[学习笔记] Android群英传：ListView 使用技巧

一.Listview常用优化技巧

1.使用ViewHolder模式提高效率

ViewHolder模式利用了ListView的视图缓存机制，避免每次调用getView()的时候去通过findViewById()实例化控件，需要在自定义的Adapter里面定义一个内部类ViewHolder即可，代码如下：

public final class ViewHolder{ public ImageView img; public TextView tv; }

完整的Adapter：

/** * 自定义Adapter * Created by LGL on 2016/3/10. */ public class MyAdapter extends BaseAdapter { private List<String> mData; private LayoutInflater mInflater; //构造方法 public MyAdapter(Context context, List<String> mData) { this.mData = mData; mInflater = LayoutInflater.from(context); } //返回长度 @Override public int getCount() { return mData.size(); } @Override public Object 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 viewHolder = null; //判断是否有缓存 if (convertView == null) { viewHolder = new ViewHolder(); //通过LayoutInflater去实例化布局 convertView = mInflater.inflate(R.layout.item, null); viewHolder.img = (ImageView) convertView.findViewById(R.id.img); viewHolder.tv = (TextView) convertView.findViewById(R.id.tv); convertView.setTag(viewHolder); } else { //通过TAG找到缓存的布局 viewHolder = (ViewHolder) convertView.getTag(); } //设置布局中要显示的东西 viewHolder.tv.setText(mData.get(position)); return convertView; } public final class ViewHolder { public ImageView img; public TextView tv; } }

使用adapter：

public class MainActivity extends AppCompatActivity { private ListView listview; private MyAdapter myAdapter; private List<String> list = new ArrayList<String>(); @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); listview = (ListView) findViewById(R.id.listview); for (int i = 0; i < 20; i++) { list.add( i + ""); } myAdapter = new MyAdapter(this, list); listview.setAdapter(myAdapter); } }

2.设置项目间分割栏、

ListView在各个项目之中，可以通过分割线进行区分的，系统也提供了两个属性来设置item间的颜色和高度

android:dividerHeight="10dp" android:divider="@android:color/holo_blue_bright"

去掉这个分割线的话：

android:divider="@null"

3.隐藏listview的滚动条

android:scrollbars="none"

4.取消ListView的item点击效果

android:listSelector="@android:color/transparent"

这里：可以直接填#00000000

5.设置ListView需要显示在第几项

listview.setSelection(15);

类似于平滑的效果了：

listview.smoothScrollBy(1,15); listview.smoothScrollByOffset(15); listview.smoothScrollToPosition(15);

6.动态修改ListView

<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical"> <ListView android:id="@+id/listview" android:layout_width="match_parent" android:layout_height="0dp" android:layout_weight="1" /> <Button android:id="@+id/add_data" android:layout_width="match_parent" android:layout_height="wrap_content" android:text="添加数据" /> </LinearLayout>

点击事件：

@Override public void onClick(View view) { switch (view.getId()) { case R.id.add_data: list.add("我是新增加的数据"); //通知刷新 myAdapter.notifyDataSetChanged(); break; } }

7.遍历所有的item

ListView作为一个ViewGroup,他提供了很多操纵子View的方法，最常用的就是getChilaAt()来获取View了

for (int j = 0; j < listview.getChildCount();j++){ View v = listview.getChildAt(12); }

8.处理空ListView

<TextView android:id="@+id/tv" android:text="没有数据" android:layout_width="match_parent" android:layout_height="wrap_content" />

在代码中设置如果是空数据的话就加载指定控件：

listview.setEmptyView(findViewById(R.id.tv));

9.ListView滑动监听

很多应用场景需要重写ListView的其实都是因为滑动监听，通过判断滑动事件来处理不同的逻辑，开发者通常还需要GestureDetector手势识别，VelocityTracker滑动速度检测来辅助监听，这里介绍两种监听方法，一种是OnTouchListener,另一中是OnScrollListener

1. OnTouchListener

OnTouchListener是View的监听事件，包括ACTION_DOWN,UP,MOVE等，通过坐标的改变老发生不同的逻辑

listview.setOnTouchListener(new View.OnTouchListener() { @Override public boolean onTouch(View view, MotionEvent motionEvent) { switch (motionEvent.getAction()) { case MotionEvent.ACTION_DOWN: //触摸时操作 break; case MotionEvent.ACTION_MOVE: //移动时操作 break; case MotionEvent.ACTION_UP: //离开时操作 break; } return true; } });

2. OnScrollListener

OnScrollListener是AbsListView的监听事件，他封装了很多ListView的相关信息

listview.setOnScrollListener(new AbsListView.OnScrollListener() { @Override public void onScrollStateChanged(AbsListView absListView, int scrollState) { switch (scrollState) { case AbsListView.OnScrollListener.SCROLL_STATE_IDLE: //滚动停止 break; case AbsListView.OnScrollListener.SCROLL_STATE_TOUCH_SCROLL: //正在滚动 break; case AbsListView.OnScrollListener.SCROLL_STATE_FLING: //手指抛动时，即手指用力滑动的时候 break; } } @Override public void onScroll(AbsListView absListView, int firstVisibleItem, int visibleItemCount, int totalItemCount) { //滚动的时候一直在调用 } });

OnScrollListener中有两个回调方法onScrollStateChanged()和onScroll() onScrollStateChanged()，有三种模式:

OnScrollListener.SCROLL_STATE_IDLE://滚动停止OnScrollListener.SCROLL_STATE_TOUCH_SCROLL://正在滚动OnScrollListener.SCROLL_STATE_FLING://手指抛动时，即手指用力滑动的时候

当用户没有做手指抛动的动作时，这个方法只会调用2次，否则就调用三次，差别就在于手指抛动的这个状态

onScroll():

firstVisibleItem:当前能看到的第一个item(包括部分显示的ListItem)的id(下标从0开始) visibleItemCount:可以看到的item(包括部分显示的ListItem)总数 totalItemCount:表示ListView的ListItem总数

if(firstVisibleItem+visibleItemCount == totalItemCount &&totalItemCount>0){ //滚动到最后一行 } if(firstVisibleItem>lastVisiblePosition){ //上滑 }else if(firstVisibleItem<lastVisiblePosition){ //下滑 }

lastVisiblePosition 为上次 firstVisibleItem;

ListView提供了获取位置信息的Api

//获取可视区域内最后一个item的id listview.getLastVisiblePosition(); //获取可视区域内第一个item的id listview.getFirstVisiblePosition();

二.ListView常用扩展

1.具有弹性的ListView

Android默认滑动到顶部或者底部只会有一个阴影，而在5.X之后改变成了半圆的阴影

在IOS上，列表是具有弹性的，即滚动到顶部或者底部，会再滚动一段距离。

在查看ListView的源码的时候会发现一个控制滑动到边缘的处理方法

@Override protected boolean overScrollBy(int deltaX, int deltaY, int scrollX, int scrollY, int scrollRangeX, int scrollRangeY, int maxOverScrollX, int maxOverScrollY, boolean isTouchEvent) { return super.overScrollBy(deltaX, deltaY, scrollX, scrollY, scrollRangeX, scrollRangeY, maxOverScrollX, maxOverScrollY, isTouchEvent); }

我們可以看到这样一个参数maxOverScrollY，就是他负责控制滑动的个数的，默认是0，我们重写ListView

/** * 弹性ListView */ public class ListViewScroll extends ListView { private int mMaxOverdistance ; public ListViewScroll(Context context, AttributeSet attrs) { super(context, attrs); //通过分辨率来调节滑动尺度 DisplayMetrics metrics = context.getResources().getDisplayMetrics(); float density = metrics.density; mMaxOverdistance = (int)(density*mMaxOverdistance); } @Override protected boolean overScrollBy(int deltaX, int deltaY, int scrollX, int scrollY, int scrollRangeX, int scrollRangeY, int maxOverScrollX, int maxOverScrollY, boolean isTouchEvent) { return super.overScrollBy(deltaX, deltaY, scrollX, scrollY, scrollRangeX, scrollRangeY, mMaxOverdistance, maxOverScrollY, isTouchEvent); } }

2.自动显示，隐藏布局的ListView

列表滑动actionbar显示或者隐藏 需要使用ToolsBar,然后一个listview

<android.support.v7.widget.Toolbar android:id="@+id/toolbar" android:layout_width="match_parent" android:layout_height="?attr/actionBarSize" android:background="?attr/colorPrimary" /> public class ScrollHideListView extends Activity { private Toolbar mToolbar; private ListView mListView; private String[] mStr = new String[20]; private int mTouchSlop; private float mFirstY; private float mCurrentY; private int direction; private ObjectAnimator mAnimator; private boolean mShow = true; View.OnTouchListener myTouchListener = new View.OnTouchListener() { @Override public boolean onTouch(View v, MotionEvent event) { switch (event.getAction()) { case MotionEvent.ACTION_DOWN: mFirstY = event.getY(); break; case MotionEvent.ACTION_MOVE: mCurrentY = event.getY(); if (mCurrentY - mFirstY > mTouchSlop) { direction = 0;// down } else if (mFirstY - mCurrentY > mTouchSlop) { direction = 1;// up } if (direction == 1) { if (mShow) { toolbarAnim(1);//hide mShow = !mShow; } } else if (direction == 0) { if (!mShow) { toolbarAnim(0);//show mShow = !mShow; } } break; case MotionEvent.ACTION_UP: break; } return false; } }; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.scroll_hide); mTouchSlop = ViewConfiguration.get(this).getScaledTouchSlop(); mToolbar = (Toolbar) findViewById(R.id.toolbar); mListView = (ListView) findViewById(R.id.listview); for (int i = 0; i < mStr.length; i++) { mStr[i] = "Item " + i; } View header = new View(this); header.setLayoutParams(new AbsListView.LayoutParams( AbsListView.LayoutParams.MATCH_PARENT, (int) getResources().getDimension( R.dimen.abc_action_bar_default_height_material))); mListView.addHeaderView(header); mListView.setAdapter(new ArrayAdapter<String>( ScrollHideListView.this, android.R.layout.simple_expandable_list_item_1, mStr)); mListView.setOnTouchListener(myTouchListener); } private void toolbarAnim(int flag) { if (mAnimator != null && mAnimator.isRunning()) { mAnimator.cancel(); } if (flag == 0) { mAnimator = ObjectAnimator.ofFloat(mToolbar, "translationY", mToolbar.getTranslationY(), 0); } else { mAnimator = ObjectAnimator.ofFloat(mToolbar, "translationY", mToolbar.getTranslationY(), -mToolbar.getHeight()); } mAnimator.start(); } }

3.聊天ListView

继承BaseAdapter的时候需要重写两个方法

@Override public int getItemViewType(int position) { return super.getItemViewType(position); } @Override public int getViewTypeCount() { return super.getViewTypeCount(); }

实体类Bean

Bean

/** * 实体类 */ public class Bean { private int type; private String text; private Bitmap icon; public Bean() { } public int getType() { return type; } public void setType(int type) { this.type = type; } public String getText() { return text; } public void setText(String text) { this.text = text; } public Bitmap getIcon() { return icon; } public void setIcon(Bitmap icon) { this.icon = icon; } }

现在就可以编写我们的Adapter了

SpeakAdapter

public class ChatAdapter extends BaseAdapter{ private List<Bean>mData; private LayoutInflater mInflater; //构造方法 public ChatAdapter(Context context,List<Bean>mData){ this.mData = mData; mInflater = LayoutInflater.from(context); } @Override public int getCount() { return mData.size(); } @Override public Object 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 viewHolder; if(convertView == null){ //区分左右 if(getItemViewType(position) == 0){ viewHolder = new ViewHolder(); convertView = mInflater.inflate(R.layout.left_item,null); viewHolder.icon = (ImageView) convertView.findViewById(R.id.left_icon); viewHolder.text = (TextView) convertView.findViewById(R.id.tv_left); }else{ viewHolder = new ViewHolder(); convertView = mInflater.inflate(R.layout.right_item,null); viewHolder.icon = (ImageView) convertView.findViewById(R.id.right_icon); viewHolder.text = (TextView) convertView.findViewById(R.id.tv_right); } convertView.setTag(viewHolder); }else{ viewHolder = (ViewHolder) convertView.getTag(); } viewHolder.icon.setImageBitmap(mData.get(position).getIcon()); viewHolder.text.setText(mData.get(position).getText()); return convertView; } @Override public int getItemViewType(int position) { Bean bean = mData.get(position); return bean.getType(); } @Override public int getViewTypeCount() { return 2; } public final class ViewHolder{ public ImageView icon; public TextView text; } }

ChatListViewActivity：

/** * 聊天ListView */ public class ChatListViewActivity extends AppCompatActivity { private ListView mListview; @Override public void onCreate(Bundle savedInstanceState, PersistableBundle persistentState) { super.onCreate(savedInstanceState, persistentState); setContentView(R.layout.activity_chat); mListview = (ListView) findViewById(R.id.listview); Bean bean1 = new Bean(); bean1.setType(0); bean1.setIcon(BitmapFactory.decodeResource(getResources(), R.mipmap.ic_launcher)); bean1.setText("我是左边"); Bean bean2 = new Bean(); bean2.setType(1); bean2.setIcon(BitmapFactory.decodeResource(getResources(), R.mipmap.ic_launcher)); bean2.setText("我是右边"); List<Bean>data = new ArrayList<Bean>(); data.add(bean1); data.add(bean2); ChatAdapter adapter = new ChatAdapter(this,data); mListview.setAdapter(adapter); } }

4.动态改变ListView的布局

public class FocusListViewAdapter extends BaseAdapter { private List<String> mData; private Context mContext; private int mCurrentItem = 0; public FocusListViewAdapter(Context context, List<String> data) { this.mContext = context; this.mData = data; } public int getCount() { return mData.size(); } public Object getItem(int position) { return mData.get(position); } public long getItemId(int position) { return position; } public View getView(int position, View convertView, ViewGroup parent) { LinearLayout layout = new LinearLayout(mContext); layout.setOrientation(LinearLayout.VERTICAL); if (mCurrentItem == position) { layout.addView(addFocusView(position)); } else { layout.addView(addNormalView(position)); } return layout; } public void setCurrentItem(int currentItem) { this.mCurrentItem = currentItem; } private View addFocusView(int i) { ImageView iv = new ImageView(mContext); iv.setImageResource(R.drawable.ic_launcher); return iv; } private View addNormalView(int i) { LinearLayout layout = new LinearLayout(mContext); layout.setOrientation(LinearLayout.HORIZONTAL); ImageView iv = new ImageView(mContext); iv.setImageResource(R.drawable.in_icon); layout.addView(iv, new LinearLayout.LayoutParams( LinearLayout.LayoutParams.WRAP_CONTENT, LinearLayout.LayoutParams.WRAP_CONTENT)); TextView tv = new TextView(mContext); tv.setText(mData.get(i)); layout.addView(tv, new LinearLayout.LayoutParams( LinearLayout.LayoutParams.WRAP_CONTENT, LinearLayout.LayoutParams.WRAP_CONTENT)); layout.setGravity(Gravity.CENTER); return layout; } listView.setOnItemClickListener(new AdapterView.OnItemClickListener() { @Override public void onItemClick(AdapterView<?> parent, View view, int position, long id) { myAdapter.setCurrentItem(position); myAdapter.notifyDataSetChanged(); } });