基于Andoird 4.2.2的同步框架源代码学习——同步发起端

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：

[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：

[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

[java]
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) { <

补充：移动开发 , Android ,