Context

不同Context的应用场景：

NO1:Application和Service可以启动一个Activity，需要创建一个新task，比如Application调用startActivity(intent)系统会报错误：

java.lang.RuntimeException: Unable to create application
com.xjp.toucheventdemo.MyApplication: android.util.AndroidRuntimeException: Calling
startActivity() from outside of an Activity context requires the FLAG_ACTIVITY_NEW_TASK
flag. Is this really what you want?

除了Activity可以创建一个Dialog，其他都可以创建Dialog会报错

Caused by: java.lang.IllegalStateException: You need to use a Theme.AppCompat theme (or descendant) with this activity.

其实可以写全局的dialog不需要依附Activity，可以用Application的Context来显示系统级别的dialog。