Glide自定义CropTransformation

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裁剪是Android开发中一个常见的需求，虽然Android的ImageView的scaleType属性提供了多种图片的展示方式，其中包括一些裁剪方式，以及Glide自带的Transformation也提供了常见的圆角，圆形，居中裁剪，但有的时候仍然不能满足我们的需求，比如说为了版本迭代老版本apk能兼容新版本的图片资源，UI迭代的情况下产品会提出裁剪图片保留图片底部区域的规则，这时候ImageView和Glide自带的裁剪就不能满足需求了，需要我们自定义Transformation实现新的裁剪规则

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裁剪类型

定义枚举：顶部裁剪，居中裁剪，底部裁剪

enum class CropType {
     TOP, CENTER, BOTTOM
}

实线BitmapTransformation接口

默认居中裁剪，支持传入自定义的裁剪像素宽高，重写必要的方法，updateDiskCacheKey，transform，equals，hashCode

private val TAG: String = CropTransformation::class.java.name

class CropTransformation(var width: Int = 0, var height: Int = 0, var cropType: CropType = CropType.CENTER) : BitmapTransformation() {
   
    override fun updateDiskCacheKey(messageDigest: MessageDigest) {
        messageDigest.update(TAG.toByteArray(CHARSET))
    }

    override fun transform(pool: BitmapPool, toTransform: Bitmap, outWidth: Int, outHeight: Int): Bitmap {
        ...
    }

    override fun equals(other: Any?): Boolean {
        return other is CropTransformation && other.width == width && other.height == height && other.cropType == cropType
    }

    override fun hashCode(): Int {
        return TAG.hashCode()
    }
}

实现核心方法transform

• transform的入参为，图片复用线程池pool，下载后要处理的图片toTransform，目标View的宽高

• 当宽或高为0时默认使用图片的宽或高

• 图片格式默认采用要处理图片的格式，否则使用ARGB_8888

• 从当前图片缓存池中取得一张复用的图片进行处理，打开alpha通道

• 计算裁剪的宽和图片的宽之比，以及高之比，取大的那个

• 计算缩放后的宽和高

• 计算绘制的开始位置，x和y坐标，x坐标为（当前裁剪的宽度 - 缩放后的宽度）/ 2，从水平居中区域开始绘制，y坐标根据当前的裁剪类型而定，TOP从0开始，CENTER为（当前裁剪的高度 - 缩放后的高度）/ 2，从垂直居中区域开始，BOTTOM直接裁剪底部区域，结束位置的x坐标为开始位置的x坐标 + 缩放后的宽度，y坐标为开始位置的y坐标 + 缩放后的高度，最后计算出绘制的矩形区域，设置从当前图片缓存池中取得的图片密度为要处理的图片toTransform的密度，利用复用的图片创建canvas，drawBitmap把要处理的图片toTransform绘制到canvas的目标矩形区域

  override fun transform(pool: BitmapPool, toTransform: Bitmap, outWidth: Int, outHeight: Int): Bitmap {
        width = if (width == 0) toTransform.width else width
        height = if (height == 0) toTransform.height else height

        val config = if (toTransform.config != null) toTransform.config else Bitmap.Config.ARGB_8888
        val bitmap = pool[width, height, config]

        bitmap.setHasAlpha(true)

        val scaleX = width.toFloat() / toTransform.width
        val scaleY = height.toFloat() / toTransform.height
        val scale = max(scaleX, scaleY)

        val scaledWidth = scale * toTransform.width
        val scaledHeight = scale * toTransform.height
        val left = (width - scaledWidth) / 2
        val top: Float = getTop(scaledHeight)
        val targetRect = RectF(left, top, left + scaledWidth, top + scaledHeight)

        bitmap.density = toTransform.density
        val canvas = Canvas(bitmap)
        canvas.drawBitmap(toTransform, null, targetRect, null)

        return bitmap
  }

private fun getTop(scaledHeight: Float): Float {
  return when (cropType) {
    CropType.TOP -> 0f
    CropType.CENTER -> (height - scaledHeight) / 2
    CropType.BOTTOM -> height - scaledHeight
  }
}

完整代码如下：

package com.kubi.kucoin.utils

import android.graphics.Bitmap
import android.graphics.Canvas
import android.graphics.RectF
import com.bumptech.glide.load.engine.bitmap_recycle.BitmapPool
import com.bumptech.glide.load.resource.bitmap.BitmapTransformation
import java.security.MessageDigest
import kotlin.math.max

/**
 * @description: 图片裁剪
 * @author: Jessie.Li
 * @email: jessie.li@corp.kucoin.com
 * @create: 2022-01-05 17:47
 **/
private val TAG: String = CropTransformation::class.java.name

class CropTransformation(var width: Int = 0, var height: Int = 0, var cropType: CropType = CropType.CENTER) : BitmapTransformation() {
    enum class CropType {
        TOP, CENTER, BOTTOM
    }

    override fun updateDiskCacheKey(messageDigest: MessageDigest) {
        messageDigest.update(TAG.toByteArray(CHARSET))
    }

    override fun transform(pool: BitmapPool, toTransform: Bitmap, outWidth: Int, outHeight: Int): Bitmap {
        width = if (width == 0) toTransform.width else width
        height = if (height == 0) toTransform.height else height

        val config = if (toTransform.config != null) toTransform.config else Bitmap.Config.ARGB_8888
        val bitmap = pool[width, height, config]

        bitmap.setHasAlpha(true)

        val scaleX = width.toFloat() / toTransform.width
        val scaleY = height.toFloat() / toTransform.height
        val scale = max(scaleX, scaleY)

        val scaledWidth = scale * toTransform.width
        val scaledHeight = scale * toTransform.height
        val left = (width - scaledWidth) / 2
        val top: Float = getTop(scaledHeight)
        val targetRect = RectF(left, top, left + scaledWidth, top + scaledHeight)

        bitmap.density = toTransform.density
        val canvas = Canvas(bitmap)
        canvas.drawBitmap(toTransform, null, targetRect, null)

        return bitmap
    }

    private fun getTop(scaledHeight: Float): Float {
        return when (cropType) {
            CropType.TOP -> 0f
            CropType.CENTER -> (height - scaledHeight) / 2
            CropType.BOTTOM -> height - scaledHeight
        }
    }

    override fun equals(other: Any?): Boolean {
        return other is CropTransformation && other.width == width && other.height == height && other.cropType == cropType
    }

    override fun hashCode(): Int {
        return TAG.hashCode()
    }
}