Android View 事件体系（3）

View的弹性滑动

   实现渐进式滑动的共同思想：将一次大的滑动分成若干次小的滑动，并在一个时间段内完成。

• Scroller

       代码示例：

public class MainActivity extends AppCompatActivity {

    @BindView(R.id.frame_layout)
    MyFrameLayout mFrameLayout;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ButterKnife.bind(this);
    }

    @OnClick(R.id.btn)
    public void onViewClicked() {
        mFrameLayout.smoothScrollTo(-200, 0);
    }
}

       MyFrameLayout代码

public class MyFrameLayout extends FrameLayout {


    private Scroller mScroller;

    public MyFrameLayout(Context context) {
        super(context);

    }

    public MyFrameLayout( Context context, AttributeSet attrs) {
        super(context, attrs);
        if (mScroller == null){
            mScroller = new Scroller(context);
        }
    }

    @Override
    public void computeScroll() {
        if (mScroller.computeScrollOffset()){
            scrollTo(mScroller.getCurrX(),mScroller.getCurrY());
            /**
             * <p>Cause an invalidate to happen on a subsequent cycle through the event loop.
             * Use this to invalidate the View from a non-UI thread.</p>
             *
             * <p>This method can be invoked from outside of the UI thread
             * only when this View is attached to a window.</p>
             *
             * @see #invalidate()
             * @see #postInvalidateDelayed(long)
             */
            postInvalidate();
        }
    }

    public void smoothScrollTo(int destX,int destY){
        /**
         * Return the scrolled left position of this view. This is the left edge of
         * the displayed part of your view. You do not need to draw any pixels
         * farther left, since those are outside of the frame of your view on
         * screen.
         *
         * @return The left edge of the displayed part of your view, in pixels.
         * getScrollX()
         */
        int ScrollX=getScrollX();
        int deltaX=destX-ScrollX;
        mScroller.startScroll(ScrollX,0,deltaX,0,2000);
        /**
         * Invalidate the whole view. If the view is visible,
         * {@link #onDraw(android.graphics.Canvas)} will be called at some point in
         * the future.
         * <p>
         * This must be called from a UI thread. To call from a non-UI thread, call
         * {@link #postInvalidate()}.
         */
        invalidate();
    }

}

       布局文件

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/layout"
    tools:context=".MainActivity">
    <Button
        android:id="@+id/btn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="缓慢滚动到指定位置"
        />
    <com.example.viewtest.MyFrameLayout
        android:layout_marginTop="10dp"
        android:id="@+id/frame_layout"
        android:layout_width="match_parent"
        android:layout_height="match_parent">
        <ImageView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:src="@mipmap/ic_launcher"/>

    </com.example.viewtest.MyFrameLayout>



</LinearLayout>

拓展：invalidate源码分析  https://cruise1008.github.io/2016/04/30/how-does-invalidate-and-requestLayout-work/

• 使用动画

        根据动画在给定的时间内完成整个动画过程的依据，可以在动画的每一帧到来时获取动画完成的比例，再根据这个比例计算出当前View所要滑动的距离。

public class MainActivity extends AppCompatActivity {


    @BindView(R.id.btn)
    Button mBtn;
    private final int startX=0;
    private final int deltaX=-100;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ButterKnife.bind(this);
        ValueAnimator valueAnimator = ValueAnimator.ofFloat(0, 1).setDuration(2000);
        valueAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
            @Override
            public void onAnimationUpdate(ValueAnimator animation) {
                float animatedFraction = animation.getAnimatedFraction();
                mBtn.scrollTo(startX+(int)(deltaX*animatedFraction),0);
            }
        });
        valueAnimator.start();
    }
}

 

• 使用延时策略

          通过发送一系列延时消息从而达到一种渐进式的效果，可以使用Handler或View的postDelayed方法，也可以使用线程的sleep方法。

public class MainActivity extends AppCompatActivity {


    @BindView(R.id.btn)
    Button mBtn;
    private static final int MESSAGE_SCROLL_TO=1;
    private static final int FRAME_COUNT=30;
    private static final int DELAYED_TIME=33;
    private int mCount=0;

    @SuppressLint("HandlerLeak")
    private Handler mHandler=new Handler(){
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what){
                case MESSAGE_SCROLL_TO:{
                    mCount++;
                    if (mCount <= FRAME_COUNT){
                        float fraction=mCount / (float) FRAME_COUNT;
                        int scrollX = (int) (fraction * 100);
                        mBtn.scrollTo(scrollX,0);
                        mHandler.sendEmptyMessageDelayed(MESSAGE_SCROLL_TO,DELAYED_TIME);
                    }
                    break;
                }

                default:
                    break;
            }
        }
    };
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ButterKnife.bind(this);
        mHandler.sendEmptyMessageDelayed(MESSAGE_SCROLL_TO,DELAYED_TIME);
    }
}

 

参考：1、 《Android 开发艺术探索》 任玉刚

           2、   invalidate源码分析  Cruise Yang：https://cruise1008.github.io/2016/04/30/how-does-invalidate-and-requestLayout-work/

 下一篇：View事件分发机制：https://blog.youkuaiyun.com/lwj_hewo/article/details/90544368