Android中的Custom View — 绘图与事件处理

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

    private val paint = Paint(Paint.ANTI_ALIAS_FLAG).apply {
        color = Color.BLUE
        strokeWidth = 4f
        style = Paint.Style.FILL
    }

    override fun onDraw(canvas: Canvas) {
        super.onDraw(canvas)
        canvas.drawCircle(width / 2f,
            height / 2f,
            50f, paint)
    }
}

override fun onMeasure(
    widthMeasureSpec: Int,
    heightMeasureSpec: Int
) {
    val desiredWidth = paddingLeft + paddingRight
        + DEFAULT_WIDTH
    val desiredHeight = paddingTop + paddingBottom
        + DEFAULT_HEIGHT

    val measuredWidth = MeasureSpec.getSize(widthMeasureSpec)
    val modeWidth = MeasureSpec.getMode(widthMeasureSpec)

    val resultWidth = when (modeWidth) {
        MeasureSpec.EXACTLY -> measuredWidth
        MeasureSpec.AT_MOST ->
            desiredWidth.coerceAtMost(measuredWidth)
        else -> desiredWidth
    }

    val resultHeight = resolveSize(desiredHeight,
        heightMeasureSpec)

    setMeasuredDimension(resultWidth, resultHeight)
}

override fun onDraw(canvas: Canvas) {
    super.onDraw(canvas)

    // 背景
    canvas.drawRect(0f, 0f,
        width.toFloat(), height.toFloat(), bgPaint)

    // 文本
    canvas.drawText("自定义视图",
        paddingLeft.toFloat(),
        paddingTop.toFloat() + textPaint.textSize,
        textPaint)

    // 圆形
    canvas.drawCircle(
        width / 2f,
        height / 2f,
        radius, circlePaint)

    // 路径（贝塞尔曲线）
    val path = Path().apply {
        moveTo(0f, height.toFloat())
        quadTo(width / 2f, 0f,
            width.toFloat(), height.toFloat())
    }
    canvas.drawPath(path, pathPaint)
}

<!-- res/values/attrs.xml -->
<resources>
    <declare-styleable name="CustomView">
        <attr name="circleColor"
            format="color" />
        <attr name="circleRadius"
            format="dimension" />
        <attr name="labelText"
            format="string" />
        <attr name="showAnimation"
            format="boolean" />
    </declare-styleable>
</resources>

init {
    val typedArray = context.obtainStyledAttributes(
        attrs,
        R.styleable.CustomView
    )

    circleColor = typedArray.getColor(
        R.styleable.CustomView_circleColor,
        Color.BLUE
    )
    circleRadius = typedArray.getDimension(
        R.styleable.CustomView_circleRadius,
        50f
    )
    labelText = typedArray.getString(
        R.styleable.CustomView_labelText
    ) ?: "Default"

    typedArray.recycle()
}

private val gestureDetector = GestureDetector(
    context, object : GestureDetector.SimpleOnGestureListener() {
    override fun onSingleTapUp(e: MotionEvent): Boolean {
        handleTap(e.x, e.y)
        return true
    }
})

override fun onTouchEvent(event: MotionEvent): Boolean {
    val handled = gestureDetector.onTouchEvent(event)

    when (event.action) {
        MotionEvent.ACTION_MOVE -> {
            currentX = event.x
            currentY = event.y
            invalidate()
            return true
        }
    }

    return handled || super.onTouchEvent(event)
}

class EfficientCustomView
    @JvmOverloads constructor(
    context: Context,
    attrs: AttributeSet? = null
) : View(context, attrs) {

    // 在构造函数中一次性创建的对象
    private val paint = Paint(Paint.ANTI_ALIAS_FLAG)
    private val rect = Rect()
    private val position = PointF()

    fun updatePosition(x: Float, y: Float) {
        position.set(x, y)
        // 仅重绘点周围的区域
        rect.set(
            (x - 10).toInt(),
            (y - 10).toInt(),
            (x + 10).toInt(),
            (y + 10).toInt()
        )
        invalidate(rect)
    }
}