Android滑動控件的滑動慣性效果實現

在Android中，實現滑動控件的滑動慣性效果通常使用Scroller類

1. 首先，創建一個自定義的滑動控件，繼承自ViewGroup（例如LinearLayout、RelativeLayout等）。

public class InertiaScrollView extends LinearLayout {
    // ...
}

2. 在自定義控件的構造方法中，初始化Scroller對象和其他相關變量。

private Scroller mScroller;
private VelocityTracker mVelocityTracker;
private int mTouchSlop;
private float mLastMotionX;
private float mLastMotionY;

public InertiaScrollView(Context context, AttributeSet attrs) {
    super(context, attrs);
    mScroller = new Scroller(context);
    ViewConfiguration config = ViewConfiguration.get(context);
    mTouchSlop = config.getScaledTouchSlop();
}

3. 重寫onInterceptTouchEvent方法，攔截觸摸事件。

@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
    final int action = ev.getAction();
    if ((action == MotionEvent.ACTION_MOVE) && (mIsBeingDragged)) {
        return true;
    }

    switch (action & MotionEvent.ACTION_MASK) {
        case MotionEvent.ACTION_MOVE: {
            final float x = ev.getX();
            final float y = ev.getY();
            final int xDiff = (int) Math.abs(x - mLastMotionX);
            final int yDiff = (int) Math.abs(y - mLastMotionY);
            if (yDiff > mTouchSlop) {
                mIsBeingDragged = true;
                mLastMotionY = y;
            }
            break;
        }

        case MotionEvent.ACTION_DOWN: {
            mIsBeingDragged = false;
            mLastMotionX = ev.getX();
            mLastMotionY = ev.getY();
            break;
        }

        case MotionEvent.ACTION_CANCEL:
        case MotionEvent.ACTION_UP: {
            mIsBeingDragged = false;
            break;
        }
    }

    return mIsBeingDragged;
}

4. 重寫onTouchEvent方法，處理觸摸事件。

@Override
public boolean onTouchEvent(MotionEvent event) {
    initVelocityTrackerIfNotExists();
    mVelocityTracker.addMovement(event);

    final int action = event.getAction();
    switch (action) {
        case MotionEvent.ACTION_DOWN: {
            if (!mScroller.isFinished()) {
                mScroller.abortAnimation();
            }
            mLastMotionX = event.getX();
            mLastMotionY = event.getY();
            break;
        }

        case MotionEvent.ACTION_MOVE: {
            final float x = event.getX();
            final float y = event.getY();
            final int deltaX = (int) (mLastMotionX - x);
            final int deltaY = (int) (mLastMotionY - y);
            mLastMotionX = x;
            mLastMotionY = y;

            scrollBy(0, deltaY);
            break;
        }

        case MotionEvent.ACTION_UP: {
            final VelocityTracker velocityTracker = mVelocityTracker;
            velocityTracker.computeCurrentVelocity(1000);
            int initialVelocity = (int) velocityTracker.getYVelocity();
            if ((Math.abs(initialVelocity) > mMinimumVelocity)) {
                fling(-initialVelocity);
            }

            if (mVelocityTracker != null) {
                mVelocityTracker.recycle();
                mVelocityTracker = null;
            }
            break;
        }

        case MotionEvent.ACTION_CANCEL: {
            if (mVelocityTracker != null) {
                mVelocityTracker.recycle();
                mVelocityTracker = null;
            }
            break;
        }
    }

    return true;
}

5. 實現fling方法，用于處理慣性滾動。

private void fling(int velocityY) {
    mScroller.fling(0, getScrollY(), 0, velocityY, 0, 0, Integer.MIN_VALUE, Integer.MAX_VALUE);
    invalidate();
}

6. 重寫computeScroll方法，用于計算滾動偏移。

@Override
public void computeScroll() {
    if (mScroller.computeScrollOffset()) {
        int oldY = getScrollY();
        int y = mScroller.getCurrY();
        if (oldY != y) {
            scrollTo(0, y);
        }
        postInvalidate();
    }
}

7. 最后，不要忘記在自定義控件的onDetachedFromWindow方法中釋放VelocityTracker對象。

@Override
protected void onDetachedFromWindow() {
    super.onDetachedFromWindow();
    if (mVelocityTracker != null) {
        mVelocityTracker.recycle();
        mVelocityTracker = null;
    }
}

現在，你已經實現了一個具有滑動慣性效果的自定義滑動控件。你可以根據需要對其進行進一步的優化和擴展。