功能需求：
点击悬浮窗中的按钮显示一个Dialog
踩坑之路：
先是Context
Dialog adDialog = new Dialog(context, R.style.DialogStyle);
adDialogView = LayoutInflater.from(context).inflate(R.layout.dialog,null);
adDialog.setContentView(adDialogView);
adDialog.show();
崩溃，log：
android.view.WindowManager$BadTokenException: Unable to add window -- token null is not for an application
换成ApplicationContext
Dialog adDialog = new Dialog(context.getApplicationContext(), R.style.DialogStyle);
adDialogView = LayoutInflater.from(context).inflate(R.layout.dialog,null);
adDialog.setContentView(adDialogView);
adDialog.show();
崩溃，log：
android.view.WindowManager$BadTokenException: Unable to add window -- token null is not for an application
换成Activity
Dialog adDialog = new Dialog(activity, R.style.DialogStyle);
adDialogView = LayoutInflater.from(context).inflate(R.layout.dialog,null);
adDialog.setContentView(adDialogView);
adDialog.show();
第一次启动APP，点击显示悬浮窗，成功！退出APP再次进入，点击悬浮窗，崩溃！log：
android.view.WindowManager$BadTokenException: Unable to add window -- token android.os.BinderProxy@41e48918 is not valid; is your activity running?
添加 TYPE_SYSTEM_ALERT
在代码中添加：adDialog.getWindow().setType(WindowManager.LayoutParams.TYPE_SYSTEM_ALERT);
Dialog adDialog = new Dialog(context.getApplicationContext(), R.style.DialogStyle);
adDialog.getWindow().setType(WindowManager.LayoutParams.TYPE_SYSTEM_ALERT);
adDialogView = LayoutInflater.from(context).inflate(R.layout.dialog,null);
adDialog.setContentView(adDialogView);
adDialog.show();
并且在manifest中增加权限：
<uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW" />
此时，无论是使用context或者activity还是context.getApplicationContext()都可以正常显示Dialog
