一、CursorAdapter介绍
CursorAdapter这个类是继承于BaseAdapter的它是一个虚类它为Cursor和ListView连接提供了桥梁
二、CursorAdapter详解
1.CursorAdapter的继承关系图
从图中可以看出CursorAdapter是继承于BaseAdapter的，它有一个直接的子类SimpleCursorAdapter
2.CursorAdapter的用法
我们首先看一下CursorAdapter的部分源码：
/**
* @see android.widget.ListAdapter#getCount()
*/
public int getCount() {
if (mDataValid && mCursor != null) {
return mCursor.getCount();
} else {
return 0;
}
}
/**
* @see android.widget.ListAdapter#getItem(int)
*/
public Object getItem( int position) {
if (mDataValid && mCursor != null) {
mCursor.moveToPosition(position);
return mCursor;
} else {
return null;
}
}
/**
* @see android.widget.ListAdapter#getItemId(int)
*/
public long getItemId( int position) {
if (mDataValid && mCursor != null) {
if ( mCursor.moveToPosition(position)) {
return mCursor.getLong( mRowIDColumn);
} else {
return 0;
}
} else {
return 0;
}
}
/**
* @see android.widget.ListAdapter# getView(int, View, ViewGroup)
*/
public View getView( int position, View convertView, ViewGroup parent) {
if (!mDataValid) {
throw new IllegalStateException( "this should only be called when the cursor is valid");
}
if (!mCursor.moveToPosition(position)) {
throw new IllegalStateException( "couldn't move cursor to position " + position);
}
View v;
if (convertView == null) {
v = newView( mContext, mCursor, parent);
} else {
v = convertView;
}
bindView(v, mContext, mCursor);
return v;
}
从源码中可以看出CursorAdapter是继承了BaseAdapter后覆盖它的getView方法在getView方法中调用了newView和bindView方法，我们在写CursorAdapter时必须实现它的两个方法
/**
* Makes a new view to hold the data pointed to by cursor.
* @param context Interface to application's global information
* @param cursor The cursor from which to get the data. The cursor is already
* moved to the correct position.
* @param parent The parent to which the new view is attached to
* @return the newly created view.
*/
public abstract View newView (Context context, Cursor cursor, ViewGroup parent);
/**
* Bind an existing view to the data pointed to by cursor
* @param view Existing view, returned earlier by newView
* @param context Interface to application's global information
* @param cursor The cursor from which to get the data. The cursor is already
* moved to the correct position.
*/
public abstract void bindView(View view, Context context, Cursor cursor);
从源码的 getView( int position, View convertView, ViewGroup parent)方法中我们可以看出：
(1)newView：并不是每次都被调用的，它只在实例化的时候调用,数据增加的时候也会调用,但是在重绘(比如修改条目里的TextView的内容)的时候不会被调用
(2)bindView：从代码中可以看出在绘制Item之前一定会调用bindView方法它在重绘的时候也同样被调用
3.
CursorAdapter还有一个重要的方法
public
void
changeCursor (Cursor cursor)
源码如下：
/**
* Change the underlying cursor to a new cursor. If there is an existing cursor it will be
* closed.
*
* @param cursor The new cursor to be used
*/
public void changeCursor (Cursor cursor) {
Cursor old = swapCursor(cursor);
if (old != null) {
old.close();
}
}
swapCursor(cusor)的源码如下：
/**
* Swap in a new Cursor, returning the old Cursor. Unlike
* {@link #changeCursor(Cursor)}, the returned old Cursor is <em>not</em>
* closed.
*
* @param newCursor The new cursor to be used.
* @return Returns the previously set Cursor, or null if there wasa not one.
* If the given new Cursor is the same instance is the previously set
* Cursor, null is also returned.
*/
public Cursor swapCursor (Cursor newCursor) {
if (newCursor == mCursor) {
return null;
}
Cursor oldCursor = mCursor;
if (oldCursor != null) {
if ( mChangeObserver != null) oldCursor.unregisterContentObserver(mChangeObserver );
if ( mDataSetObserver != null) oldCursor.unregisterDataSetObserver(mDataSetObserver );
}
mCursor = newCursor;
if (newCursor != null) {
if ( mChangeObserver != null) newCursor.registerContentObserver(mChangeObserver );
if ( mDataSetObserver != null) newCursor.registerDataSetObserver(mDataSetObserver );
mRowIDColumn = newCursor.getColumnIndexOrThrow("_id" );
mDataValid = true;
// notify the observers about the new cursor
notifyDataSetChanged();
} else {
mRowIDColumn = -1;
mDataValid = false;
// notify the observers about the lack of a data set
notifyDataSetInvalidated();
}
return oldCursor;
}
从源码中可以看出调用此方法后会把当前的mCursor置为新传过来的cursor把原来的cursor返回去并关掉
作用：当我们的Cursor变化时调用此方法
adapter.changeCursor(cursor)，它的功能类似于
adapter.notifyDataSetChanged()方法
4.
之前的疑惑
之前我一直对cursor是怎么移动的疑惑，比方说cursor中有40条数据，那么它是怎样一行一行移动cursor把这40条数据显示出来的，看过源码后发现其实很简单，
之前我一直对cursor是怎么移动的疑惑，比方说cursor中有40条数据，那么它是怎样一行一行移动cursor把这40条数据显示出来的，看过源码后发现其实很简单，
它在
getCount()方法中
return
mCursor
.getCount();然后在getView方法的时候调用了
mCursor
.moveToPosition(position)其实和BaseAdapter的原理是一样的,这样就可以一条一条的绘制条目了。
三、源码小案例：
1.案例功能
在EditText中输入姓名和电话，点击保存后会显示在下面的listView中
2.代码片段
(1)MyCursorAdapter的主要代码：
@Override
public View newView(Context context, Cursor cursor, ViewGroup parent) {
ViewHolder viewHolder= new ViewHolder();
LayoutInflater inflater=(LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE );
View view=inflater.inflate(R.layout.item_contacts ,parent,false);
viewHolder. tv_name=(TextView) view.findViewById(R.id.tv_showusername );
viewHolder. tv_phonenumber=(TextView) view.findViewById(R.id.tv_showusernumber );
view.setTag(viewHolder);
Log. i("cursor" ,"newView=" +view);
return view;
}
@Override
public void bindView(View view, Context context, Cursor cursor) {
Log. i("cursor" ,"bindView=" +view);
ViewHolder viewHolder=(ViewHolder) view.getTag();
//从数据库中查询姓名字段
String name=cursor.getString(cursor.getColumnIndex(PersonInfo.NAME));
//从数据库中查询电话字段
String phoneNumber=cursor.getString(cursor.getColumnIndex(PersonInfo.PHONENUMBER));
viewHolder. tv_name.setText(name);
viewHolder. tv_phonenumber.setText(phoneNumber);
}
调用newView方法实例化条目,然后调用bindView绘制条目,当只绘制时不会调用newView方法。
(2)点击保存按钮执行的方法
private void setClickListener() {
btn_save.setOnClickListener( new OnClickListener() {
public void onClick(View v) {
userName=et_name.getText().toString();
userPhoneNumber=et_phonenumber .getText().toString();
if( userName.equals( "")){
Toast. makeText(MainActivity.this, "用户名不能为空!",0).show();
return;
}
if( userPhoneNumber.equals( "")){
Toast. makeText(MainActivity.this,"电话不能为空", 0).show();
return;
}
ContentValues contentValues= new ContentValues();
contentValues.put(PersonInfo. NAME, userName);
contentValues.put(PersonInfo.PHONENUMBER ,userPhoneNumber );
//把EditText中的文本插入数据库
dataBase.insert(PersonInfo. PERSON_INFO_TABLE, null,contentValues);
//根据 _id 降序插叙数据库保证最后插入的在最上面
Cursor myCursor = dataBase.query(PersonInfo. PERSON_INFO_TABLE, null, null, null, null, null, orderBy);
//Cursor改变调用chanageCursor()方法
myCursorAdapter.changeCursor(myCursor);
}
});
}