Android笔记（十五）ContentProvider源码浅析_settings.secure.getintforuser

ContentProvider作为四大组件之一，由于业务上用到的地方不多,目前业务是系统界面，属于系统应用，最适合使用ContentProvider来进行少量数据存储，我们业务中涉及到的Settings.system和Settings.Secure等数据库，就是通过ContentProvider来封装、用ContentResolver来访问的

//通过ContentResolver来访问Settings.Secure
 final boolean userSetup = 0 != Settings.Secure.getIntForUser(
                    mContext.getContentResolver(),
                    Settings.Secure.USER_SETUP_COMPLETE,
                    0 /*default */,
                    mCurrentUserId);
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//通过ContentResolver来访问Settings.Global
Settings.Global.HEADS_UP_OFF != Settings.Global.getInt(
                    mContext.getContentResolver(), Settings.Global.HEADS_UP_NOTIFICATIONS_ENABLED,
                    Settings.Global.HEADS_UP_OFF);
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除了上面的用法，平时也很少自己自定义一个ContentProvider。现在来Read the fucking code，稍微深入了解下：
我们知道ContentProvider提供跨进程的数据传输，通过ContentResolver.registerContentObserver()的方式来监听数据变化，一旦ContentProvider中数据改变（也可能是被其他进程修改），就是触发ContentObserver中的回调方法

 @Override
        public void onChange(boolean selfChange) {
        //读取ContentProvider中的值操作
        }
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那么ContentProvider、ContentResolver、ContentObserver三者之间存在着什么样的联系？
在Context实例化之后，会初始化mContentResolver

 mContentResolver = new ApplicationContentResolver(this, mainThread, user);
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这个值就是我们getContentResolver()获得的ContentResolver对象。
ContentResolver有下面几个方法：

• delete()
• query()
• update()
• insert()
  以delete()为例

    public final int delete(@RequiresPermission.Write @NonNull Uri url, @Nullable String where,
            @Nullable String[] selectionArgs) {
        Preconditions.checkNotNull(url, "url");
        IContentProvider provider = acquireProvider(url);//这里获取ContentProvider代理
        if (provider == null) {
            throw new IllegalArgumentException("Unknown URL " + url);
        }
        try {
            long startTime = SystemClock.uptimeMillis();
            int rowsDeleted = provider.delete(mPackageName, url, where, selectionArgs);
            long durationMillis = SystemClock.uptimeMillis() - startTime;
            maybeLogUpdateToEventLog(durationMillis, url, "delete", where);
            return rowsDeleted;
        } catch (RemoteException e) {
            // Arbitrary and not worth documenting, as Activity
            // Manager will kill this process shortly anyway.
            return -1;
        } finally {
            releaseProvider(provider);
        }
    }
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看上面注释，发现调用了下面这个抽象方法

protected abstract IContentProvider acquireProvider(Context c, String name); 
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再去ApplicationContentProvider中查看，最终发现是在ActivityThread中installProvider方法来加载Provider

public final IContentProvider acquireProvider(
            Context c, String auth, int userId, boolean stable) {
        final IContentProvider provider = acquireExistingProvider(c, auth, userId, stable);
        if (provider != null) {
            return provider;
        }

        // There is a possible race here.  Another thread may try to acquire
        // the same provider at the same time.  When this happens, we want to ensure
        // that the first one wins.
        // Note that we cannot hold the lock while acquiring and installing the
        // provider since it might take a long time to run and it could also potentially
        // be re-entrant in the case where the provider is in the same process.
        IActivityManager.ContentProviderHolder holder = null;
        try {
            holder = ActivityManagerNative.getDefault().getContentProvider(
                    getApplicationThread(), auth, userId, stable);
        } catch (RemoteException ex) {
            throw ex.rethrowFromSystemServer();
        }
        if (holder == null) {
            Slog.e(TAG, "Failed to find provider info for " + auth);
            return null;
        }

        // Install provider will increment the reference count for us, and break
        // any ties in the race.
        holder = installProvider(c, holder, holder.info,
                true /*noisy*/, holder.noReleaseNeeded, stable);
        return holder.provider;
    }
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