android - 无法在 Android 中设置自定义 ImageView 的图像资源

Question

我根据我在 this 中学到的经验创建了一个自定义 View 项目。和 this codelab .
在我的项目中，我不仅尝试在 View 上绘制，还尝试在自定义 ImageView 上绘制.因此，我创建了一个自定义 ImageView并按照上述官方代码实验室中的所有步骤进行操作。
这是我的自定义 ImageView类(class):

// Stroke width for the the paint.
private const val STROKE_WIDTH = 12f

class MyImageView @JvmOverloads constructor(
    context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0
) : AppCompatImageView(context, attrs, defStyleAttr) {

    private var path = Path()
    private val drawColor = ResourcesCompat.getColor(resources, R.color.colorPaint, null)

    private lateinit var extraCanvas : Canvas
    private lateinit var extraBitmap : Bitmap

    private val touchTolerance = ViewConfiguration.get(context).scaledTouchSlop

    private var currentX = 0f
    private var currentY = 0f

    private var motionTouchEventX = 0f
    private var motionTouchEventY = 0f

    // Set up the paint with which to draw.
    private val paint = Paint().apply {
        color = drawColor
        // Smooths out edges of what is drawn without affecting shape.
        isAntiAlias = true
        // Dithering affects how colors with higher-precision than the device are down-sampled.
        isDither = true
        style = Paint.Style.STROKE // default: FILL
        strokeJoin = Paint.Join.ROUND // default: MITER
        strokeCap = Paint.Cap.ROUND // default: BUTT
        strokeWidth = STROKE_WIDTH // default: Hairline-width (really thin)
    }

    init{
        init()
    }

    private fun init(){


        setOnTouchListener(OnTouchListener { _, event ->
            event?.let {
                motionTouchEventX = it.x
                motionTouchEventY = it.y

                when(it.action){
                    MotionEvent.ACTION_DOWN -> touchStart()
                    MotionEvent.ACTION_MOVE -> touchMove()
                    MotionEvent.ACTION_UP -> touchUp()
                }
                return@OnTouchListener true
            }
            false
        })
    }

    override fun onDraw(canvas: Canvas?) {
        canvas?.drawBitmap(extraBitmap, 0f, 0f, null)
    }

    override fun onSizeChanged(w: Int, h: Int, oldw: Int, oldh: Int) {
        super.onSizeChanged(w, h, oldw, oldh)

        if (::extraBitmap.isInitialized) extraBitmap.recycle()
        extraBitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888)
        extraCanvas = Canvas(extraBitmap)
    }


    private fun touchStart() {
        path.reset()
        path.moveTo(motionTouchEventX, motionTouchEventY)
        currentX = motionTouchEventX
        currentY = motionTouchEventY
    }

    private fun touchMove() {
        val dx = Math.abs(motionTouchEventX - currentX)
        val dy = Math.abs(motionTouchEventY - currentY)
        if (dx >= touchTolerance || dy >= touchTolerance) {
            // QuadTo() adds a quadratic bezier from the last point,
            // approaching control point (x1,y1), and ending at (x2,y2).
            path.quadTo(currentX, currentY, (motionTouchEventX + currentX) / 2, (motionTouchEventY + currentY) / 2)
            currentX = motionTouchEventX
            currentY = motionTouchEventY
            // Draw the path in the extra bitmap to save it.
            extraCanvas.drawPath(path, paint)
        }
        // Invalidate() is inside the touchMove() under ACTION_MOVE because there are many other
        // types of motion events passed into this listener, and we don't want to invalidate the
        // view for those.
        invalidate()
    }

    private fun touchUp() {
        // Reset the path so it doesn't get drawn again.
        path.reset()
    }
}

这就是我的 MainActivity 的 XML 布局好像
(在 codelab 中没有人，他们只是以编程方式使用自定义 View ):

<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto">


    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".MainActivity">

        <com.celik.abdullah.drawingonimageview.MyImageView
            android:id="@+id/myImageView"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />

    </androidx.constraintlayout.widget.ConstraintLayout>
</layout>

这是我的 MainActivity.kt类(class):

class MainActivity : AppCompatActivity() {

    private lateinit var binding: ActivityMainBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = DataBindingUtil.setContentView(this, R.layout.activity_main)
        binding.myImageView.setImageResource(R.drawable.ic_launcher_foreground)
    }
}

我的问题是:未显示我要绘制的图像资源。
绘图部分的工作方式类似于代码实验室。但是 R.drawable.ic_launcher_foreground我只想用于测试目的的 drawable 没有显示在屏幕上。为什么 ？

我希望有人可以提供帮助。

Answer 1

在自定义 View 中， Canvas 仅用于绘制 extraBitmap正在由 extraCanvas 更新所以MyImageView只处理 extraBitmap 的绘图.
setImageResource属于ImageView内部处理资源到可绘制对象的转换并将其绘制在其内部的 Canvas 上的类 onDraw()但是 onDraw正在被不处理接收位图绘制的自定义 View 覆盖，因此解决方案是调用 super.onDraw作为:

override fun onDraw(canvas: Canvas?) {
    super.onDraw(canvas)
    // ^^^^^^^^^^^^^^^^ add this to execute imageview's onDraw for image handling
    canvas?.drawBitmap(extraBitmap, 0f, 0f, null)
}

或者，您可以 override setImageResource并通过 setImageResource 添加绘制接收到的资源的代码 canvas上的方法内 onDraw的 MyImageView .