无法在 Android 中设置自定义图像视图的图像资源

Question

我根据在 this and this codelab 中学到的经验创建了一个自定义视图项目。在我的项目中，我尝试不仅在视图上绘制，而且在自定义 ImageView 上绘制。因此，我创建了一个自定义 ImageView 并执行了上述官方代码实验室中的所有步骤。这是我的习惯 ImageView 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 XML 布局 MainActivity 的样子（在代码实验室中没有人，他们只是以编程方式使用自定义视图）：

<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 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：没有显示我要绘制的图像资源。绘图部分的工作方式与代码实验室中的一样。但是我想仅用于测试目的的 R.drawable.ic_launcher_foreground 可绘制对象没有显示在屏幕上。为什么？

希望有人能帮忙。

Answer 1

在自定义视图中，canvas 仅用于绘制由 extraCanvas 更新的 extraBitmap，因此 MyImageView 仅处理 extraBitmap 的绘制extraBitmap.

setImageResource 属于 ImageView class，它在内部处理资源到可绘制对象的转换并将其绘制在 canvas 内部 onDraw() 但是onDraw 被不处理接收到的位图绘图的自定义视图覆盖，因此解决方案是将 super.onDraw 调用为：

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 并添加代码以通过 setImageResource 方法在 onDraw 的 canvas 中绘制接收到的资源 MyImageView.

Answer 2

您也可以使用 setBackgroundResource(..):

来实现

binding.myImageView.setBackgroundResource(R.drawable.ic_launcher_foreground)

所以您的 MainActivity 看起来像：

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

使用 purple 绘制颜色，稍微绘制一下并使用 ic_launcher_foreground 作为背景图像后，结果会是这样的：

不要忘记，正如@Pavneet_Singh所建议的：

call super on the onDraw method you override (as in super.onDraw(canvas))

无法在 Android 中设置自定义图像视图的图像资源

Can not set image resource of a custom imageview in Android

android

android-imageview

android-canvas

android-custom-view

android-paint