viewpager 循环滑动

要实现 ViewPager 的循环滑动，可以通过以下步骤：

1. 继承 ViewPager 类，重写 `onTouchEvent` 方法，使其支持循环滑动。

public class LoopViewPager extends ViewPager {

    public LoopViewPager(Context context) {
        super(context);
    }

    public LoopViewPager(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        int action = event.getAction();
        if (action == MotionEvent.ACTION_DOWN) {
            stopAutoScroll();
        } else if (action == MotionEvent.ACTION_UP || action == MotionEvent.ACTION_CANCEL) {
            startAutoScroll();
        }
        return super.onTouchEvent(event);
    }
}

2. 重写 `onMeasure` 方法，使其支持 wrap_content。

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    int height = 0;
    for (int i = 0; i < getChildCount(); i++) {
        View child = getChildAt(i);
        child.measure(widthMeasureSpec, MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED));
        int h = child.getMeasuredHeight();
        if (h > height) {
            height = h;
        }
    }
    heightMeasureSpec = MeasureSpec.makeMeasureSpec(height, MeasureSpec.EXACTLY);
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}

3. 重写 `setAdapter` 方法，使其支持循环滑动。

@Override
public void setAdapter(PagerAdapter adapter) {
    super.setAdapter(adapter);
    setCurrentItem(0);
}

@Override
public void setCurrentItem(int item) {
    int realCount = getRealCount();
    if (realCount == 0) {
        super.setCurrentItem(item);
        return;
    }
    int position = getRealPosition(item);
    super.setCurrentItem(position);
}

private int getRealCount() {
    PagerAdapter adapter = getAdapter();
    if (adapter == null) {
        return 0;
    }
    return adapter.getCount();
}

private int getRealPosition(int position) {
    int realCount = getRealCount();
    if (realCount == 0) {
        return 0;
    }
    return position % realCount;
}

4. 在 `onPageSelected` 回调中处理循环滑动的逻辑。

@Override
public void onPageSelected(int position) {
    int realCount = getRealCount();
    if (realCount == 0) {
        return;
    }
    int realPosition = getRealPosition(position);
    if (realPosition == 0) {
        setCurrentItem(realCount, false);
    } else if (realPosition == realCount - 1) {
        setCurrentItem(1, false);
    }
}

5. 在 `startAutoScroll` 和 `stopAutoScroll` 方法中处理自动滑动的逻辑。

private void startAutoScroll() {
    stopAutoScroll();
    mHandler.postDelayed(mAutoScrollTask, mInterval);
}

private void stopAutoScroll() {
    mHandler.removeCallbacks(mAutoScrollTask);
}

private Runnable mAutoScrollTask = new Runnable() {
    @Override
    public void run() {
        int currentItem = getCurrentItem();
        setCurrentItem(currentItem + 1, true);
        mHandler.postDelayed(this, mInterval);
    }
};

完整的实现代码如下：

public class LoopViewPager extends ViewPager {

    private static final int DEFAULT_INTERVAL = 3000;

    private Handler mHandler = new Handler();
    private int mInterval = DEFAULT_INTERVAL;

    public LoopViewPager(Context context) {
        super(context);
        init();
    }

    public LoopViewPager(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    private void init() {
        setPageTransformer(true, new DefaultTransformer());
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        int action = event.getAction();
        if (action == MotionEvent.ACTION_DOWN) {
            stopAutoScroll();
        } else if (action == MotionEvent.ACTION_UP || action == MotionEvent.ACTION_CANCEL) {
            startAutoScroll();
        }
        return super.onTouchEvent(event);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int height = 0;
        for (int i = 0; i < getChildCount(); i++) {
            View child = getChildAt(i);
            child.measure(widthMeasureSpec, MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED));
            int h = child.getMeasuredHeight();
            if (h > height) {
                height = h;
            }
        }
        heightMeasureSpec = MeasureSpec.makeMeasureSpec(height, MeasureSpec.EXACTLY);
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }

    @Override
    public void setAdapter(PagerAdapter adapter) {
        super.setAdapter(adapter);
        setCurrentItem(0);
    }

    @Override
    public void setCurrentItem(int item) {
        int realCount = getRealCount();
        if (realCount == 0) {
            super.setCurrentItem(item);
            return;
        }
        int position = getRealPosition(item);
        super.setCurrentItem(position);
    }

    @Override
    public void setCurrentItem(int item, boolean smoothScroll) {
        int realCount = getRealCount();
        if (realCount == 0) {
            super.setCurrentItem(item, smoothScroll);
            return;
        }
        int position = getRealPosition(item);
        super.setCurrentItem(position, smoothScroll);
    }

    @Override
    public int getCurrentItem() {
        int realCount = getRealCount();
        if (realCount == 0) {
            return super.getCurrentItem();
        }
        int position = super.getCurrentItem();
        return getRealPosition(position);
    }

    private int getRealCount() {
        PagerAdapter adapter = getAdapter();
        if (adapter == null) {
            return 0;
        }
        return adapter.getCount();
    }

    private int getRealPosition(int position) {
        int realCount = getRealCount();
        if (realCount == 0) {
            return 0;
        }
        return position % realCount;
    }

    @Override
    public void onPageSelected(int position) {
        int realCount = getRealCount();
        if (realCount == 0) {
            return;
        }
        int realPosition = getRealPosition(position);
        if (realPosition == 0) {
            setCurrentItem(realCount, false);
        } else if (realPosition == realCount - 1) {
            setCurrentItem(1, false);
        }
    }

    public void setInterval(int interval) {
        mInterval = interval;
    }

    public void startAutoScroll() {
        stopAutoScroll();
        mHandler.postDelayed(mAutoScrollTask, mInterval);
    }

    public void stopAutoScroll() {
        mHandler.removeCallbacks(mAutoScrollTask);
    }

    private Runnable mAutoScrollTask = new Runnable() {
        @Override
        public void run() {
            int currentItem = getCurrentItem();
            setCurrentItem(currentItem + 1, true);
            mHandler.postDelayed(this, mInterval);
        }
    };

    private static class DefaultTransformer implements ViewPager.PageTransformer {
        @Override
        public void transformPage(View page, float position) {
            if (position < -1) {
                page.setAlpha(0);
            } else if (position <= 1) {
                float scaleFactor = Math.max(0.75f, 1 - Math.abs(position - 0.125f));
                page.setScaleX(scaleFactor);
                page.setScaleY(scaleFactor);
            } else {
                page.setAlpha(0);
            }
        }
    }
}

使用方式：

<com.example.LoopViewPager
    android:id="@+id/view_pager"
    android:layout_width="match_parent"
    android:layout_height="wrap_content" />

LoopViewPager viewPager = findViewById(R.id.view_pager);
PagerAdapter adapter = new MyPagerAdapter();
viewPager.setAdapter(adapter);
viewPager.setInterval(3000);
viewPager.startAutoScroll();