ListView中getView的原理＋如何在ListView中放置多個item

ListView 和 Adapter 的基礎

工做原理:

1. ListView 針對List中每一個item，要求 adapter 「給我一個視圖」 (getView)。
2. 一個新的視圖被返回並顯示

若是咱們有上億個項目要顯示怎麼辦？爲每一個項目建立一個新視圖？NO!這不可能！

實際上Android爲你緩存了視圖。

Android中有個叫作Recycler的構件，下圖是他的工做原理：

1. 若是你有10億個項目(item)，其中只有可見的項目存在內存中，其餘的在Recycler中。
2. ListView先請求一個type1視圖(getView)而後請求其餘可見的項目。convertView在getView中是空(null)的。
3. 當item1滾出屏幕，而且一個新的項目從屏幕低端上來時，ListView再請求一個type1視圖。convertView此時不是空值了，它的值是item1。你只需設定新的數據而後返回convertView，沒必要從新建立一個視圖。

請看下面的示例代碼，這裏在getView中使用了System.out進行輸出

 

public class MultipleItemsList extends ListActivity {
 
    private MyCustomAdapter mAdapter;
 
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mAdapter = new MyCustomAdapter();
        for (int i = 0; i < 50; i++) {
            mAdapter.addItem("item " + i);
        }
        setListAdapter(mAdapter);
    }
 
    private class MyCustomAdapter extends BaseAdapter {
 
        private ArrayList mData = new ArrayList();
        private LayoutInflater mInflater;
 
        public MyCustomAdapter() {
            mInflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        }
 
        public void addItem(final String item) {
            mData.add(item);
            notifyDataSetChanged();
        }
 
        @Override
        public int getCount() {
            return mData.size();
        }
 
        @Override
        public String getItem(int position) {
            return mData.get(position);
        }
 
        @Override
        public long getItemId(int position) {
            return position;
        }
 
        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            System.out.println("getView " + position + " " + convertView);
            ViewHolder holder = null;
            if (convertView == null) {
                convertView = mInflater.inflate(R.layout.item1, null);
                holder = new ViewHolder();
                holder.textView = (TextView)convertView.findViewById(R.id.text);
                convertView.setTag(holder);
            } else {
                holder = (ViewHolder)convertView.getTag();
            }
            holder.textView.setText(mData.get(position));
            return convertView;
        }
 
    }
 
    public static class ViewHolder {
        public TextView textView;
    }
}

執行程序，而後在Logcat中查看日誌

 

getView 被調用 9 次 ，convertView 對於全部的可見項目是空值（以下）

 

 

02-05 13:47:32.559: INFO/System.out(947): getView 0 null

02-05 13:47:32.570: INFO/System.out(947): getView 1 null

02-05 13:47:32.589: INFO/System.out(947): getView 2 null

02-05 13:47:32.599: INFO/System.out(947): getView 3 null

02-05 13:47:32.619: INFO/System.out(947): getView 4 null

02-05 13:47:32.629: INFO/System.out(947): getView 5 null

02-05 13:47:32.708: INFO/System.out(947): getView 6 null

02-05 13:47:32.719: INFO/System.out(947): getView 7 null

02-05 13:47:32.729: INFO/System.out(947): getView 8 null

 

而後稍微向下滾動List，直到item10出現：

 

convertView仍然是空值，由於recycler中沒有視圖（item1的邊緣仍然可見，在頂端）

 

 

02-05 13:48:25.169: INFO/System.out(947): getView 9 null

 

再滾動List

 

convertView不是空值了！item1離開屏幕到Recycler中去了，而後item11被建立

 

 

02-05 13:48:42.879: INFO/System.out(947): getView 10 android.widget.LinearLayout@437430f8

 

再滾動：

02-05 14:01:31.069: INFO/System.out(947): getView 11 android.widget.LinearLayout@437447d0

02-05 14:01:31.142: INFO/System.out(947): getView 12 android.widget.LinearLayout@43744ff8

02-05 14:01:31.279: INFO/System.out(947): getView 13 android.widget.LinearLayout@43743fa8

02-05 14:01:31.350: INFO/System.out(947): getView 14 android.widget.LinearLayout@43745820

02-05 14:01:31.429: INFO/System.out(947): getView 15 android.widget.LinearLayout@43746048

02-05 14:01:31.550: INFO/System.out(947): getView 16 android.widget.LinearLayout@43746870

02-05 14:01:31.669: INFO/System.out(947): getView 17 android.widget.LinearLayout@43747098

02-05 14:01:31.839: INFO/System.out(947): getView 18 android.widget.LinearLayout@437478c0

02-05 14:03:30.900: INFO/System.out(947): getView 19 android.widget.LinearLayout@43748df0

02-05 14:03:32.069: INFO/System.out(947): getView 20 android.widget.LinearLayout@437430f8

convertView 如咱們所期待的非空了，在item11離開屏幕以後，它的視圖(@437430f8)做爲convertView容納item21了

不一樣的項目佈局(item layout)

咱們再舉一個稍微複雜的例子，在上例的list中加入一些分隔線

你須要作這些:

1. 重(@Override)寫 getViewTypeCount() – 返回你有多少個不一樣的佈局
2. 重寫 getItemViewType(int) – 由position返回view type id
3. 根據view item的類型，在getView中建立正確的convertView

如下是代碼： 

 

public class MultipleItemsList extends ListActivity {
 
    private MyCustomAdapter mAdapter;
 
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mAdapter = new MyCustomAdapter();
        for (int i = 1; i < 50; i++) {
            mAdapter.addItem("item " + i);
            if (i % 4 == 0) {
                mAdapter.addSeparatorItem("separator " + i);
            }
        }
        setListAdapter(mAdapter);
    }
 
    private class MyCustomAdapter extends BaseAdapter {
 
        private static final int TYPE_ITEM = 0;
        private static final int TYPE_SEPARATOR = 1;
        private static final int TYPE_MAX_COUNT = TYPE_SEPARATOR + 1;
 
        private ArrayList mData = new ArrayList();
        private LayoutInflater mInflater;
 
        private TreeSet mSeparatorsSet = new TreeSet();
 
        public MyCustomAdapter() {
            mInflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        }
 
        public void addItem(final String item) {
            mData.add(item);
            notifyDataSetChanged();
        }
 
        public void addSeparatorItem(final String item) {
            mData.add(item);
            // save separator position
            mSeparatorsSet.add(mData.size() - 1);
            notifyDataSetChanged();
        }
 
        @Override
        public int getItemViewType(int position) {
            return mSeparatorsSet.contains(position) ? TYPE_SEPARATOR : TYPE_ITEM;
        }
 
        @Override
        public int getViewTypeCount() {
            return TYPE_MAX_COUNT;
        }
 
        @Override
        public int getCount() {
            return mData.size();
        }
 
        @Override
        public String getItem(int position) {
            return mData.get(position);
        }
 
        @Override
        public long getItemId(int position) {
            return position;
        }
 
        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            ViewHolder holder = null;
            int type = getItemViewType(position);
            System.out.println("getView " + position + " " + convertView + " type = " + type);
            if (convertView == null) {
                holder = new ViewHolder();
                switch (type) {
                    case TYPE_ITEM:
                        convertView = mInflater.inflate(R.layout.item1, null);
                        holder.textView = (TextView)convertView.findViewById(R.id.text);
                        break;
                    case TYPE_SEPARATOR:
                        convertView = mInflater.inflate(R.layout.item2, null);
                        holder.textView = (TextView)convertView.findViewById(R.id.textSeparator);
                        break;
                }
                convertView.setTag(holder);
            } else {
                holder = (ViewHolder)convertView.getTag();
            }
            holder.textView.setText(mData.get(position));
            return convertView;
        }
 
    }
 
    public static class ViewHolder {
        public TextView textView;
    }
}

 

運行程序，你會看到每4個item一個分割線

看看日誌，無異常，全部的convertView都是空的

02-05 15:19:03.080: INFO/System.out(1035): getView 0 null type = 0

02-05 15:19:03.112: INFO/System.out(1035): getView 1 null type = 0

02-05 15:19:03.130: INFO/System.out(1035): getView 2 null type = 0

02-05 15:19:03.141: INFO/System.out(1035): getView 3 null type = 0

02-05 15:19:03.160: INFO/System.out(1035): getView 4 null type = 1

02-05 15:19:03.170: INFO/System.out(1035): getView 5 null type = 0

02-05 15:19:03.180: INFO/System.out(1035): getView 6 null type = 0

02-05 15:19:03.190: INFO/System.out(1035): getView 7 null type = 0

02-05 15:19:03.210: INFO/System.out(1035): getView 8 null type = 0

02-05 15:19:03.210: INFO/System.out(1035): getView 9 null type = 1

滾動list：

 

02-05 15:19:54.160: INFO/System.out(1035): getView 10 null type = 0

02-05 15:19:57.440: INFO/System.out(1035): getView 11 android.widget.LinearLayout@43744528 type = 0

02-05 15:20:01.310: INFO/System.out(1035): getView 12 android.widget.LinearLayout@43744eb0 type = 0

02-05 15:20:01.880: INFO/System.out(1035): getView 13 android.widget.LinearLayout@437456d8 type = 0

02-05 15:20:02.869: INFO/System.out(1035): getView 14 null type = 1

02-05 15:20:06.489: INFO/System.out(1035): getView 15 android.widget.LinearLayout@43745f00 type = 0

02-05 15:20:07.749: INFO/System.out(1035): getView 16 android.widget.LinearLayout@43747170 type = 0

02-05 15:20:10.250: INFO/System.out(1035): getView 17 android.widget.LinearLayout@43747998 type = 0

02-05 15:20:11.661: INFO/System.out(1035): getView 18 android.widget.LinearLayout@437481c0 type = 0

02-05 15:20:13.180: INFO/System.out(1035): getView 19 android.widget.LinearLayout@437468a0 type = 1

02-05 15:20:16.900: INFO/System.out(1035): getView 20 android.widget.LinearLayout@437489e8 type = 0

02-05 15:20:25.690: INFO/System.out(1035): getView 21 android.widget.LinearLayout@4374a8d8 type = 0

convertView對於分割線是空的，直到第一個分割線可見，當其離開屏幕，視圖去到Recycler而且convertView開始起做用。

本文翻譯自http://android.amberfog.com/?p=296

代碼下載：MultipleItemsList.zip – source code