Android自定义Drawable的LinearGradient渐变背景显示纯色问题

渐变失效显示纯色是两个代码逻辑错误导致的：

• LinearGradient初始化时机错误：你在类成员声明位置创建渐变实例时，textWidth还未完成赋值，依赖textWidth计算的intrinsicWidth此时返回默认零值，相当于渐变的起止x坐标差极小，没有足够的横向拉伸距离，最终只会渲染单个色值的纯色。
• Paint属性冲突：你给paint设置shader后又赋值了纯白色paint.color = -0x1，部分Android系统版本上，Paint同时设置color和shader时，会优先使用color值做纯色填充，直接覆盖shader效果。

class TestDrawable(textSize: Int = 16) : Drawable() {
    private val rect = RectF()
    private val paint = Paint(Paint.ANTI_ALIAS_FLAG)
    private val textPaint = TextPaint(Paint.ANTI_ALIAS_FLAG)
    private val textWidth: Int
    private val text: String
    private lateinit var backgroundGradient: LinearGradient

    override fun draw(canvas: Canvas) {
        rect.set(bounds)
        canvas.drawRoundRect(
            rect,
            AndroidUtilities.dp(2f).toFloat(),
            AndroidUtilities.dp(2f).toFloat(),
            paint
        )
        canvas.drawText(
            text,
            rect.left + AndroidUtilities.dp(5f),
            rect.top + AndroidUtilities.dp(12f),
            textPaint
        )
    }

    override fun onBoundsChange(bounds: Rect) {
        super.onBoundsChange(bounds)
        // 绘制边界变化时同步更新渐变尺寸，适配实际宽高
        backgroundGradient = LinearGradient(
            0f, 0f, bounds.width().toFloat(), 0f,
            intArrayOf(-0xb73320, -0xafa523, -0x41bf40, -0x457d5),
            floatArrayOf(0.06f, 0.34f, 0.73f, 1f),
            Shader.TileMode.CLAMP
        )
        paint.shader = backgroundGradient
    }

    override fun getIntrinsicWidth(): Int {
        return textWidth + AndroidUtilities.dp((5 * 2).toFloat())
    }

    override fun getIntrinsicHeight(): Int {
        return AndroidUtilities.dp(16f)
    }

    init {
        textPaint.textSize = AndroidUtilities.dp(textSize.toFloat()).toFloat()
        textPaint.typeface = AndroidUtilities.getTypeface("fonts/rmedium.ttf")
        textPaint.color = -0x1000000

        paint.style = Paint.Style.FILL
        // 删除paint.color = -0x1 避免纯色覆盖渐变

        text = "plus".uppercase()
        textWidth = ceil(textPaint.measureText(text).toDouble()).toInt()

        // 等textWidth计算完成后再初始化渐变，保证尺寸参数正确
        backgroundGradient = LinearGradient(
            0f, 0f, intrinsicWidth.toFloat(), 0f,
            intArrayOf(-0xb73320, -0xafa523, -0x41bf40, -0x457d5),
            floatArrayOf(0.06f, 0.34f, 0.73f, 1f),
            Shader.TileMode.CLAMP
        )
        paint.shader = backgroundGradient
        backgroundGradient.setLocalMatrix(Matrix())
    }
}

• 调整渐变初始化顺序：将LinearGradient的创建逻辑移到init块中textWidth赋值完成之后，确保创建渐变时能拿到正确的宽度参数
• 移除Paint的纯色设置：删除paint.color = -0x1代码，避免纯色值覆盖shader的渐变绘制效果
• 新增onBoundsChange回调处理：当Drawable实际绘制区域和固有尺寸不一致时，自动更新渐变的终点坐标，保证渐变始终完整铺满整个绘制区域，不会出现拉伸错位或截断问题

内容的提问来源于stack exchange，提问作者Drics