Android自定義View相關總結

繼承View重寫onDraw方法

通常用于實作復雜的、不規則的效果，需要自己支持wrap_content和padding，自定義圓形進度條組件的kotlin代碼

class MyCircleProgress : View {
    private var mWidth = 0
    private var mHeight = 0
    private var progress = 0.6

    constructor(context: Context) : super(context) {

    }

    constructor(context: Context, attr: AttributeSet) : super(context, attr) {

    }

    private fun dp2px(dp: Int): Int {
        return TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, dp.toFloat(), Resources.getSystem().displayMetrics).toInt()
    }

    override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec)
        val widthSize = MeasureSpec.getSize(widthMeasureSpec)
        val widthMode = MeasureSpec.getMode(widthMeasureSpec)
        val heightSize = MeasureSpec.getSize(heightMeasureSpec)
        val heightMode = MeasureSpec.getMode(heightMeasureSpec)
        when(widthMode) {
        	// 處理match_parent和固定寬度的情況
            MeasureSpec.UNSPECIFIED, MeasureSpec.EXACTLY -> mWidth = widthSize
            // 處理wrap_content
            MeasureSpec.AT_MOST -> mWidth = dp2px(200)
        }
        when(heightMode) {
            MeasureSpec.UNSPECIFIED, MeasureSpec.EXACTLY -> mHeight = heightSize
            MeasureSpec.AT_MOST -> mHeight = dp2px(200)
        }
        setMeasuredDimension(mWidth, mHeight)
    }

    override fun onDraw(canvas: Canvas?) {
        canvas?.apply {
            val paint  = Paint().apply {
            	// Paint設定的顏色默認是有透明度的，且透明度為0
            	// 并且在kotlin中0xff00ffff超過了int的范圍，所以這里要呼叫toInt()
                color = 0xff00ffff.toInt()
                isAntiAlias = true
                this.strokeWidth = 10F
                style = Paint.Style.STROKE
            }
            // 設定圓弧的繪制區域，處理padding
            val rectF = RectF(10F + paddingLeft, 10F + paddingTop,
                (mWidth - 10 - paddingRight).toFloat(), (mHeight - 10 - paddingBottom).toFloat())
            // 繪制圓弧，表示進度
            drawArc(rectF, -90F, (360 * progress).toFloat(), false, paint)
            paint.color = 0xff000000.toInt()
            paint.style = Paint.Style.FILL
            paint.textSize = 50F
            // 繪制文字，顯示進度百分比
            drawText("${(100 * progress).toInt()} %", (mWidth / 2).toFloat(), (mHeight / 2).toFloat(), paint)
        }
    }

}

繼承特定的View重寫onDraw方法

用于擴展某種已有View的功能，比如TextView、AppCompatImageView，不需要自己支持wrap_content和padding，自定義圓形ImageView的kotlin代碼

class MyImageView : androidx.appcompat.widget.AppCompatImageView {

    constructor(context: Context) : super(context) {

    }

    constructor(context: Context, attr: AttributeSet) : super(context, attr) {

    }

    override fun onDraw(canvas: Canvas?) {
        val paint = Paint().apply {
            color = 0xffffffff.toInt()
            isAntiAlias = true
        }
        val xfermode = PorterDuffXfermode(PorterDuff.Mode.SRC_IN)

        canvas?.apply {
            saveLayer(0F, 0F, width.toFloat(), height.toFloat(), null, Canvas.ALL_SAVE_FLAG)
            // 直接使用clipPath會有鋸齒問題
            // canvas.clipPath(path)
            canvas.drawCircle((width / 2).toFloat(), (height / 2).toFloat(), (width / 2 - 1).toFloat(), paint)
            val bitmap = drawable.toBitmap()
            val bitmapRect = Rect(0, 0, bitmap.width, bitmap.height)
            val canvasRect = Rect(0, 0, width, height)
            paint.xfermode = xfermode

            drawBitmap(bitmap, bitmapRect, canvasRect, paint)
            restore()
        }
    }
}

最終效果

除了以上兩種方法，我們還可以繼承ViewGroup和特定的ViewGroup（比如FrameLayout）來實作自定義View，在實際開發中，常常需要實作幾種View組合在一起的效果，可以繼承特定的ViewGroup來實作（一般不直接繼承ViewGroup，除非必要）