Android CursorAdapter的使用詳解

一、CursorAdapter介紹

CursorAdapter這個類是繼承於BaseAdapter的它是一個虛類它為Cursor和ListView連接提供了橋梁

二、CursorAdapter詳解

1.CursorAdapter的繼承關系圖

從圖中可以看出CursorAdapter是繼承於BaseAdapter的，它有一個直接的子類SimpleCursorAdapter

2.CursorAdapter的用法

我們首先看一下CursorAdapter的部分源碼：

/**
     * @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;
    }

  從源碼中可以看出CursorAdapter是繼承了BaseAdapter后覆蓋它的getView方法在getView方法中調用了newView和bindView方法，我們在寫CursorAdapter時必須實現它的兩個方法
 /**
     * 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);

從源碼的 getView(  int position, View convertView, ViewGroup parent)方法中我們可以看出：

(1)newView：並不是每次都被調用的，它只在實例化的時候調用,數據增加的時候也會調用,但是在重繪(比如修改條目里的TextView的內容)的時候不會被調用

(2)bindView：從代碼中可以看出在繪制Item之前一定會調用bindView方法它在重繪的時候也同樣被調用

3.CursorAdapter還有一個重要的方法  public  void changeCursor (Cursor cursor)

源碼如下：

/**
     * 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)的源碼如下：

/**
     * 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;
    }

從源碼中可以看出調用此方法后會把當前的mCursor置為新傳過來的cursor把原來的cursor返回去並關掉

作用：當我們的Cursor變化時調用此方法

adapter.changeCursor(cursor)，它的功能類似於adapter.notifyDataSetChanged()方法

4.之前的疑惑
 之前我一直對cursor是怎么移動的疑惑，比方說cursor中有40條數據，那么它是怎樣一行一行移動cursor把這40條數據顯示出來的，看過源碼后發現其實很簡單，

它在getCount()方法中 return mCursor.getCount();然后在getView方法的時候調用了mCursor.moveToPosition(position)其實和BaseAdapter的原理是一樣的,這樣就可以一條一條的繪制條目了。

三、源碼小案例：

1.案例功能

 

 

在EditText中輸入姓名和電話，點擊保存后會顯示在下面的listView中

2.代碼片段

(1)MyCursorAdapter的主要代碼：

 

@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();
       //從數據庫中查詢姓名字段
           String name=cursor.getString(cursor.getColumnIndex(PersonInfo.NAME));
       //從數據庫中查詢電話字段
           String phoneNumber=cursor.getString(cursor.getColumnIndex(PersonInfo.PHONENUMBER));
           
           viewHolder. tv_name.setText(name);
           viewHolder. tv_phonenumber.setText(phoneNumber);
     }

調用newView方法實例化條目,然后調用bindView繪制條目,當只繪制時不會調用newView方法。

 (2)點擊保存按鈕執行的方法

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, "用戶名不能為空!",0).show();
                      return;
                    }
                    if( userPhoneNumber.equals( "")){
                     Toast. makeText(MainActivity.this,"電話不能為空", 0).show();
                      return;
                    }
                   
                    ContentValues contentValues= new ContentValues();
                    contentValues.put(PersonInfo. NAME, userName);
                    contentValues.put(PersonInfo.PHONENUMBER ,userPhoneNumber );
                    //把EditText中的文本插入數據庫
                    dataBase.insert(PersonInfo. PERSON_INFO_TABLE, null,contentValues);
                    //根據 _id 降序插敘數據庫保證最后插入的在最上面
                    Cursor myCursor          = dataBase.query(PersonInfo. PERSON_INFO_TABLE, null, null, null, null, null, orderBy);
                    //Cursor改變調用chanageCursor()方法
                    myCursorAdapter.changeCursor(myCursor);
                }
           });
     }

如果數據庫操作不熟可以看此博客

源碼戳HERE