user-interface - 如何以编程方式平滑地滚动页面

Question

我正在使用这段代码以编程方式滚动我的寻呼机

public void MoveNext(View view) {
    pager.setCurrentItem(pager.getCurrentItem() + 1);
}

public void MovePrevious(View view) {

    pager.setCurrentItem(pager.getCurrentItem() - 1);

}

代码运行完美，但转换速度太快。我如何引入延迟以使滚动更顺利？

Answer 1

我相信有两种方法可以实现这个目标，我已经通过 ViewPager 的 fakeDrag() 进行了尝试，但效果并不完美，幸运的是，另一种方法可以通过模拟触摸 Action 事件和使用 ObjectAnimator 指定动画持续时间，然后控制滚动速度。

public class ViewPagerActivity extends FragmentActivity
        implements View.OnClickListener, Animator.AnimatorListener {

    private ViewPager mViewPager;
    private View btnTriggerNext;
    private View btnTriggerPrev;

    @Override
    protected void onCreate(...) {
        super...;
        setContentView(R.layout.layout_xml);

        mViewPager = findViewById(...);

        btnTriggerNext = findViewById(R.id.btnTriggerNext);
        btnTriggerNext.setOnClickListener(this);

        btnTriggerPrev = findViewById(R.id.btnTriggerPrev);
        btnTriggerPrev.setOnClickListener(this);
    }

    private boolean mIsInAnimation;
    private long mMotionBeginTime;
    private float mLastMotionX;

    @Override
    public void onClick(View v) {
        if (mIsInAnimation) return;
        ObjectAnimator anim;

        if (v == btnTriggerPrev) {
            if (!hasPrevPage()) return;
            anim = ObjectAnimator.ofFloat(this, "motionX", 0, mViewPager.getWidth());
        }
        else if (v == btnTriggerNext) {
            if (!hasNextPage()) return;
            anim = ObjectAnimator.ofFloat(this, "motionX", 0, -mViewPager.getWidth());
        }
        else return;

        anim.setInterpolator(new LinearInterpolator());
        anim.addListener(this);
        anim.setDuration(300);
        anim.start();
    }

    public void setMotionX(float motionX) {
        if (!mIsInAnimation) return;
        mLastMotionX = motionX;
        final long time = SystemClock.uptimeMillis();
        simulate(MotionEvent.ACTION_MOVE, mMotionBeginTime, time);
    }

    @Override
    public void onAnimationEnd(Animator animation) {
        mIsInAnimation = false;
        final long time = SystemClock.uptimeMillis();
        simulate(MotionEvent.ACTION_UP, mMotionBeginTime, time);
    }

    @Override
    public void onAnimationStart(Animator animation) {
        mLastMotionX = 0;
        mIsInAnimation = true;
        final long time = SystemClock.uptimeMillis();
        simulate(MotionEvent.ACTION_DOWN, time, time);
        mMotionBeginTime = time;
    }

    // method from http://stackoverflow.com/a/11599282/1294681
    private void simulate(int action, long startTime, long endTime) {
        // specify the property for the two touch points
        MotionEvent.PointerProperties[] properties = new MotionEvent.PointerProperties[1];
        MotionEvent.PointerProperties pp = new MotionEvent.PointerProperties();
        pp.id = 0;
        pp.toolType = MotionEvent.TOOL_TYPE_FINGER;

        properties[0] = pp;

        // specify the coordinations of the two touch points
        // NOTE: you MUST set the pressure and size value, or it doesn't work
        MotionEvent.PointerCoords[] pointerCoords = new MotionEvent.PointerCoords[1];
        MotionEvent.PointerCoords pc = new MotionEvent.PointerCoords();
        pc.x = mLastMotionX;
        pc.pressure = 1;
        pc.size = 1;
        pointerCoords[0] = pc;

        final MotionEvent ev = MotionEvent.obtain(
                startTime, endTime, action, 1, properties,
                pointerCoords, 0,  0, 1, 1, 0, 0, 0, 0);

        mViewPager.dispatchTouchEvent(ev);
    }

    private boolean hasPrevPage() {
        return mViewPager.getCurrentItem() > 0;
    }

    private boolean hasNextPage() {
        return mViewPager.getCurrentItem() + 1 < mViewPager.getAdapter().getCount();
    }

    @Override
    public void onAnimationCancel(Animator animation) {
    }

    @Override
    public void onAnimationRepeat(Animator animation) {
    }
}

因为它是模拟触摸事件，所以请使用适当的持续时间(小于600ms会很好)进行滚动，当滚动进行时，放下手指会停止滚动并导致一些错误。