【Kotlin设计模式】Kotlin实现装饰器模式

前言

装饰器模式（Decorator Pattern），用于动态地为对象添加新功能，而无需修改其结构，通过使用不用装饰类及这些装饰类的排列组合，可以实现不同的功能和效果，但是这样的效果就是会增加很多类，过度使用增加程序的复杂性。

适配器模式主要包括以下几个角色：

1、组件接口（Component）：定义一个对象接口，可以有基本功能或抽象功能。
2、具体组件（ConcreteComponent）：实现组件接口的基本功能。
3、装饰器（Decorator）：持有一个组件对象的引用，并实现组件接口。
4、具体装饰器（ConcreteDecorator）：实现了装饰器，并为组件对象添加了具体的附加功能。

在这里插入图片描述

实现

以下例子实现装饰器模式，定义图像处理接口BitmapProcessor，提供处理图片方法方法，具体组件实现基本的 BitmapProcessor，提供原始的 Bitmap对象。


interface BitmapProcessor {fun process(bitmap: Bitmap): Bitmap
}class BasicBitmapProcessor(private val bitmap: Bitmap) : BitmapProcessor {override fun process(bitmap: Bitmap): Bitmap {return this.bitmap}
}

实现一个通用的装饰器，持有一个 BitmapProcessor 对象的引用。


abstract class BitmapProcessDecorator(private val process: BitmapProcessor) : BitmapProcessor {override fun process(bitmap: Bitmap): Bitmap {return process.process(bitmap)}
}

实现具体装饰器继承，选择图片、缩放图片功能。


class RotateDecorator(process: BitmapProcessor, private val angle:Float): BitmapProcessDecorator(process) {override fun process(bitmap: Bitmap): Bitmap {val matrix = Matrix().apply {postRotate(angle)}return Bitmap.createBitmap(bitmap, 0, 0, bitmap.width, bitmap.height, matrix, true)}
}class ScaleDecorator(process: BitmapProcessor, private val sx: Float, private val sy: Float) : BitmapProcessDecorator(process) {override fun process(bitmap: Bitmap): Bitmap {val matrix = Matrix().apply {postScale(sx, sy)}return Bitmap.createBitmap(bitmap, 0, 0, bitmap.width, bitmap.height, matrix, true)}
}

客户端创建对应的装饰器


//创建原始图片对象
val originalBitmap = BitmapFactory.decodeResource(resources, R.drawable.ic_launcher_foreground)//创建具体图片处理对象
val basicProcessor = BasicBitmapProcessor(originalBitmap)//创建图片旋转装饰器
val rotateDecorator = RotateDecorator(basicProcessor, 45.0f)
rotateDecorator.process(originalBitmap)   //旋转90度//创建图片缩放装饰器
val scaleDecorator = ScaleDecorator(basicProcessor, 2.0f, 2.0f)
scaleDecorator.process(originalBitmap)    //缩放2倍//处理图片，显示
val processorBitmap= basicProcessor.process(originalBitmap)
mBinding.image.setImageBitmap(processorBitmap)

在Android中，经常使用的控件RecyclerView中就提供了装饰器模式的应用，ItemDecoration它允许你在不修改原始 RecyclerView 的情况下为每个项添加装饰比如分割线、边距等。


/*** An ItemDecoration allows the application to add a special drawing and layout offset* to specific item views from the adapter's data set. This can be useful for drawing dividers* between items, highlights, visual grouping boundaries and more.*/
public abstract static class ItemDecoration {/*** Draw any appropriate decorations into the Canvas supplied to the RecyclerView.* Any content drawn by this method will be drawn before the item views are */public void onDraw(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull State state) {onDraw(c, parent);}/*** Draw any appropriate decorations into the Canvas supplied to the RecyclerView.* Any content drawn by this method will be drawn after the item views are drawn*/public void onDrawOver(@NonNull Canvas c, @NonNull RecyclerView parent,@NonNull State state) {onDrawOver(c, parent);}/*** Retrieve any offsets for the given item. Each field of <code>outRect</code> specifies* the number of pixels that the item view should be inset by, similar to padding or margin.* The default implementation sets the bounds of outRect to 0 and returns.** <p>* If this ItemDecoration does not affect the positioning of item views, it should set* all four fields of <code>outRect</code> (left, top, right, bottom) to zero* before returning.** <p>* If you need to access Adapter for additional data, you can call* {@link RecyclerView#getChildAdapterPosition(View)} to get the adapter position of the* View.** @param outRect Rect to receive the output.* @param view    The child view to decorate* @param parent  RecyclerView this ItemDecoration is decorating* @param state   The current state of RecyclerView.*/public void getItemOffsets(@NonNull Rect outRect, @NonNull View view,@NonNull RecyclerView parent, @NonNull State state) {getItemOffsets(outRect, ((LayoutParams) view.getLayoutParams()).getViewLayoutPosition(),parent);}
}

下面实现为RecyclerView的条目添加分割线，继承ItemDecoration类，实现onDrawOver方法。


class DividerItemDecoration(context: Context) : RecyclerView.ItemDecoration() {private val divider: Drawable?init {val styledAttributes = context.obtainStyledAttributes(intArrayOf(android.R.attr.listDivider))divider = styledAttributes.getDrawable(0)styledAttributes.recycle()}override fun onDrawOver(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {val left = parent.paddingLeftval right = parent.width - parent.paddingRightval childCount = parent.childCountfor (i in 0 until childCount - 1) {val child = parent.getChildAt(i)val params = child.layoutParams as RecyclerView.LayoutParamsval top = child.bottom + params.bottomMarginval bottom = top + divider!!.intrinsicHeightdivider.setBounds(left, top, right, bottom)divider.draw(c)}}
}