Android自定義View模仿即刻點贊數位切換效果範例

class LikeView @JvmOverloads constructor(
    context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0
) : View(context, attrs, defStyleAttr) {

    private val paint = Paint().also {
        it.isAntiAlias = true
        it.textSize = 200f
    }

    private val textRect0 = Rect(300, 100, 800, 300)
    private val textRect1 = Rect(300, 300, 800, 500)
    private val textRect2 = Rect(300, 500, 800, 700)

    private var nextNumberAlpha: Int = 0
        set(value) {
            field = value
            invalidate()
        }

    private var currentNumberAlpha: Int = 255
        set(value) {
            field = value
            invalidate()
        }

    private var offsetPercent = 0f
        set(value) {
            field = value
            invalidate()
        }

    private val fontMetrics: FontMetrics = paint.fontMetrics
    private var currentNumber = 99
    private var nextNumber = 0
    private var motionLess = currentNumber.toString()
    private var currentMotion = ""
    private var nextMotion = ""

    private val animator: ObjectAnimator by lazy {
        val nextNumberAlphaAnimator = PropertyValuesHolder.ofInt("nextNumberAlpha", 0, 255)
        val offsetPercentAnimator = PropertyValuesHolder.ofFloat("offsetPercent", 0f, 1f)
        val currentNumberAlphaAnimator = PropertyValuesHolder.ofInt("currentNumberAlpha", 255, 0)
        val animator = ObjectAnimator.ofPropertyValuesHolder(
            this,
            nextNumberAlphaAnimator,
            offsetPercentAnimator,
            currentNumberAlphaAnimator
        )
        animator.duration = 200
        animator.interpolator = DecelerateInterpolator()
        animator.addListener(
            onEnd = {
                currentNumber = nextNumber
            }
        )
        animator
    }

    override fun onDraw(canvas: Canvas) {
        paint.alpha = 255

        paint.color = Color.LTGRAY
        canvas.drawRect(textRect0, paint)

        paint.color = Color.RED
        canvas.drawRect(textRect1, paint)

        paint.color = Color.GREEN
        canvas.drawRect(textRect2, paint)

        paint.color = Color.BLACK
        if (motionLess.isNotEmpty()) {
            drawText(canvas, motionLess, textRect1, 0f)
        }

        if (nextMotion.isEmpty() || currentMotion.isEmpty()) {
            return
        }

        val textHorizontalOffset =
            if (motionLess.isNotEmpty()) paint.measureText(motionLess) else 0f
        if (nextNumber > currentNumber) {
            paint.alpha = currentNumberAlpha
            drawText(canvas, currentMotion, textRect1, textHorizontalOffset, -offsetPercent)
            paint.alpha = nextNumberAlpha
            drawText(canvas, nextMotion, textRect2, textHorizontalOffset, -offsetPercent)
        } else {
            paint.alpha = nextNumberAlpha
            drawText(canvas, nextMotion, textRect0, textHorizontalOffset, offsetPercent)
            paint.alpha = currentNumberAlpha
            drawText(canvas, currentMotion, textRect1, textHorizontalOffset, offsetPercent)
        }
    }

    private fun drawText(
        canvas: Canvas,
        text: String,
        rect: Rect,
        textHorizontalOffset: Float = 0f,
        offsetPercent: Float = 0f
    ) {
        canvas.drawText(
            text,
            rect.left.toFloat() + textHorizontalOffset,
            rect.top + (rect.bottom - rect.top) / 2f - (fontMetrics.bottom + fontMetrics.top) / 2f + offsetPercent * 200,
            paint
        )
    }


    override fun onDetachedFromWindow() {
        super.onDetachedFromWindow()
        animator.end()
    }

    fun plus() {
        if (currentNumber == Int.MAX_VALUE) {
            return
        }
        nextNumber = currentNumber + 1

        processText(findEqualsStringIndex())

        if (animator.isRunning) {
            return
        }
        animator.start()
    }

    fun minus() {
        if (currentNumber == 0) {
            return
        }
        nextNumber = currentNumber - 1
        processText(findEqualsStringIndex())
        if (animator.isRunning) {
            return
        }
        animator.start()
    }

    private fun findEqualsStringIndex(): Int {
        var equalIndex = -1
        val nextNumberStr = nextNumber.toString()
        val currentNumberStr = currentNumber.toString()

        val endIndex = min(currentNumberStr.length, nextNumberStr.length) - 1

        for (index in 0..endIndex) {
            if (nextNumberStr[index] != currentNumberStr[index]) {
                break
            }
            equalIndex = index
        }
        return equalIndex
    }

    private fun processText(index: Int) {
        val currentNumberStr = currentNumber.toString()
        val nextNumberStr = nextNumber.toString()
        if (index == -1) {
            motionLess = ""
            currentMotion = currentNumberStr
            nextMotion = nextNumberStr
        } else {
            motionLess = currentNumberStr.substring(0, index + 1)
            currentMotion = currentNumberStr.substring(index + 1)
            nextMotion = nextNumberStr.substring(index + 1)
        }
    }
}