自定义View之SportView

private val SPORT_RADIUS  = 150.dp

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

    private val mPaint = Paint(Paint.ANTI_ALIAS_FLAG)

    private val text = "aaaa"

    //存储文字边界
    private val bounds = Rect()

    private val fontMetrics = Paint.FontMetrics()

    @RequiresApi(Build.VERSION_CODES.LOLLIPOP)
    override fun onDraw(canvas: Canvas) {
        super.onDraw(canvas)

        //绘制圆环
        mPaint.color = Color.GRAY
        mPaint.style = Paint.Style.STROKE
        mPaint.strokeWidth = 20.dp
        canvas.drawCircle((width/2).toFloat(), (height/2).toFloat(),SPORT_RADIUS,mPaint)

        //绘制进度条
        mPaint.color = Color.GREEN
        mPaint.strokeCap = Paint.Cap.ROUND
        canvas.drawArc(width/2- SPORT_RADIUS,height/2- SPORT_RADIUS,width/2+ SPORT_RADIUS,height/2+ SPORT_RADIUS,-90f,225f,false,mPaint)

        //绘制文字
        mPaint.style = Paint.Style.FILL
        mPaint.textSize = 100.dp
        //设置文字左右居中
        mPaint.textAlign = Paint.Align.CENTER
        //文字默认会在纵向基准线baseline的上方
//        mPaint.getTextBounds(text,0,text.length,bounds)
//        Log.d("SportView",bounds.toString())
        //适用于静态的文字居中效果
//        val centerY = height/2f-(bounds.top+bounds.bottom)/2
        //ascent 文字核心部分的顶部
        // descent 文字核心部分的底部
        // top bottom 文字顶部和底部的极限值
        //动态文字居中策略，为了防止文字跳动
        mPaint.getFontMetrics(fontMetrics)
        //核心部分居中
        val centerY = height/2f - (mPaint.ascent()+mPaint.descent())/2
        canvas.drawText(text,width/2f,centerY,mPaint)
    }
}

总结

静态的文字居中效果
使用height/2f-(bounds.top+bounds.bottom)/2
动态文字居中策略，为了防止文字跳动
使用 height/2f - (mPaint.ascent()+mPaint.descent())/2