Action+ IntentService架構
這一部分給大家總結一下mms裏面對於後臺任務的處理。正常情況下，一個互聯網應用可能會涉及到n多的後臺任務要運行，短信應用也不例外，例如插入短信到數據庫，刪除短信，標記爲已讀，發送短信，接收短信，下載彩信等。這些都是耗時任務，並且他們之間有些還有先後順序要求，如果沒有一個良好的後臺任務管理框架，拿維護起來可就要了親命了。
這裏我們就來看一下mms應用中的“Action+IntentService架構”是怎麼解決這個問題的。
先簡單看一下Action：
public abstract class Action implements Parcelable {
..........................................
public void start() {
DataModel.startActionService(this);
}
//該Action要執行的具體後臺任務放在這裏，該方法需要我們繼承的
//Action子類實現
protected Object executeAction() {
return null;
}
..........................................
//需要在該Action之後執行的Action放到mBackgroundActions中，在該Action執行完
//之後會將這些mBackgroundActions取出來執行
private final List<Action> mBackgroundActions = new LinkedList<Action>();
protected void requestBackgroundWork(final Action backgroundAction) {
LogUtil.i(TAG, "requestBackgroundWork");
mBackgroundActions.add(backgroundAction);
}
protected Bundle doBackgroundWork() throws DataModelException {
return null;
}
..........................................
}我們再來以短信發送過程爲例來說明：
這個過程涉及到兩個Action，InsertNewMessageAction和SendMessageAction，分別對應的是將短信插入數據庫和真正發送的過程，都是耗時任務，所以都需要Action來處理。在這裏我們就看到SendMessageAction需要在InsertNewMessageAction之後執行，所以被放在了mBackgroundActions中了。
而實際上，我統計了一下，整個mms應用中有大概30個Action，包括DeleteConversationAction、DeleteMessageAction、
DownloadMmsAction、DumpDatabaseAction、HandleLowStorageAction、MarkAsReadAction、ReceiveSmsMessageAction、UpdateMessagePartSizeAction等。這些紛繁的後臺任務因爲有了Action架構，就顯得不那麼亂了。
再來看一下IntentService.
Action本身只是一個後臺任務邏輯載體，並沒有維護線程來執行任務，所以真正的任務執行是在IntentService裏面。有經驗的Android工程師應該都知道IntentService，因爲Service本身是運行在UI Thread中的，所以一般我們執行後臺任務都要再單獨開線程，而Android框架爲了方便，直接又封裝了一個帶Thread的Service，即IntentService。並且IntentService是即用即銷燬，所以我們連生命週期都不要維護，只需要實現它的onHandleIntent方法執行自己的後臺邏輯即可。
我們來看一下在mms源碼中對IntentService的使用：
public class ActionServiceImpl extends IntentService {
/**
*提供一個靜態全局方法，方便外界直接對自己實例化
*/
private static void startServiceWithIntent(final Intent intent) {
final Context context = Factory.get().getApplicationContext();
final int opcode = intent.getIntExtra(EXTRA_OP_CODE, 0);
// Increase refCount on wake lock - acquiring if necessary
if (VERBOSE) {
LogUtil.v(TAG, "acquiring wakelock for opcode " + opcode);
}
sWakeLock.acquire(context, intent, opcode);
intent.setClass(context, ActionServiceImpl.class);
// TODO: Note that intent will be quietly discarded if it exceeds available rpc
// memory (in total around 1MB). See this article for background
// http://developer.android.com/reference/android/os/TransactionTooLargeException.html
// Perhaps we should keep large structures in the action monitor?
if (context.startService(intent) == null) {
LogUtil.e(TAG,
"ActionService.startServiceWithIntent: failed to start service for intent "
+ intent);
sWakeLock.release(intent, opcode);
}
}
protected void onHandleIntent(final Intent intent) {
if (intent == null) {
// Shouldn't happen but sometimes does following another crash.
LogUtil.w(TAG, "ActionService.onHandleIntent: Called with null intent");
return;
}
final int opcode = intent.getIntExtra(EXTRA_OP_CODE, 0);
sWakeLock.ensure(intent, opcode);
try {
Action action;
final Bundle actionBundle = intent.getBundleExtra(EXTRA_ACTION_BUNDLE);
actionBundle.setClassLoader(getClassLoader());
switch(opcode) {
case OP_START_ACTION: {
action = (Action) actionBundle.getParcelable(BUNDLE_ACTION);
//真正執行Action
executeAction(action);
break;
}
case OP_RECEIVE_BACKGROUND_RESPONSE: {
action = (Action) actionBundle.getParcelable(BUNDLE_ACTION);
final Bundle response = intent.getBundleExtra(EXTRA_WORKER_RESPONSE);
processBackgroundResponse(action, response);
break;
}
case OP_RECEIVE_BACKGROUND_FAILURE: {
action = (Action) actionBundle.getParcelable(BUNDLE_ACTION);
processBackgroundFailure(action);
break;
}
default:
throw new RuntimeException("Unrecognized opcode in ActionServiceImpl");
}
//執行Action的BackgroundAction
action.sendBackgroundActions(mBackgroundWorker);
} finally {
// Decrease refCount on wake lock - releasing if necessary
sWakeLock.release(intent, opcode);
}
}
}
// Doing this work in the background so that we're not competing with sync
// which could bring the deleted message back to life between the time we deleted
// it locally and deleted it in telephony (sync is also done on doBackgroundWork).
//
// Previously this block of code deleted from telephony first but that can be very
// slow (on the order of seconds) so this was modified to first delete locally, trigger
// the UI update, then delete from telephony.
@Override
protected Bundle doBackgroundWork() {
｝
另外，ActionMonitor也值得一說
public class ActionMonitor {
protected int mState;
static void setState(final Action action, final int expectedOldState,
final int newState) {
int oldMonitorState = expectedOldState;
int newMonitorState = newState;
final ActionMonitor monitor
= ActionMonitor.lookupActionMonitor(action.actionKey);
if (monitor != null) {
oldMonitorState = monitor.mState;
monitor.updateState(action, expectedOldState, newState);
newMonitorState = monitor.mState;
}
.............................................
}
public interface ActionCompletedListener {
abstract void onActionSucceeded(ActionMonitor monitor,
final Action action, final Object data, final Object result);
abstract void onActionFailed(ActionMonitor monitor, final Action action,
final Object data, final Object result);
}
｝每一個Action對應一個ActionMonitor，用於維護Action所處於的狀態，這樣就可以在每個狀態與外界交互，比如在Action執行完之後回調onActionSucceeded方法。
當然，這裏都是抽絲剝繭的說了一下整個架構，而具體的細節其實還有很多，包括各種執行失敗的處理交互，各種任務的前後關聯，這裏面內容還是比較多的，但限於篇幅這裏就不展開了，感興趣的同學可以直接查看mms源碼。