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A facility for the application to tell the system about something it wants to be doing in the background

A facility for an application to expose some of its functionality to other applications.

It allows third-party application servers to send lightweight messages to their Android applications. The messaging service is not designed for sending a lot of user content via the messages. Rather, it should be used to tell the application that there is new data on the server, so that the application can fetch it. C2DM makes no guarantees about delivery or the order of messages. So, for example, while you might use this feature to tell an instant messaging application that the user has new messages, you probably would not use it to pass the actual messages. An application on an Android device doesn’t need to be running to receive messages. The system will wake up the application via Intent broadcast when the the message arrives, as long as the application is set up with the proper broadcast receiver and permissions.

It uses an existing connection for Google services. This requires users to set up their Google account on their mobile devices.

C2DM imposes the following limitations: The message size limit is 1024 bytes. Google limits the number of messages a sender sends in aggregate, and the number of messages a sender sends to a specific device

OSGi (Open Service Gateway Initiative) is Java framework for developing and deploying modular software programs and libraries

基于Unix（Linux）的系统都有一个很优秀的传统，就是倡导非常轻便的进程间通信（IPC）机制；倡导进程通过IPC来互相协作；倡导功能单一，小巧而强壮的进程，而不是又大又复杂的“万金油”

Android的组件和进程间通信都建立在一种基于称为Intent的消息基础之上。Intent就是一种消息，它包含了两个重要的内容：1. 消息的目的，即这个消息是发给哪个组件的？（消息的目的中不会包含“消息是发给哪个进程”这样的信息，这里Android有意淡化进程的概念，而只让我们关心组件，因为了解太多关于进程的具体信息会加大复杂度，而又如何做到进程间的消息传递呢？下文会说到一种Android中关于这点比较特别的设计方式，我认为是一种简捷有用又符合手机特点的设计）；2. 消息所携带的数据内容，即需要传递给目标的数据。下面是一个简单的利用Intent来启动一个Service并向其传递数据的代码示例

这种模式的消息由于已经确切知道了消息目标的确切信息，所以只适用于同一进程内的不同组件之间通信，例如打开一个子窗体，和同一进程中的service通信等。对应的，隐式消息就一般用于跨进程的通信了，隐式消息没有确定的消息目的地，除了数据外，隐式消息只是包含了一些用于表征消息特征的描述字段

如果你呼叫 startService() 多次，每一次的呼叫都會被轉成一個 message，並放在 mServiceLooper 的 message queue 中，等待被服務。一個 message 所對應的工作被完成後，才會繼續服務下一個工作。所以，這些等待被服務的工作，並不是一起並行 (Concurrent) 的，而是循序執行。

從研究這個 IntentService 的原始碼，我們可以學到如何運用簡單的 pattern (Service + Handler + HandlerThread)，幫我們更簡易與有系統地，完成我們所想要做的事

最後我們先前說過，在 IntentServcie 中等待被服務的工作，並不會被一起並行，而是循序執行。如果你今天想要這些等待被服務的工作，能夠一起被並行，在研讀完這個 IntentService 的原始碼後，你自己知不知道如何寫個可支援並行工作的 IntentService？ 提示：可以用 Service + AsyncTask 的組合

When onReceive() returns

by the system at any time

I’ve run into the following generalized question a lot recently: I have an event that occurs in the background. I want to update my activity, if the activity is on the screen. Otherwise, I want to raise a Notification.

Hence, the recipe for the activity-or-Notification pattern is: Define an action string you will use when the event occurs that you want to go to the activity or notification (e.g., com.commonsware.java.packages.are.fun.EVENT). Dynamically register a BroadcastReceiever in your activity, with an IntentFilter set up for the aforementioned action string and with a positive priority (the default priority for a filter is 0). This receiver should then have the activity do whatever it needs to do to update the UI based on this event. The receiver should also call abortBroadcast() to prevent others from getting it. Be sure to register the receiver in onStart() or onResume() and unregister the receiver in the corresponding onStop or onPause() method. Register in your manifest a BroadcastReceiver, with an <intent-filter> set up for the aforementioned action string. This receiver should raise the Notification. In your service (e.g., an IntentService), when the event occurs, call sendOrderedBroadcast(). And that’s it. If the activity is on-screen, its receiver will be registered, so it will get the event, process it, and cancel the broadcast. If the activity is not on-screen, its receiver will not be registered, so the event will go to the default handler, in the form of your manifest-registered BroadcastReceiver, which will raise the Notification.

For those of you that haven’t tried Dropbox, it’s a service that makes it remarkably easy to share files between multiple computers (it means you don’t have to keep emailing files between your home and work computers, or rely on a clunky USB stick). It’s also great for collaboration — we’ve been using it around the TechCrunch office to share files for years now

Google Chrome to Phone Extension is a project consisting of a Chrome Extension, Android App, and supporting AppEngine server that enables users to send links from their Chrome desktop browser to their Android device using Android's Cloud to Device Messaging service.

Current Instapaper and former Tumblr developer Marco Arment wonders out loud if Google Android phones can ever achieve the levels of usability and polish Apple’s iPhone has arguably had since day one

Android will continue to exhibit what Google does best: great low-level engineering and tight integration with Google’s other services. But it’s never going to be Apple-like in user experience, polish, or design. Attention to detail, like most facets of truly good design, can’t be (and never is) added later. It’s an entire development philosophy, methodology, and culture

So the basic idea is: launch an AsyncTask making your service call, show a nifty progress dialog while the task thread is running, and have the task’s result be posted back to your activity once it completes. Cool, but what if the user decides to rotate the screen while your task is running? Or a phone call comes in, interrupting your app, and Android decides to kill it? Both these actions will effectively terminateyour activity, and recreate it when resuming (yes, a screen rotation kills your activity, very clever, isn’t it?). Unfortunately, any AsyncTask that was still running now holds a stale reference to your activity, because the restarted activity will be an entirely different object in memory (and it will go through onCreate(), as if the activity had started for the first time). I’m not entirely sure whether AsyncTask will actually post back the data to the old activity object (if it was a weak reference, it may already have been garbage collected), but in any case, your “new” activity will never see it, because it’s a different instance.

AsyncTask要点 1、必须从主线程调用，或者线程中有Handler或Looper。 2、不要在一个可能会被另外一个AsyncTask调用的库里面使用AsyncTask（AsyncTask是不可重入的） 3、如果从一个activity中调用，activity进程可能会在AsyncTask结束前退出，例如： 用户退出了activity 系统内存不足 系统暂存了activity的状态留待后用 系统干掉了你的线程 如果AsyncTask中的工作很重要，应该使用……

IntentService 的好处 Acitivity的进程，当处理Intent的时候，会产生一个对应的Service Android的进程处理器现在会尽可能的不kill掉你 非常容易使用

总结 离开主线程！ 磁盘和网络操作不是马上就能完的 了解sqlite在干嘛 进度展示很好