Xfermode的完全使用解析

作用：将不同的绘制内容结合到一起
先绘制 一张图片

package com.example.viewtest.view

import android.content.Context
import android.graphics.Bitmap
import android.graphics.BitmapFactory
import android.graphics.Canvas
import android.graphics.Paint
import android.util.AttributeSet
import android.view.View
import com.example.viewtest.R
import com.example.viewtest.ext.dp

private val IMAGE_WIDTH = 200.dp
private val IMAGE_PADDING = 20.dp
class AvatarView(context: Context, attrs: AttributeSet) : View(context, attrs) {
    
    private val paint = Paint(Paint.ANTI_ALIAS_FLAG)

    override fun onDraw(canvas: Canvas) {
        canvas.drawBitmap(getAvatar(IMAGE_WIDTH.toInt()), IMAGE_PADDING, IMAGE_PADDING, paint)
    }

    fun getAvatar(width: Int): Bitmap {
        val options = BitmapFactory.Options()
        // 第一次只读上下左右四个角的尺寸，这个速度是超级快的
        options.inJustDecodeBounds = true
        BitmapFactory.decodeResource(resources, R.drawable.ic_choice_select, options)
        options.inJustDecodeBounds = false
        // 本身的大小
        options.inDensity = options.outWidth
        // 实际想要的大小
        options.inTargetDensity = width
        // 无论原图有多大，都能读出一个很小，很快的图
        return BitmapFactory.decodeResource(resources, R.drawable.ic_choice_select, options)
    }
}

然后再绘制一个圆形，重叠在一起，圆形之外的部分透明显示

package com.example.viewtest.view

import android.content.Context
import android.graphics.Bitmap
import android.graphics.BitmapFactory
import android.graphics.Canvas
import android.graphics.Paint
import android.graphics.PorterDuff
import android.graphics.PorterDuffXfermode
import android.graphics.RectF
import android.util.AttributeSet
import android.view.View
import com.example.viewtest.R
import com.example.viewtest.ext.dp

private val IMAGE_WIDTH = 200.dp
private val IMAGE_PADDING = 20.dp
private val XFERMODE = PorterDuffXfermode(PorterDuff.Mode.SRC_IN)
class AvatarView(context: Context, attrs: AttributeSet) : View(context, attrs) {

    private val paint = Paint(Paint.ANTI_ALIAS_FLAG)
    private val bounds = RectF(IMAGE_PADDING, IMAGE_PADDING, IMAGE_PADDING + IMAGE_WIDTH,IMAGE_PADDING + IMAGE_WIDTH )

    override fun onDraw(canvas: Canvas) {
        val count = canvas.saveLayer(bounds, null)
        canvas.drawOval(bounds, paint)
        paint.xfermode = XFERMODE
        canvas.drawBitmap(getAvatar(IMAGE_WIDTH.toInt()), IMAGE_PADDING, IMAGE_PADDING, paint)
        paint.xfermode = null
        canvas.restoreToCount(count)
    }

    private fun getAvatar(width: Int): Bitmap {
        val options = BitmapFactory.Options()
        // 第一次只读上下左右四个角的尺寸，这个速度是超级快的
        options.inJustDecodeBounds = true
        BitmapFactory.decodeResource(resources, R.drawable.qr_download, options)
        options.inJustDecodeBounds = false
        // 本身的大小
        options.inDensity = options.outWidth
        // 实际想要的大小
        options.inTargetDensity = width
        // 无论原图有多大，都能读出一个很小，很快的图
        return BitmapFactory.decodeResource(resources, R.drawable.qr_download, options)
    }
}