PendingIntent是一個Intent的描述、包裝，給予了這個PendingIntent 的組件在指定的事件發生或指定的時間到達時啟動Activty、Service或者Broadcast。
根據是要啟動Activity、Service還是Broadcast分別對應一個獲取PendingIntent的方法
public static PendingIntent getActivity(Context context, int requestCode,
Intent intent, int flags)
public static PendingIntent getBroadcast(Context context, int requestCode
Intent intent, int flags)
public static PendingIntent getService(Context context, int requestCode,
Intent intent, int flags)
三個函數的參數都相同，其中最后一個參數flags在文檔中是這樣解析的：
flags May be FLAG_ONE_SHOT, FLAG_NO_CREATE, FLAG_CANCEL_CURRENT, FLAG_UPDATE_CURRENT, or any of the flags as supported by Intent.fillIn() to control which unspecified parts of the intent that can be supplied when the actual send happens.
flags:May be FLAG_ONE_SHOT,LAG_NO_CREATE,LAG_CANCEL_CURRENT, FLAG_UPDATE_CURRENT, or any of the flags as supported by Intent.fillIn() to control which unspecified parts of the intent that can be supplied when the actual send happens.
目前為止只提供FLAG_ONE_SHOT, FLAG_NO_CREATE, FLAG_CANCEL_CURRENT, FLAG_UPDATE_CURRENT這四個flag
FLAG_ONE_SHOT:this PendingIntent can only be used once. If set, after send() is called on it, it will be automatically canceled for you and any future attempt to send through it will fail.
利用 FLAG_ONE_SHOT獲取的PendingIntent只能使用一次，即使再次利用上面三個方法重新獲取，再使用PendingIntent也將失敗。
FLAG_NO_CREATE:if the described PendingIntent does not already exist, then simply return null instead of creating it.
利用FLAG_NO_CREAT獲取PendingIntent，若描述的Intent不存在則返回NULL值.
FLAG_CANCEL_CURRENT:if the described PendingIntent already exists, the current one is canceled before generating a new one. You can use this to retrieve a new PendingIntent when you are only changing the extra data in the Intent; by canceling the previous pending intent, this ensures that only entities given the new data will be able to launch it. If this assurance is not an issue, consider FLAG_UPDATE_CURRENT.
如果描述的PendingIntent已經存在，則在使用新的Intent之前會先取消掉當前的。你可以通過這個去取回，并且通過取消先前的Intent，更新在Intent中的數據。這能確保對象被給予新的數據。如果無法保證唯一，考慮使用flag_update_current。
FLAG_UPDATE_CURRENT: if the described PendingIntent already exists, then keep it but its replace its extra data with what is in this new Intent. This can be used if you are creating intents where only the extras change, and don't care that any entities that received your previous PendingIntent will be able to launch it with your new extras even if they are not explicitly given to it.
如果描述的Intent存在，想繼續持有他但是需要修改部分數據。這就可以被使用如果你創建了一個新的Intent當你只想修改部分數據，而且不想關注他那些以前就存在的Intent會收到新的更新數據甚至不是特別的給它的。
上面4個flag中最經常使用的是FLAG_UPDATE_CURRENT，因為描述的Intent有 更新的時候需要用到這個flag去更新你的描述。
使用 FLAG_CANCEL_CURRENT也能做到更新extras，只不過是先把前面的extras清除，另外FLAG_CANCEL_CURRENT和 FLAG_UPDATE_CURRENT的區別在于能否新new一個Intent，FLAG_UPDATE_CURRENT能夠新new一個 Intent，而FLAG_CANCEL_CURRENT則不能，只能使用第一次的Intent。
此外還需要注意參數：
int requestCode ： Private request code for the sender (currently not used).
PendingIntent contentIntent = PendingIntent.getActivity(context,
num, intent, PendingIntent.FLAG_UPDATE_CURRENT);
對于FLAG_UPDATE_CURRENT,如果上面的requestCode 為常量，則對于先后出現的若干Notification,則所有對應的Intent里面的extra被更新為最新的，就是全部同一為最后一次的。
相反，如果requestCode 每次不一樣，對于FLAG_CANCEL_CURRENT,則只響應最前面的第一條Notifiacation,后面所有的不響應
