);
android.view.WindowManager.LayoutParams p = dialog.getWindow().getAttributes(); //获取对话框当前的参数值
p.width = (int) (dm.widthPixels*0.8);
p.gravity = Gravity.CENTER;
dialog.getWindow().setAttributes(p); //设置生效