android 使用setShadowLayer创建简单的阴影

现在Android项目使用阴影的需求越来越多了，下面我价绍一下创建阴影的代码。一般来说ui切图的阴影虽然好看舒服一点，但是不利于动态定制，如果代码里能自己定制多好啊。主要的使用的api是paint.setShaodwLayout。

/**
 * 设置简单的阴影
 */
class SimpleShadowBuilder {

    private var shadowRadius: Float = 2f.dpToPxFloat()
    private var solidColor: Int = Color.WHITE
    private var shadowColor: Int = Color.DKGRAY
    private var dx = 0f
    private var dy = 0f

    private var width: Int = -1
    private var height: Int = -1

    private var shape: Shape = OvalShape()

    fun setShadowRadius(shadowRadius: Float): SimpleShadowBuilder {
        this.shadowRadius = shadowRadius.coerceAtLeast(0f)
        return this
    }

    fun setShadowColor(color: Int): SimpleShadowBuilder {
        shadowColor = color
        return this
    }

    fun setSolidColor(color: Int): SimpleShadowBuilder {
        this.solidColor = color
        return this
    }

    fun setShadowDx(dx: Float): SimpleShadowBuilder {
        this.dx = dx
        return this
    }

    fun setShadowDy(dy: Float): SimpleShadowBuilder {
        this.dy = dy
        return this
    }

    fun width(width: Int): SimpleShadowBuilder {
        this.width = width
        return this
    }

    fun height(height: Int): SimpleShadowBuilder {
        this.height = height
        return this
    }

    fun setShape(shape: Shape): SimpleShadowBuilder {
        this.shape = shape
        return this
    }

    fun roundShape(cornerRadius: Float): SimpleShadowBuilder {
        val outerRadius = floatArrayOf(
            cornerRadius, cornerRadius, cornerRadius, cornerRadius,
            0f, 0f, 0f, 0f
        )
        this.shape = RoundRectShape(outerRadius, null, null)
        return this
    }

    private fun build(): Drawable {
        val shadowDrawable = ShapeDrawable()

        val paint = shadowDrawable.paint
        paint.style = Paint.Style.FILL
        paint.color = solidColor
        paint.isAntiAlias = true
        paint.isFilterBitmap = true
        paint.isDither = true

        paint.setShadowLayer(
            shadowRadius,
            dx.coerceAtMost(shadowRadius),
            dy.coerceAtMost(shadowRadius),
            shadowColor
        )
        shadowDrawable.shape = this.shape

        var contentDr: Drawable = shadowDrawable

        if (strokeWidth > 0 && strokeColor != Color.TRANSPARENT) {
            val gradientDrawable = GradientDrawable()
            gradientDrawable.setColor(Color.TRANSPARENT)
            gradientDrawable.setStroke(strokeWidth, strokeColor)
            if (shape is RoundRectShape && outerRadius != null) {
                val target = FloatArray(8)
                outerRadius?.copyInto(target)
                gradientDrawable.cornerRadii = target
            }
            contentDr = LayerDrawable(arrayOf(shadowDrawable, gradientDrawable))
        }

        return if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            InsetDrawable(contentDr, shadowRadius)
        } else {
            InsetDrawable(contentDr, shadowRadius.toInt())
        }
    }

    fun into(view: View) {
        val parent = view.parent as? ViewGroup
        parent?.clipChildren = false
        parent?.clipToPadding = false

        view.setLayerType(View.LAYER_TYPE_SOFTWARE, null)
        val background = build()
        view.post {
            val shapeDr = (background as InsetDrawable).drawable
            val inset = ceil(shadowRadius).toInt()
            val dxInt = dx.toInt().coerceAtMost(inset)
            val dyInt = dy.toInt().coerceAtMost(inset)
            shapeDr?.setBounds(
                inset - dxInt,
                inset - dyInt,
                (view.measuredWidth - inset) - dxInt,
                (view.measuredHeight - inset - dyInt)
            )
        }
        view.background = background
    }
}

下面贴上使用效果

总的来说，上述方法虽然简单易用，但是存在一定的局限性，会导致硬件加速被关闭。在一些简单的情境下，可以尝试使用该方法。但是如果想要达到更好的效果，建议使用 Google 的 MaterialShapeDrawable。然而，笔者在实践中发现，即便使用 MaterialShapeDrawable，效果也并不是十分理想。希望未来能够找到更加令人满意的解决方案。

如果大家有更好的方案，可以一起讨论一下哦。