Android数据库内容变化的监听 _数据库

古人已用三冬足，年少今开万卷余。这篇文章主要讲述Android数据库内容变化的监听相关的知识，希望能为你提供帮助。
首先介绍内容监测的基本模式
基于uri的内容监测的基本模式被android.content.ContentResolver实现。
它为基于Uri的内容监测的提供了一个平台。（其实如果有必要，我们可以自己实现一个）
ContentResolver为此提供了三个方法：
注册监听器到某个uri
public final void registerContentObserver (Uri uri, boolean notifyForDescendents, ContentObserver observer)
Register an observer class that gets callbacks when data identified by a given content URI changes.
Parameters
uri The URI to watch for changes. This can be a specific row URI, or a base URI for a whole class of content.
notifyForDescendents If true changes to URIs beginning with uri will also cause notifications to be sent.
If false only changes to the exact URI specified by uri will cause notifications to be sent.
If true, than any URI values at or below the specified URI will also trigger a match.
observer The object that receives callbacks when changes occur.
取消被注册的监听器
public final void unregisterContentObserver (ContentObserver observer)
Unregisters a change observer.
参数
observer The previously registered observer that is no longer needed.
通知所有注册到uri的监听器，告诉他们内容发生了改变。
public void notifyChange (Uri uri, ContentObserver observer)
Notify registered observers that a row was updated. To register, call registerContentObserver(). By default, CursorAdapter objects will get this notification.
参数
observer The observer that originated the change, may be null .用以说明observer是第一个发现内容改变的ContentObserver,可为null.
通知所有注册到uri的监听器，告诉他们内容发生了改变。
public void notifyChange (Uri uri, ContentObserver observer, boolean syncToNetwork)
Notify registered observers that a row was updated. To register, call registerContentObserver(). By default, CursorAdapter objects will get this notification.
参数
observer The observer that originated the change, may be null 用以说明observer是第一个发现内容改变的ContentObserver,可为null
syncToNetwork If true, attempt to sync the change to the network.
注1:"基于uri的内容监测的基本模式被android.content.ContentResolver实现",严格来说ContentResolver至少提供了接口。
真正的实现在android.content.ContentService.
其实ContentResolver为基于Uri的内容监测所提供的方法只是调用ContentService相关的方法
注2:"注册监听器到uri"只是说如果notifyChange的uri和registerContentObserver中的uri相匹配，则调用observer的方法onChange。
内容监听器ContentObserver主要有三个方法
public boolean deliverSelfNotifications ()
Returns true if this observer is interested in notifications for changes made through the cursor the observer is registered with.
注：这个函数的使用还是puzzle.
public final void dispatchChange (boolean selfChange)
注：这个是为提供用handler执行onChange的一个接口。所以一般比没必要重载它。
public void onChange (boolean selfChange)
This method is called when a change occurs to the cursor that is being observed.
参数
selfChange true if the update was caused by a call to commit on the cursor that is being observed.
注1:这个就是我们需要重载的函数，在里面可以实现对内容改变的个性化响应。
关于ContentObserver的更多内容请参阅《内容监听器ContentObserver》。
基本使用是这样的：
首先使用registerContentObserver注册observer监听器到uri，然后在内容发生改变时，就调用notifyChange通知系统所有注册到该uri的监听器，告诉他们内容发生了改变。
这时相应的监听器的dispatchChange方法被调用，在dispatchChange中onChange被调用。
最后如果不想让observer监听器uri相关的内容监听，可以调用unregisterContentObserver来取消注册。

对数据库改变的监测是如何实现的呢？
在providers对数据进行改变后，会通过getContext().getContentResolver().notifyChange的发生相应的uri的内容发生了改变
比如：com.android.providers.contacts中的ContactsProvider2
ContactsProvider2.java文件
public class ContactsProvider2 extends SQLiteContentProvider implements OnAccountsUpdateListener {
-------------------省略------------------
@Override
public Uri insert(Uri uri, ContentValues values) {
waitForAccess();
return super.insert(uri, values);
}
@Override
public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) {
-------------------省略------------------
waitForAccess();
return super.update(uri, values, selection, selectionArgs);
}

@Override
public int delete(Uri uri, String selection, String[] selectionArgs) {
waitForAccess();
return super.delete(uri, selection, selectionArgs);
}
}
-------------------省略------------------
protected void notifyChange() {
notifyChange(mSyncToNetwork);
mSyncToNetwork = false;
}

protected void notifyChange(boolean syncToNetwork) {
getContext().getContentResolver().notifyChange(ContactsContract.AUTHORITY_URI, null,
syncToNetwork);
}
-------------------省略------------------
}
SQLiteContentProvider.java文件
public abstract class SQLiteContentProvider extends ContentProvider
implements SQLiteTransactionListener {
@Override
public Uri insert(Uri uri, ContentValues values) {
Uri result = null;
boolean applyingBatch = applyingBatch();
if (!applyingBatch) {
mDb = mOpenHelper.getWritableDatabase();
mDb.beginTransactionWithListener(this);
try {
result = insertInTransaction(uri, values);
if (result != null) {
mNotifyChange = true;
}
mDb.setTransactionSuccessful();
} finally {
mDb.endTransaction();
}

onEndTransaction();
} else {
result = insertInTransaction(uri, values);
if (result != null) {
mNotifyChange = true;
}
}
return result;
}
public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) {
-------------------省略------------------
}
public int delete(Uri uri, String selection, String[] selectionArgs) {
-------------------省略------------------
}
-------------------省略------------------
protected abstract void notifyChange();
-------------------省略------------------
protected void onEndTransaction() {
if (mNotifyChange) {
mNotifyChange = false;
notifyChange();
}
}
-------------------省略------------------
}
那么Cursor是如何实现对基于uri的数据库内容变化进行监听呢？
只要把一个监听器ContentObserver对uri使用ContentResolver的registerContentObserver方法进行注册。
如果相应uri的数据库内容变化发变化，先通知ContentObserver，再又让它通知Cursor。
当然为了方便系统把ContentObserver设计成了内部类。
具体可见在Cursor接口的一个实现android.database.AbstractCursor。
AbstractCursor.java文件请见附件1
同时Cursor为我们提供了setNotificationUri(ContentResolver cr, Uri notifyUri)让它内部类的ContentObserver注册到ContentResolver的某个Uri上.
具体代码参见附件1。
那么外部如何监听Cursor呢？。
Cursor为外部监测数据库的变化提供了以下接口：
abstract void registerContentObserver(ContentObserver observer)
Register an observer that is called when changes happen to the content backing this cursor.
注1：当数据库内容变化时通知observer，具体见附件1的onChange(boolean selfChange)函数.
注2：这个监听器observer主要是在setNotificationUri(ContentResolver cr, Uri notifyUri)中notifyUri对应的数据库内容发生变化时被通知的（间接），
abstract void unregisterContentObserver(ContentObserver observer)
Unregister an observer that has previously been registered with this cursor via registerContentObserver(ContentObserver).
另外，Cursor还为我们提供了接口，让外部可以通过注册一个DataSetObserver来对Cursor的requery(), deactivate(), close()行为进行监听。 abstract void registerDataSetObserver(DataSetObserver observer)
Register an observer that is called when changes happen to the contents of the this cursors data set, for example, when the data set is changed via requery(), deactivate(), or close().
注1：当Cursor发生requery(), deactivate(), or close()时通知DataSetObserver observer abstract void unregisterDataSetObserver(DataSetObserver observer)
Unregister an observer that has previously been registered with this cursor via registerContentObserver(ContentObserver). DataSetObserver的主要方法

Public Methods
void	onChanged()This method is called when the entire data set has changed, most likely through a call to `requery()` on a `Cursor`.
void	onInvalidated()This method is called when the entire data becomes invalid, most likely through a call to `deactivate()` or `close()` on a `Cursor`.

那么CursorAdapter是如何实现对基于uri的数据库内容变化进行监听呢？
它当然是借助Cursor来实现的。
但是CursorAdapter与ContentObserver相对应对外提供（可以重载）的接口是：
onContentChanged（）
当时CursorAdapter与DataSetObserver相对应对外提供（可以重载）的接口是：
onChanged()-> notifyDataSetChanged()//这个消息并完全不是从Cursor传递来，它会在CursorAdapter内手动被调用
onInvalidated()-> notifyDataSetInvalidated()//这个消息并不是从Cursor传递来。
具体可以参照附件2的android.widget.CursorAdapter源代码
CursorAdapter的notifyDataSetChanged()和notifyDataSetInvalidated()事件可以继续往外传递 BaseAdapter为此提供了以下系列方法。

void	notifyDataSetChanged()Notifies the attached observers that the underlying data has been changed and any View reflecting the data set should refresh itself.
void	notifyDataSetInvalidated()Notifies the attached observers that the underlying data is no longer valid or available.
void	registerDataSetObserver(DataSetObserver observer)Register an observer that is called when changes happen to the data used by this adapter.
void	unregisterDataSetObserver(DataSetObserver observer)Unregister an observer that has previously been registered with this adapter via `registerDataSetObserver(DataSetObserver)`.

android.widget.BaseAdapter的部分源代码如下：public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter { private final DataSetObservable mDataSetObservable = new DataSetObservable(); public boolean hasStableIds() { return false; } public void registerDataSetObserver(DataSetObserver observer) { mDataSetObservable.registerObserver(observer); } public void unregisterDataSetObserver(DataSetObserver observer) { mDataSetObservable.unregisterObserver(observer); } /** * Notifies the attached View that the underlying data has been changed * and it should refresh itself. */ public void notifyDataSetChanged() { mDataSetObservable.notifyChanged(); } public void notifyDataSetInvalidated() { mDataSetObservable.notifyInvalidated(); } -------------------省略------------------ } 注意：对于BaseAdapter，外部可以强行调用notifyDataSetChanged和notifyDataSetInvalidated来通知DataSetObserver observer【Android数据库内容变化的监听】再分享一下我老师大神的人工智能教程吧。零基础！通俗易懂！风趣幽默！还带黄段子！希望你也加入到我们人工智能的队伍中来！https://www.cnblogs.com/captainbed