關鍵組件：

ContentResolver

ContentService

SyncManager

SyncManager.ActiveSyncContext

SyncManager.SyncOperation

SyncManager.SyncHandler

ContentResolver

外部的應用程序通過調用ContentResolve.requestSync()靜態方法發起同步：

[java]

/**

* @param account which account should be synced

* @param authority which authority should be synced

* @param extras any extras to pass to the SyncAdapter.

*/

public static void requestSync(Account account, String authority, Bundle extras) {

validateSyncExtrasBundle(extras);

try {

getContentService().requestSync(account, authority, extras);

} catch (RemoteException e) {

}

}

/**

* @param account which account should be synced

* @param authority which authority should be synced

* @param extras any extras to pass to the SyncAdapter.

*/

public static void requestSync(Account account, String authority, Bundle extras) {

validateSyncExtrasBundle(extras);

try {

getContentService().requestSync(account, authority, extras);

} catch (RemoteException e) {

}

}

方法接收三個參數：

- account：需要同步的帳號

- authority：需要進行同步的authority

- extras：需要傳遞給sync adapter的附加數據

在這里，getContentService()方法返回系統服務ContentService的代理對象，然后通過它遠程調用ContentService.requestSync()。

ContentService

ContentService是Android的系統服務，它提供一系列數據同步及數據訪問等相關的操作。它的行為在IContentService.aidl中描述。

這里，通過遠程調用ContentService.requestSync()方法來啟動針對指定帳號（account）的指定內容（authority）的同步：

[java]

public void requestSync(Account account, String authority, Bundle extras) {

...

try {

SyncManager syncManager = getSyncManager();

if (syncManager != null) {

syncManager.scheduleSync(account, userId, authority, extras, 0 /* no delay */,

false /* onlyThoseWithUnkownSyncableState */);

}

}

...

}

public void requestSync(Account account, String authority, Bundle extras) {

...

try {

SyncManager syncManager = getSyncManager();

if (syncManager != null) {

syncManager.scheduleSync(account, userId, authority, extras, 0 /* no delay */,

false /* onlyThoseWithUnkownSyncableState */);

}

}

...

}

在這個方法中，會獲取一個SyncManager類的實例。顧名思義，SyncManager管理與同步相關的處理。

SyncManager

[java]

public void scheduleSync(Account requestedAccount, int userId, String requestedAuthority,

Bundle extras, long delay, boolean onlyThoseWithUnkownSyncableState) {

...

final boolean backgroundDataUsageAllowed = !mBootCompleted ||

getConnectivityManager().getBackgroundDataSetting();

...

// 產生一個同步帳戶列表。對于手動同步，列表中僅有一個AccountUser元素，它封裝了需要同步的帳號以及對應的應用程序（userId）

AccountAndUser[] accounts;

if (requestedAccount != null && userId != UserHandle.USER_ALL) {

accounts = new AccountAndUser[] { new AccountAndUser(requestedAccount, userId) };

}

...

for (AccountAndUser account : accounts) {

// 在這里，會掃描系統中所有提供了sync adapter的service：根據intent filter

// 然后從得到service info中取得各自的authority。service info從對應服務的meta-data標簽中指定的sync adapter描述文件中解析出來。

final HashSet syncableAuthorities = new HashSet();

for (RegisteredServicesCache.ServiceInfo syncAdapter :

mSyncAdapters.getAllServices(account.userId)) {

syncableAuthorities.add(syncAdapter.type.authority);

}

...

for (String authority : syncableAuthorities) {

// 檢查帳戶是否能夠同步

int isSyncable = mSyncStorageEngine.getIsSyncable(account.account, account.userId,

authority);

if (isSyncable == 0) {

continue;

}

final RegisteredServicesCache.ServiceInfo syncAdapterInfo;

syncAdapterInfo = mSyncAdapters.getServiceInfo(

SyncAdapterType.newKey(authority, account.account.type), account.userId);

...

if (isSyncable < 0) {

Bundle newExtras = new Bundle();

newExtras.putBoolean(ContentResolver.SYNC_EXTRAS_INITIALIZE, true);

...

// 部署同步操作

scheduleSyncOperation(

new SyncOperation(account.account, account.userId, source, authority,

newExtras, 0, backoffTime, delayUntil, allowParallelSyncs));

}

...

}

}

}

public void scheduleSync(Account requestedAccount, int userId, String requestedAuthority,

Bundle extras, long delay, boolean onlyThoseWithUnkownSyncableState) {

...

final boolean backgroundDataUsageAllowed = !mBootCompleted ||

getConnectivityManager().getBackgroundDataSetting();

...

// 產生一個同步帳戶列表。對于手動同步，列表中僅有一個AccountUser元素，它封裝了需要同步的帳號以及對應的應用程序（userId）

AccountAndUser[] accounts;

if (requestedAccount != null && userId != UserHandle.USER_ALL) {

accounts = new AccountAndUser[] { new AccountAndUser(requestedAccount, userId) };

}

...

for (AccountAndUser account : accounts) {

// 在這里，會掃描系統中所有提供了sync adapter的service：根據intent filter

// 然后從得到service info中取得各自的authority。service info從對應服務的meta-data標簽中指定的sync adapter描述文件中解析出來。

final HashSet syncableAuthorities = new HashSet();

for (RegisteredServicesCache.ServiceInfo syncAdapter :

mSyncAdapters.getAllServices(account.userId)) {

syncableAuthorities.add(syncAdapter.type.authority);

}

...

for (String authority : syncableAuthorities) {

// 檢查帳戶是否能夠同步

int isSyncable = mSyncStorageEngine.getIsSyncable(account.account, account.userId,

authority);

if (isSyncable == 0) {

continue;

}

final RegisteredServicesCache.ServiceInfo syncAdapterInfo;

syncAdapterInfo = mSyncAdapters.getServiceInfo(

SyncAdapterType.newKey(authority, account.account.type), account.userId);

...

if (isSyncable < 0) {

Bundle newExtras = new Bundle();

newExtras.putBoolean(ContentResolver.SYNC_EXTRAS_INITIALIZE, true);

...

// 部署同步操作

scheduleSyncOperation(

new SyncOperation(account.account, account.userId, source, authority,

newExtras, 0, backoffTime, delayUntil, allowParallelSyncs));

}

...

}

}

}

這里，首先從系統中篩選出符合限定條件的service的信息，然后發起對應的同步。

首先為每一個同步操作生成一個SyncOperation實例，它封裝了同步操作需要的全部信息：

[java]

public class SyncOperation implements Comparable {

public final Account account;

public final int userId;

public int syncSource;

public String authority;

public final boolean allowParallelSyncs;

public Bundle extras;

public final String key;

public long earliestRunTime;

public boolean expedited;

public SyncStorageEngine.PendingOperation pendingOperation;

public Long backoff;

public long delayUntil;

public long effectiveRunTime;

public class SyncOperation implements Comparable {

public final Account account;

public final int userId;

public int syncSource;

public String authority;

public final boolean allowParallelSyncs;

public Bundle extras;

public final String key;

public long earliestRunTime;

public boolean expedited;

public SyncStorageEngine.PendingOperation pendingOperation;

public Long backoff;

public long delayUntil;

public long effectiveRunTime;

然后調用scheduleSyncOperation方法：

[java]

public void scheduleSyncOperation(SyncOperation syncOperation) {

boolean queueChanged;

synchronized (mSyncQueue) {

queueChanged = mSyncQueue.add(syncOperation);

}

if (queueChanged) {

...

sendCheckAlarmsMessage();

}

...

}

public void scheduleSyncOperation(SyncOperation syncOperation) {

boolean queueChanged;

synchronized (mSyncQueue) {

queueChanged = mSyncQueue.add(syncOperation);

}

if (queueChanged) {

...

sendCheckAlarmsMessage();

}

...

}

首先將SyncOperation實例插入隊列mSyncQueue然后向SyncManager中定義的SyncHandler發送消息，通知其隊列發生變化：

[java]

private void sendCheckAlarmsMessage() {

...

mSyncHandler.removeMessages(SyncHandler.MESSAGE_CHECK_ALARMS);

mSyncHandler.sendEmptyMessage(SyncHandler.MESSAGE_CHECK_ALARMS);

}

private void sendCheckAlarmsMessage() {

...

mSyncHandler.removeMessages(SyncHandler.MESSAGE_CHECK_ALARMS);

mSyncHandler.sendEmptyMessage(SyncHandler.MESSAGE_CHECK_ALARMS);

}

隨后，SyncHandler處理這個消息：

[java] v

public void handleMessage(Message msg) {

...

try {

...

switch (msg.what) {

...

case SyncHandler.MESSAGE_CHECK_ALARMS:

...

nextPendingSyncTime = maybeStartNextSyncLocked();

break;

}

}

...

}

public void handleMessage(Message msg) {

...

try {

...

switch (msg.what) {

...

case SyncHandler.MESSAGE_CHECK_ALARMS:

...

nextPendingSyncTime = maybeStartNextSyncLocked();

break;

}

}

...

}

這里，maybeStartNextSyncLocked()方法經過一系列的檢查，確認執行同步的全部條件已經達到之后，對SyncOperation進行分發：

[java]

private long maybeStartNextSyncLocked() {

...

dispatchSyncOperation(candidate);

}

return nextReadyToRunTime;

}

private long maybeStartNextSyncLocked() {

...

dispatchSyncOperation(candidate);

}

return nextReadyToRunTime;

}

接下來，將綁定到提供sync adapter的應用程序中對應的service：

[java]

private boolean dispatchSyncOperation(SyncOperation op) {

...

// connect to the sync adapter

SyncAdapterType syncAdapterType = SyncAdapterType.newKey(op.authority, op.account.type);

final RegisteredServicesCache.ServiceInfo syncAdapterInfo;

syncAdapterInfo = mSyncAdapters.getServiceInfo(syncAdapterType, op.userId);

...

ActiveSyncContext activeSyncContext =

new ActiveSyncContext(op, insertStartSyncEvent(op), syncAdapterInfo.uid);

activeSyncContext.mSyncInfo = mSyncStorageEngine.addActiveSync(activeSyncContext);

mActiveSyncContexts.add(activeSyncContext);

...

if (!activeSyncContext.bindToSyncAdapter(syncAdapterInfo, op.userId)) {

Log.e(TAG, "Bind attempt failed to " + syncAdapterInfo);

closeActiveSyncContext(activeSyncContext);

return false;

}

return true;

}

private boolean dispatchSyncOperation(SyncOperation op) {

...

// connect to the sync adapter

SyncAdapterType syncAdapterType = SyncAdapterType.newKey(op.authority, op.account.type);

final RegisteredServicesCache.ServiceInfo syncAdapterInfo;

syncAdapterInfo = mSyncAdapters.getServiceInfo(syncAdapterType, op.userId);

...

ActiveSyncContext activeSyncContext =

new ActiveSyncContext(op, insertStartSyncEvent(op), syncAdapterInfo.uid);

activeSyncContext.mSyncInfo = mSyncStorageEngine.addActiveSync(activeSyncContext);

mActiveSyncContexts.add(activeSyncContext);

...

if (!activeSyncContext.bindToSyncAdapter(syncAdapterInfo, op.userId)) {

Log.e(TAG, "Bind attempt failed to " + syncAdapterInfo);

closeActiveSyncContext(activeSyncContext);

return false;

}

return true;

}

與前面的AccountManager非常的雷同，這里通過ActiveSyncContext類來完成service的綁定：

[java]

class ActiveSyncContext extends ISyncContext.Stub

implements ServiceConnection, IBinder.DeathRecipient {

...

public void onServiceConnected(ComponentName name, IBinder service) {

Message msg = mSyncHandler.obtainMessage();

msg.what = SyncHandler.MESSAGE_SERVICE_CONNECTED;

msg.obj = new ServiceConnectionData(this, ISyncAdapter.Stub.asInterface(service));

mSyncHandler.sendMessage(msg);

}

...

boolean bindToSyncAdapter(RegisteredServicesCache.ServiceInfo info, int userId) {

if (Log.isLoggable(TAG, Log.VERBOSE)) {

Log.d(TAG, "bindToSyncAdapter: " + info.componentName + ", connection " + this);

}

Intent intent = new Intent();

intent.setAction("android.content.SyncAdapter");

intent.setComponent(info.componentName);

intent.putExtra(Intent.EXTRA_CLIENT_LABEL,

com.android.internal.R.string.sync_binding_label);

intent.putExtra(Intent.EXTRA_CLIENT_INTENT, PendingIntent.getActivityAsUser(

mContext, 0, new Intent(Settings.ACTION_SYNC_SETTINGS), 0,

null, new UserHandle(userId)));

mBound = true;

final boolean bindResult = mContext.bindService(intent, this,

Context.BIND_AUTO_CREATE | Context.BIND_NOT_FOREGROUND

| Context.BIND_ALLOW_OOM_MANAGEMENT,

mSyncOperation.userId);

if (!bindResult) {

mBound = false;

}

return bindResult;

}

...

}

class ActiveSyncContext extends ISyncContext.Stub

implements ServiceConnection, IBinder.DeathRecipient {

...

public void onServiceConnected(ComponentName name, IBinder service) {

Message msg = mSyncHandler.obtainMessage();

msg.what = SyncHandler.MESSAGE_SERVICE_CONNECTED;

msg.obj = new ServiceConnectionData(this, ISyncAdapter.Stub.asInterface(service));

mSyncHandler.sendMessage(msg);

}

...

boolean bindToSyncAdapter(RegisteredServicesCache.ServiceInfo info, int userId) {

if (Log.isLoggable(TAG, Log.VERBOSE)) {

Log.d(TAG, "bindToSyncAdapter: " + info.componentName + ", connection " + this);

}

Intent intent = new Intent();

intent.setAction("android.content.SyncAdapter");

intent.setComponent(info.componentName);

intent.putExtra(Intent.EXTRA_CLIENT_LABEL,

com.android.internal.R.string.sync_binding_label);

intent.putExtra(Intent.EXTRA_CLIENT_INTENT, PendingIntent.getActivityAsUser(

mContext, 0, new Intent(Settings.ACTION_SYNC_SETTINGS), 0,

null, new UserHandle(userId)));

mBound = true;

final boolean bindResult = mContext.bindService(intent, this,

Context.BIND_AUTO_CREATE | Context.BIND_NOT_FOREGROUND

| Context.BIND_ALLOW_OOM_MANAGEMENT,

mSyncOperation.userId);

if (!bindResult) {

mBound = false;

}

return bindResult;

}

...

}其中，bindToSyncAdapter()中創建相應的Intent，發起綁定。

然后，因為本類實現了ServiceConnection接口，所以當綁定成功時，將回調本類的onServiceConnected()方法。在這個回調中，向SyncHandler發送一條MESSAGE_SERVICE_CONNECTED消息。

緊接著，輪到SyncHandler來處理消息：

case SyncHandler.MESSAGE_SERVICE_CONNECTED: {

ServiceConnectionData msgData = (ServiceConnectionData)msg.obj;

if (Log.isLoggable(TAG, Log.VERBOSE)) {

Log.d(TAG, "handleSyncHandlerMessage: MESSAGE_SERVICE_CONNECTED: "

+ msgData.activeSyncContext);

}

// check that this isn't an old message

if (isSyncStillActive(msgData.activeSyncContext)) {

runBoundToSyncAdapter(msgData.activeSyncContext, msgData.syncAdapter);

}

break;

}

case SyncHandler.MESSAGE_SERVICE_CONNECTED: {

ServiceConnectionData msgData = (ServiceConnectionData)msg.obj;

if (Log.isLoggable(TAG, Log.VERBOSE)) {

Log.d(TAG, "handleSyncHandlerMessage: MESSAGE_SERVICE_CONNECTED: "

+ msgData.activeSyncContext);

}

// check that this isn't an old message

if (isSyncStillActive(msgData.activeSyncContext)) {

runBoundToSyncAdapter(msgData.activeSyncContext, msgData.syncAdapter);

}

break;

}

這里主要就是調用了runBoundToSyncAdapter()方法：

[java]

private void runBoundToSyncAdapter(final ActiveSyncContext activeSyncContext,

ISyncAdapter syncAdapter) {

activeSyncContext.mSyncAdapter = syncAdapter;

final SyncOperation syncOperation = activeSyncContext.mSyncOperation;

try {

...

syncAdapter.startSync(activeSyncContext, syncOperation.authority,

syncOperation.account, syncOperation.extras);

}

...

}