定制视图与触摸事件

override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
    // 1. 解析父容器给的约束
    val widthMode = MeasureSpec.getMode(widthMeasureSpec)
    val widthSize = MeasureSpec.getSize(widthMeasureSpec)
    val heightMode = MeasureSpec.getMode(heightMeasureSpec)
    val heightSize = MeasureSpec.getSize(heightMeasureSpec)
 
    // 2. 根据模式决定自己的尺寸
    val desiredWidth = when (widthMode) {
        MeasureSpec.EXACTLY -> widthSize         // 精确值：如 match_parent 或具体 dp
        MeasureSpec.AT_MOST -> min(200, widthSize) // 上限：如 wrap_content
        MeasureSpec.UNSPECIFIED -> 200            // 无限制：如 ScrollView 内的 View
        else -> 200
    }
    val desiredHeight = 200  // 简化：固定高度
 
    // 3. 必须调用 setMeasuredDimension
    setMeasuredDimension(desiredWidth, desiredHeight)
}

override fun onDraw(canvas: Canvas) {
    super.onDraw(canvas)
 
    val paint = Paint().apply {
        isAntiAlias = true  // 抗锯齿，让边缘平滑
    }
 
    // 1. 画矩形
    paint.color = Color.BLUE
    paint.style = Paint.Style.FILL
    canvas.drawRect(50f, 50f, 150f, 150f, paint)
 
    // 2. 画圆形
    paint.color = Color.RED
    paint.style = Paint.Style.STROKE  // 空心
    paint.strokeWidth = 8f
    canvas.drawCircle(300f, 100f, 60f, paint)
 
    // 3. 画文字
    paint.color = Color.BLACK
    paint.style = Paint.Style.FILL
    paint.textSize = 48f
    canvas.drawText("Hello View", 50f, 250f, paint)
 
    // 4. 画路径（直线 + 圆弧）
    paint.color = Color.GREEN
    paint.style = Paint.Style.STROKE
    paint.strokeWidth = 5f
    val path = Path().apply {
        moveTo(50f, 300f)
        lineTo(200f, 300f)
        lineTo(200f, 400f)
    }
    canvas.drawPath(path, paint)
 
    // 5. 画图片
    val bitmap = BitmapFactory.decodeResource(resources, R.drawable.ic_launcher)
    canvas.drawBitmap(bitmap, 300f, 250f, paint)
}

// 把原点平移到 View 中心
canvas.save()                       // 保存当前状态
canvas.translate(width / 2f, height / 2f)
canvas.rotate(45f)                  // 旋转 45 度
paint.color = Color.BLUE
canvas.drawRect(-50f, -50f, 50f, 50f, paint)  // 以新原点为中心画矩形
canvas.restore()                    // 恢复之前的状态

override fun onTouchEvent(event: MotionEvent): Boolean {
    val x = event.x
    val y = event.y
 
    when (event.action) {
        MotionEvent.ACTION_DOWN -> {
            // 手指按下：记录起始位置
            startX = x
            startY = y
            return true  // 返回 true 表示"我要处理这个事件"，后续的事件也会发过来
        }
        MotionEvent.ACTION_MOVE -> {
            // 手指移动：计算偏移量
            dx = x - startX
            dy = y - startY
            invalidate()  // 触发重绘
        }
        MotionEvent.ACTION_UP -> {
            // 手指抬起：处理点击或动作结束
            performClick()
        }
    }
    return super.onTouchEvent(event)
}

class MyCustomView(context: Context, attrs: AttributeSet?) : View(context, attrs) {
 
    private val gestureDetector = GestureDetector(context, object : GestureDetector.SimpleOnGestureListener() {
        override fun onDown(e: MotionEvent): Boolean {
            return true  // 必须返回 true，否则所有手势都不触发
        }
 
        override fun onSingleTapUp(e: MotionEvent): Boolean {
            // 单击
            showToast("单击了")
            return true
        }
 
        override fun onDoubleTap(e: MotionEvent): Boolean {
            // 双击
            showToast("双击了")
            return true
        }
 
        override fun onLongPress(e: MotionEvent) {
            // 长按
            showToast("长按了")
        }
 
        override fun onFling(
            e1: MotionEvent?, e2: MotionEvent?,
            velocityX: Float, velocityY: Float
        ): Boolean {
            // 快速滑动
            if (abs(velocityX) > abs(velocityY)) {
                showToast("水平快速滑动")
            }
            return true
        }
 
        override fun onScroll(
            e1: MotionEvent?, e2: MotionEvent?,
            distanceX: Float, distanceY: Float
        ): Boolean {
            // 拖动
            scrollX += distanceX.toInt()
            invalidate()
            return true
        }
    })
 
    override fun onTouchEvent(event: MotionEvent): Boolean {
        // 先把事件传给 GestureDetector，再处理自己的逻辑
        val handled = gestureDetector.onTouchEvent(event)
        return handled || super.onTouchEvent(event)
    }
}

<declare-styleable name="MyCustomView">
    <attr name="circleColor" format="color" />
    <attr name="circleRadius" format="dimension" />
    <attr name="labelText" format="string" />
</declare-styleable>

class MyCustomView(context: Context, attrs: AttributeSet?) : View(context, attrs) {
    var circleColor: Int = Color.BLUE
    var circleRadius: Float = 60f
    var labelText: String = ""
 
    init {
        context.obtainStyledAttributes(attrs, R.styleable.MyCustomView).apply {
            circleColor = getColor(R.styleable.MyCustomView_circleColor, Color.BLUE)
            circleRadius = getDimension(R.styleable.MyCustomView_circleRadius, 60f)
            labelText = getString(R.styleable.MyCustomView_labelText) ?: ""
            recycle()  // 用完后记得 recycle，否则内存泄漏
        }
    }
}

<com.example.MyCustomView
    android:layout_width="200dp"
    android:layout_height="200dp"
    app:circleColor="#FF0000"
    app:circleRadius="40dp"
    app:labelText="自定义" />