关键组件：
- ContentResolver
- ContentService
- SyncManager
- SyncManager.ActiveSyncContext
- SyncManager.SyncOperation
- SyncManager.SyncHandler
ContentResolver
外部的应用程序通过调用ContentResolve.requestSync()静态方法发起同步：
/**
* @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) {
}
}
方法接收三个参数：
- account：需要同步的帐号
- authority：需要进行同步的authority
- extras：需要传递给sync adapter的附加数据
在这里，getContentService()方法返回系统服务ContentService的代理对象，然后通过它远程调用ContentService.requestSync()。
ContentService
ContentService是Android的系统服务，它提供一系列数据同步及数据访问等相关的操作。它的行为在IContentService.aidl中描述。
这里，通过远程调用ContentService.requestSync()方法来启动针对指定帐号（account）的指定内容（authority）的同步：
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 */);
}
}
...
}
在这个方法中，会获取一个SyncManager类的实例。顾名思义，SyncManager管理与同步相关的处理。
SyncManager
public void scheduleSync(Account requestedAccount, int userId, String requestedAuthority,
Bundle extras, long delay, boolean onlyThoseWithUnkownSyncableState) {
...
final boolean backgroundDataUsageAllowed = !mBootCompleted ||
getConnectivityManager().getBackgroundDataSetting();
... // 产生一个同步帐户列表。对于手动同步，列表中仅有一个AccountUser元素，它封装了需要同步的帐号以及对应的应用程序（userId）
AccountAndUser[] accounts;
if (requestedAccount != null && userId != UserHandle.USER_ALL) {
accounts = new AccountAndUser[] { new AccountAndUser(requestedAccount, userId) };
}
...
for (AccountAndUser account : accounts) {
// 在这里，会扫描系统中所有提供了sync adapter的service：根据intent filter
// 然后从得到service info中取得各自的authority。service info从对应服务的meta-data标签中指定的sync adapter描述文件中解析出来。
final HashSet<String> syncableAuthorities = new HashSet<String>();
for (RegisteredServicesCache.ServiceInfo<SyncAdapterType> syncAdapter :
mSyncAdapters.getAllServices(account.userId)) {
syncableAuthorities.add(syncAdapter.type.authority);
} ... for (String authority : syncableAuthorities) {
// 检查帐户是否能够同步
int isSyncable = mSyncStorageEngine.getIsSyncable(account.account, account.userId,
authority);
if (isSyncable == 0) {
continue;
}
final RegisteredServicesCache.ServiceInfo<SyncAdapterType> 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));
}
...
}
}
}
这里，首先从系统中筛选出符合限定条件的service的信息，然后发起对应的同步。
首先为每一个同步操作生成一个SyncOperation实例，它封装了同步操作需要的全部信息：
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;
然后调用scheduleSyncOperation方法：
public void scheduleSyncOperation(SyncOperation syncOperation) {
boolean queueChanged;
synchronized (mSyncQueue) {
queueChanged = mSyncQueue.add(syncOperation);
}
if (queueChanged) {
...
sendCheckAlarmsMessage();
}
...
}
首先将SyncOperation实例插入队列mSyncQueue然后向SyncManager中定义的SyncHandler发送消息，通知其队列发生变化：
private void sendCheckAlarmsMessage() {
...
mSyncHandler.removeMessages(SyncHandler.MESSAGE_CHECK_ALARMS);
mSyncHandler.sendEmptyMessage(SyncHandler.MESSAGE_CHECK_ALARMS);
}
随后，SyncHandler处理这个消息：
public void handleMessage(Message msg) {
...
try {
...
switch (msg.what) {
...
case SyncHandler.MESSAGE_CHECK_ALARMS:
...
nextPendingSyncTime = maybeStartNextSyncLocked();
break;
}
}
...
}
这里，maybeStartNextSyncLocked()方法经过一系列的检查，确认执行同步的全部条件已经达到之后，对SyncOperation进行分发：
private long maybeStartNextSyncLocked() {
...
dispatchSyncOperation(candidate);
}
return nextReadyToRunTime;
}
接下来，将绑定到提供sync adapter的应用程序中对应的service：
private boolean dispatchSyncOperation(SyncOperation op) {
...
// connect to the sync adapter
SyncAdapterType syncAdapterType = SyncAdapterType.newKey(op.authority, op.account.type);
final RegisteredServicesCache.ServiceInfo<SyncAdapterType> 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;
}
与前面的AccountManager非常的雷同，这里通过ActiveSyncContext类来完成service的绑定：
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()中创建相应的Intent，发起绑定。
然后，因为本类实现了ServiceConnection接口，所以当绑定成功时，将回调本类的onServiceConnected()方法。在这个回调中，向SyncHandler发送一条MESSAGE_SERVICE_CONNECTED消息。
紧接着，轮到SyncHandler来处理消息：
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;
}
这里主要就是调用了runBoundToSyncAdapter()方法：
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);
}
...
}
这里，对传入syncAdapter实例（实际上是AbstractThreadedSyncAdpter.ISyncAdapterImpl服务的代理对象）调用startSync()方法。这样，通过IPC即可调用对应的应用程序执行同步了。详见本系列上一篇文章。