Android ContentObserver 学习

当我们查询数据中某个值是否变化时，通常很耗时费力，数据量大时可能还需要开辟线程来做这种事，急需一种及时反馈机制来处理这种问题，ContentObserver的提供的很

好的解决方案。

        ContentObserver为内容观察着，目的就是捕捉内容的变化，当所观察的Uri发生变化时，就会被触发，当然有内容观察者就离不开内容提供者。

内容观察者实现

下面以Android源码中的ProvisionedObserver展开分析：

class ProvisionedObserver extends ContentObserver {
        public ProvisionedObserver(Handler handler) {
            super(handler);
        }

        public void onChange(boolean selfChange) {
            final boolean wasProvisioned = mProvisioned;
            final boolean isProvisioned = deviceIsProvisioned();
            // latch: never unprovision
            mProvisioned = wasProvisioned || isProvisioned;
            if (MORE_DEBUG) {
                Slog.d(TAG, "Provisioning change: was=" + wasProvisioned
                        + " is=" + isProvisioned + " now=" + mProvisioned);
            }

            synchronized (mQueueLock) {
                if (mProvisioned && !wasProvisioned && mEnabled) {
                    // we're now good to go, so start the backup alarms
                    if (MORE_DEBUG) Slog.d(TAG, "Now provisioned, so starting backups");
                    startBackupAlarmsLocked(FIRST_BACKUP_INTERVAL);
                }
            }
        }
    }

ProvisionedObserver作为ContentObserver的子类主要实现了一个onChange，构造时传入了一个hander，方法其实就是观察的内容发生变化时会被触发调用，但是handler的

作用被没有体现出来，我们再看ContentObserver类的方法dispatchChange

/**
     * Dispatches a change notification to the observer. Includes the changed
     * content Uri when available and also the user whose content changed.
     * <p>
     * If a {@link Handler} was supplied to the {@link ContentObserver} constructor,
     * then a call to the {@link #onChange} method is posted to the handler's message queue.
     * Otherwise, the {@link #onChange} method is invoked immediately on this thread.
     * </p>
     *
     * @param selfChange True if this is a self-change notification.
     * @param uri The Uri of the changed content, or null if unknown.
     * @param userId The user whose content changed.
     */
    private void dispatchChange(boolean selfChange, Uri uri, int userId) {
        if (mHandler == null) {
            onChange(selfChange, uri, userId);
        } else {
            mHandler.post(new NotificationRunnable(selfChange, uri, userId));
        }
    }?

如果传入的handler是null，则会调用父类自身的onChange，反之就会通过NotificationRunnable调用到ProvisionedObserver的onChange方法，而dispatchChange是在内部

类Transport 的onChange方法中完成

private static final class Transport extends IContentObserver.Stub {
        private ContentObserver mContentObserver;

        public Transport(ContentObserver contentObserver) {
            mContentObserver = contentObserver;
        }

        @Override
        public void onChange(boolean selfChange, Uri uri, int userId) {
            ContentObserver contentObserver = mContentObserver;
            if (contentObserver != null) {
                contentObserver.dispatchChange(selfChange, uri, userId);
            }
        }

        public void releaseContentObserver() {
            mContentObserver = null;
        }
    }?

似乎分析到这里，发现接截止在Transport.onChange

注册流程分析

下面看看ProvisionedObserver的注册

final ContentResolver resolver = context.getContentResolver();
mProvisionedObserver = new ProvisionedObserver(mBackupHandler);
resolver.registerContentObserver(
		Settings.Global.getUriFor(Settings.Global.DEVICE_PROVISIONED),
		false, mProvisionedObserver);

接着进入文件ContentResolver.java的registerContentObserver

    /**
     * Register an observer class that gets callbacks when data identified by a
     * given content URI changes.
     *
     * @param uri The URI to watch for changes. This can be a specific row URI, or a base URI
     * for a whole class of content.
     * @param notifyForDescendents If <code>true</code> changes to URIs beginning with <code>uri</code>
     * will also cause notifications to be sent. If <code>false</code> only changes to the exact URI
     * specified by <em>uri</em> will cause notifications to be sent. If <code>true</code>, any URI values
     * at or below the specified URI will also trigger a match.
     * @param observer The object that receives callbacks when changes occur.
     * @see #unregisterContentObserver
     */
    public final void registerContentObserver(Uri uri, boolean notifyForDescendents,
            ContentObserver observer)
    {
        registerContentObserver(uri, notifyForDescendents, observer, UserHandle.myUserId());
    }

    /** @hide - designated user version */
    public final void registerContentObserver(Uri uri, boolean notifyForDescendents,
            ContentObserver observer, int userHandle)
    {
        try {
            getContentService().registerContentObserver(uri, notifyForDescendents,
                    observer.getContentObserver(), userHandle);
        } catch (RemoteException e) {
        }
    }

getContentService().registerContentObserver(uri, notifyForDescendents,   observer.getContentObserver(), userHandle); 会进入ContentService.registerContentObserve中

public void registerContentObserver(Uri uri, boolean notifyForDescendants,
            IContentObserver observer, int userHandle) {
        if (observer == null || uri == null) {
            throw new IllegalArgumentException("You must pass a valid uri and observer");
        }

        enforceCrossUserPermission(userHandle,
                "no permission to observe other users' provider view");

        if (userHandle < 0) {
            if (userHandle == UserHandle.USER_CURRENT) {
                userHandle = ActivityManager.getCurrentUser();
            } else if (userHandle != UserHandle.USER_ALL) {
                throw new InvalidParameterException("Bad user handle for registerContentObserver: "
                        + userHandle);
            }
        }

        synchronized (mRootNode) {
            mRootNode.addObserverLocked(uri, observer, notifyForDescendants, mRootNode,
                    Binder.getCallingUid(), Binder.getCallingPid(), userHandle);
            if (false) Log.v(TAG, "Registered observer " + observer + " at " + uri +
                    " with notifyForDescendants " + notifyForDescendants);
        }
    }

其中observer就是ProvisionedObserver.getContentObserver,如下

    /**
     * Gets access to the binder transport object. Not for public consumption.
     *
     * {@hide}
     */
    public IContentObserver getContentObserver() {
        synchronized (mLock) {
            if (mTransport == null) {
                mTransport = new Transport(this);
            }
            return mTransport;
        }
    }

ContentService.registerContentObserver传入的observer也即是一个Transport，然后添加到mRootNode中

public void addObserverLocked(Uri uri, IContentObserver observer,
                boolean notifyForDescendants, Object observersLock,
                int uid, int pid, int userHandle) {
            addObserverLocked(uri, 0, observer, notifyForDescendants, observersLock,
                    uid, pid, userHandle);
        }

        private void addObserverLocked(Uri uri, int index, IContentObserver observer,
                boolean notifyForDescendants, Object observersLock,
                int uid, int pid, int userHandle) {
            // If this is the leaf node add the observer
            if (index == countUriSegments(uri)) {
                mObservers.add(new ObserverEntry(observer, notifyForDescendants, observersLock,
                        uid, pid, userHandle));
                return;
            }

            // Look to see if the proper child already exists
            String segment = getUriSegment(uri, index);
            if (segment == null) {
                throw new IllegalArgumentException("Invalid Uri (" + uri + ") used for observer");
            }
            int N = mChildren.size();
            for (int i = 0; i < N; i++) {
                ObserverNode node = mChildren.get(i);
                if (node.mName.equals(segment)) {
                    node.addObserverLocked(uri, index + 1, observer, notifyForDescendants,
                            observersLock, uid, pid, userHandle);
                    return;
                }
            }

            // No child found, create one
            ObserverNode node = new ObserverNode(segment);
            mChildren.add(node);
            node.addObserverLocked(uri, index + 1, observer, notifyForDescendants,
                    observersLock, uid, pid, userHandle);
        }

在这个过程中，新增的内容观察者全部添加ContentService.mObservers链表中，等待触发。

触发流程

上面已经分析了，内容观察者的继承，注册，触发则有内容提供的源头来发起，也有ContentProvider来触发。通常数据变化发生在insert，delete，update三个操作中。

ContentProvider在完成insert，delete，update之后就会调用getContext().getContentResolver().notifyChange(url, null)来通知内容发生变化，至此整个注册，触发形成闭环。

ContentService.notifyChange触发函数如下：

/**
     * Notify observers of a particular user's view of the provider.
     * @param userHandle the user whose view of the provider is to be notified.  May be
     *     the calling user without requiring any permission, otherwise the caller needs to
     *     hold the INTERACT_ACROSS_USERS_FULL permission.  Pseudousers USER_ALL and
     *     USER_CURRENT are properly interpreted; no other pseudousers are allowed.
     */
    @Override
    public void notifyChange(Uri uri, IContentObserver observer,
            boolean observerWantsSelfNotifications, boolean syncToNetwork,
            int userHandle) {
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            Log.v(TAG, "Notifying update of " + uri + " for user " + userHandle
                    + " from observer " + observer + ", syncToNetwork " + syncToNetwork);
        }

        // Notify for any user other than the caller's own requires permission.
        final int callingUserHandle = UserHandle.getCallingUserId();
        if (userHandle != callingUserHandle) {
            mContext.enforceCallingOrSelfPermission(Manifest.permission.INTERACT_ACROSS_USERS,
                    "no permission to notify other users");
        }

        // We passed the permission check; resolve pseudouser targets as appropriate
        if (userHandle < 0) {
            if (userHandle == UserHandle.USER_CURRENT) {
                userHandle = ActivityManager.getCurrentUser();
            } else if (userHandle != UserHandle.USER_ALL) {
                throw new InvalidParameterException("Bad user handle for notifyChange: "
                        + userHandle);
            }
        }

        final int uid = Binder.getCallingUid();
        // This makes it so that future permission checks will be in the context of this
        // process rather than the caller's process. We will restore this before returning.
        long identityToken = clearCallingIdentity();
        try {
            ArrayList<ObserverCall> calls = new ArrayList<ObserverCall>();
            synchronized (mRootNode) {
                mRootNode.collectObserversLocked(uri, 0, observer, observerWantsSelfNotifications,
                        userHandle, calls);
            }
            final int numCalls = calls.size();
            for (int i=0; i<numCalls; i++) {
                ObserverCall oc = calls.get(i);
                try {
                    oc.mObserver.onChange(oc.mSelfChange, uri, userHandle);
                    if (Log.isLoggable(TAG, Log.VERBOSE)) {
                        Log.v(TAG, "Notified " + oc.mObserver + " of " + "update at " + uri);
                    }
                } catch (RemoteException ex) {
                    synchronized (mRootNode) {
                        Log.w(TAG, "Found dead observer, removing");
                        IBinder binder = oc.mObserver.asBinder();
                        final ArrayList<ObserverNode.ObserverEntry> list
                                = oc.mNode.mObservers;
                        int numList = list.size();
                        for (int j=0; j<numList; j++) {
                            ObserverNode.ObserverEntry oe = list.get(j);
                            if (oe.observer.asBinder() == binder) {
                                list.remove(j);
                                j--;
                                numList--;
                            }
                        }
                    }
                }
            }
            if (syncToNetwork) {
                SyncManager syncManager = getSyncManager();
                if (syncManager != null) {
                    syncManager.scheduleLocalSync(null /* all accounts */, callingUserHandle, uid,
                            uri.getAuthority());
                }
            }
        } finally {
            restoreCallingIdentity(identityToken);
        }
    }

从观察者列表中获取内容观察者(Transport), 然后调用onChange.

相关类图

注册流程图