Android CursorAdapter的使用详解

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 ;
        }
    }

    /**
     * @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 ;
            }
        } else {
            return ;
        }
    }

    /**
     * @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 = -;
            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，

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!",).show();
                      return;
                    }
                    if( userPhoneNumber.equals( "")){
                     Toast. makeText(MainActivity.this,"电话不能为空", ).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);
                }
           });
     }

如果数据库操作不熟可以看此博客

源码戳HERE