用ViewPager2实现简易的计数器

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这是一个仿物理计数器的APP，利用手机的音量按钮实现计数，音量加就是累加一个数，音量减就是减去一个数。功能极其简单，这回仍旧用了ViewPager2的上下滚动功能。只当是练练手了。好了，不多解释了，直接上代码，相信你们都看得明白。

class MainActivity : AppCompatActivity() {
    private var count = 0

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        initVPCounter()
        btnReset.setOnClickListener {
            count = 0
            updateCounter()
        }
    }

    override fun onKeyDown(keyCode: Int, event: KeyEvent): Boolean {
        return when (keyCode) {
            KeyEvent.KEYCODE_VOLUME_UP -> {
                count++
                updateCounter()
                true
            }
            KeyEvent.KEYCODE_VOLUME_DOWN -> {
                if (count > 0)
                    count--
                updateCounter()
                true
            }
            else -> super.onKeyDown(keyCode, event)
        }
    }

    private fun initVPCounter() {
        arrayOf(vpCount1, vpCount2, vpCount3, vpCount4).forEach {
            buildCounter(it)
        }
    }

    private fun buildCounter(vpCount: ViewPager2) {
        vpCount.isUserInputEnabled = false // 禁止手滑控件
        vpCount.offscreenPageLimit = 1
        vpCount.orientation = ORIENTATION_VERTICAL // 垂直方向
        vpCount.adapter = object : RecyclerView.Adapter<RecyclerView.ViewHolder>() {
            override fun onCreateViewHolder(
                parent: ViewGroup,
                viewType: Int
            ): RecyclerView.ViewHolder {
                val view = LayoutInflater.from(parent.context)
                    .inflate(R.layout.item_counter, parent, false)
                return object : RecyclerView.ViewHolder(view) {}
            }

            override fun getItemCount() = 11

            override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
                holder.itemView.tvCounterNumber.text = (position % 10).toString()
            }
        }
        vpCount.registerOnPageChangeCallback(object : ViewPager2.OnPageChangeCallback() {
            override fun onPageScrollStateChanged(state: Int) {
                // 当滚动结束并且滚动到最后一页时再跳转到第一页，跳转时无动画
                if (state == ViewPager2.SCROLL_STATE_IDLE && vpCount.currentItem == 10) {
                    vpCount.setCurrentItem(0, false)
                }
            }
        })
    }

    private fun updateCounter() {
        vpCount1.currentItem = count / 1000
        vpCount2.currentItem = count % 1000 / 100
        vpCount3.currentItem = count % 100 / 10
        vpCount4.currentItem = count % 10
    }
}

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