RatingBar

效果分析：

star_normal.png

star_select.png

准备两张星星图，一张默认，一张选中。初始的时候绘制默认的 5 颗星星，然后根据手势绘制选中的星星。

自定义属性

<declare-styleable name="KRatingbar">
        //要绘制的个数
        <attr name="starNum" format="integer"/>
        //星星之间的间距
        <attr name="starPadding" format="integer" />
        //默认星星的 resId
        <attr name="starNormal" format="reference" />
        //选中星星的 resId
        <attr name="starSelect" format="reference" />
</declare-styleable>

具体实现

class KRatingbar @JvmOverloads constructor(
    context: Context,
    attributes: AttributeSet?,
    defStyle: Int = 0
) : View(context, attributes, defStyle) {

    private var mCount = 5
    private var mNormalBitmap: Bitmap
    private var mSelectBitmap: Bitmap
    private var mStarPadding = dp2Px(5, resources)
    private var mCurSelect = -1

    init {
        val ta = context.obtainStyledAttributes(attributes, R.styleable.KRatingbar)
        mCount = ta.getInt(R.styleable.KRatingbar_starNum, 5)
        mStarPadding = ta.getInt(R.styleable.KRatingbar_starPadding, dp2Px(5, resources))
        val normalId = ta.getResourceId(R.styleable.KRatingbar_starNormal, -1)
        if (normalId == 1.unaryMinus()) throw RuntimeException("starNormal can't be empty")
        mNormalBitmap = BitmapFactory.decodeResource(resources, normalId)
        val selectId = ta.getResourceId(R.styleable.KRatingbar_starSelect, -1)
        if (selectId == 1.unaryMinus()) throw RuntimeException("starSelect can't be empty")
        mSelectBitmap = BitmapFactory.decodeResource(resources, selectId)
        ta.recycle()
    }

    override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec)
        //图片宽度*绘制的个数+星星间的间隙+paddingLeft+paddingRight
        val width =
            mNormalBitmap.width * mCount + mStarPadding * (mCount - 1) + paddingLeft + paddingRight
        //图片的高度+paddingTop+paddingBottom
        val height = mNormalBitmap.height + paddingTop + paddingBottom
        setMeasuredDimension(width, height)
    }

    override fun onDraw(canvas: Canvas) {
        super.onDraw(canvas)
        for (i in 0 until mCount) {
            var x = i * mNormalBitmap.width
            if (i != 0) {
                //平移画布，其实就是在绘制间隙
                canvas.translate(mStarPadding.toFloat(), 0f)
            }
            if (i < mCurSelect) {
                //绘制选中的星星
                canvas.drawBitmap(mSelectBitmap, x.toFloat(), 0f, null)
            } else {
                //绘制默认星星
                canvas.drawBitmap(mNormalBitmap, x.toFloat(), 0f, null)
            }

        }
    }

    override fun onTouchEvent(event: MotionEvent): Boolean {
        when (event.action) {
            MotionEvent.ACTION_DOWN, MotionEvent.ACTION_MOVE, MotionEvent.ACTION_UP -> {
                //根据手指按下的 x 坐标去计算按压的是第几个星星
                //这里要 +1，因为 index 是从 0 开始的
                val count = event.x.toInt() / (mNormalBitmap.width+mStarPadding) + 1
                //当前选中的和之前一样不再重复绘制，count !=1 是为了处理，按压第一个时，无法滑动选中的 bug
                if (mCurSelect == count && count != 1) {
                    return false
                }
                //越界时为 0 
                if (count < 0) {
                    mCurSelect = 0
                }
                //越界时默认为最大的绘制个数
                if (count > mCount) {
                    mCurSelect = mCount
                }
                //当前选中了第几个
                mCurSelect = count
                //重绘
                invalidate()
            }
        }
        //这里要返回 true 才会处理 onTouchEvent 事件
        return true
    }

}

项目地址