ListView 拉出回弹效果 overscroll，阻尼效果

本文链接：https://blog.youkuaiyun.com/androiddevelop/article/details/8155350

本文介绍了Android 9后新增的API实现ListView拉出回弹，即overscroll效果。原理涉及overSrollBy方法、边界判断、mScrollX和mScrollY变量以及重绘过程。通过自定义ListView和Activity示例，展示了如何直接使用API来实现这一阻尼效果。

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一 Android 9 之后才新增的API. 原理是

1. 在View中增加了overSrollBy方法,用于记录x, y 轴上滚动的之

2. 在AbsListView的onTouchEvent中判断是否到达边界(顶部或底部) ,然后调用view.overScrollBy ,传入 mScrollY等参数

3. overScrollBy 最终赋值给View的mScrollX, mScrollY 两个变量

4. 在AbsListView中调用完overScrollBy之后,调用invalidate重绘

二下面是一个直接使用API的例子

1. 自定义ListView

public class BounceListView extends ListView{
    private static final int MAX_Y_OVERSCROLL_DISTANCE = 200;
     
    private Context mContext;
    private int mMaxYOverscrollDistance;
     
    public BounceListView(Context context){
        super(context);
        mContext = context;
        initBounceListView();
    }
     
    public BounceListView(Context context, AttributeSet attrs){
        super(context, attrs);
        mContext = context;
        initBounceListView();
    }
     
    public BounceListView(Context context, AttributeSet attrs, int defStyle){
        super(context, attrs, defStyle);
        mContext = context;
        initBounceListView();
    }
     
    private void initBounceListView(){
        //get the density of the screen and do some maths with it on the max overscroll distance
        //variable so that you get similar behaviors no matter what the screen size
         
        final DisplayMetrics metrics = mContext.getResources().getDisplayMetrics();
        final float density = metrics.density;
         
        mMaxYOverscrollDistance = (int) (density * MAX_Y_OVERSCROLL_DISTANCE);
    }
     
    @Override
    protected boolean overScrollBy(int deltaX, int deltaY, int scrollX, int scrollY, int scrollRangeX, int scrollRangeY, int maxOverScrollX, int maxOverScrollY, boolean isTouchEvent){ 
        //This is where the magic happens, we have replaced the incoming maxOverScrollY with our own custom variable mMaxYOverscrollDistance; 
        return super.overScrollBy(deltaX, deltaY, scrollX, scrollY, scrollRangeX, scrollRangeY, maxOverScrollX, mMaxYOverscrollDistance, isTouchEvent);  
    }
     
}

2. Activity

public class MainActivity extends Activity {

	@Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        
        LinearLayout linearLayout = new LinearLayout(this);
        linearLayout.setLayoutParams(new LayoutParams(LayoutParams.FILL_PARENT, LayoutParams.FILL_PARENT));
        
        setContentView(linearLayout);
        
        
        BounceListView bounceListView = new BounceListView(this);
        
        String[] data = new String[30];
        for (int i = 0; i < data.length; i++) {
			data[i] = "天天记录 " + i;
 		}
        
        ArrayAdapter<String> arrayAdapter = 
        		new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1, data);
        
        bounceListView.setAdapter(arrayAdapter);
        
        linearLayout.addView(bounceListView);
    }
	
	
    
}

资料:

Android Overscroll: Revisited

转载请注明出处：http://blog.youkuaiyun.com/love_world_/article/details/8155350

2013-04-08 更新，这种效果也叫“阻尼效果”