如何以编程方式为按钮创建选择器

Question

How to create a selector programmatically (Kotlin) for an ImageButton with the following requirements.

• Only the background scales by 25% while the image remains the same size while the button is at the Pressed state.
• The background colour of the button at the Pressed state is different from that of the normal (default) state.
• able to set the background colour programmatically for the button so that the same button can be reused.

Thank you for all the help in advance

Answer 1

You will need to recreate the following drawable programmatically:

<selector>
    <item android:drawable="@android:color/holo_blue_light" android:state_pressed="true" />
    <item>
        <inset android:drawable="@android:color/holo_red_light" android:inset="12.5%" />
    </item>
</selector>

This StateListDrawable uses an InsetDrawable to set the background color to something smaller than the view width and height.

In the following layout

<androidx.constraintlayout.widget.ConstraintLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@android:color/darker_gray">

    <View
        android:layout_width="300dp"
        android:layout_height="300dp"
        android:background="@android:color/holo_green_light"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <ImageButton
        android:id="@+id/imageButton"
        android:layout_width="300dp"
        android:layout_height="300dp"
        android:background="@drawable/selector_drawable"
        android:src="@drawable/ic_launcher_foreground"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

the background behaves as follows:

The green square is there just to show the true extent of the ImageButton .

Here is the code to create the StateListDrawable programmatically and to assign it to the ImageButton :

val blue = ContextCompat.getColor(requireContext(), android.R.color.holo_blue_light)  
val red = ContextCompat.getColor(requireContext(), android.R.color.holo_red_light)  
  
// Create the inset drawable that is inset 12.5% on each side. This will be the default state.  
val colorDrawable = ColorDrawable(red)  
val insetDrawable = InsetDrawable(colorDrawable, 0.12f)  
  
// Create the drawable that will be the pressed state background.  
val pressedStateDrawable = ColorDrawable(blue)  
val bg = StateListDrawable()  
bg.addState(intArrayOf(android.R.attr.state_pressed), pressedStateDrawable)  
bg.addState(intArrayOf(), insetDrawable)  
binding.imageButton.background = bg

---

---

If you want to animate the background, you can use a ScaleDrawable with a ValueAnimator . Here is some sample code:

val baseLevel = 7500
val topLevel = 10000
val animationDuration = 250L
val redColor = ContextCompat.getColor(requireContext(), android.R.color.holo_red_light)
val colorDrawable = ColorDrawable(redColor)
val blueColor = ContextCompat.getColor(requireContext(), android.R.color.holo_blue_light)
val scaleDrawable = ScaleDrawable(colorDrawable, Gravity.CENTER, 1f, 1f)
scaleDrawable.level = baseLevel
binding.imageButton.background = scaleDrawable
binding.imageButton.setOnClickListener {
    ValueAnimator.ofInt(baseLevel, topLevel).apply {
        duration = animationDuration
        doOnStart {
            (scaleDrawable.drawable as ColorDrawable).color = blueColor
            scaleDrawable.level = baseLevel
        }
        addUpdateListener {
            scaleDrawable.level = it.animatedValue as Int
            binding.imageButton.invalidate()
        }
        doOnEnd {
            (scaleDrawable.drawable as ColorDrawable).color = redColor
            scaleDrawable.level = baseLevel
        }
        start()
    }
}

---

---

A still better way (IMO) is to encapsulate the expanding drawable as a custom drawable.

ExpandingBackgroundDrawable.kt

class ExpandingBackgroundDrawable(
    matchStates: IntArray,
    activeColor: Int,
    insetPercentage: Float,
    animationDuration: Long
) : Drawable(), Animatable, TimeAnimator.TimeListener {

    private val mMatchStates = matchStates
    private val mInsetPercentage = insetPercentage
    private val mAnimDuration = animationDuration

    private var mRunning = false
    private var mStartTime = 0L
    private val mTimeAnimator: TimeAnimator = TimeAnimator().also {
        it.setTimeListener(this)
    }

    private var mPaint: Paint = Paint(Paint.ANTI_ALIAS_FLAG).apply {
        color = activeColor
        style = Paint.Style.FILL
    }

    private val mRectMin = RectF()
    private val mRectCurrent = RectF()

    // Notify this Drawable when the View's state changes.
    override fun isStateful() = true

    override fun onStateChange(states: IntArray): Boolean {
        return if (StateSet.stateSetMatches(mMatchStates, states)) {
            start()
            true
        } else {
            false
        }
    }

    override fun onBoundsChange(bounds: Rect) {
        super.onBoundsChange(bounds)
        mRectMin.set(
            0f, 0f, bounds.right - mInsetPercentage * bounds.right * 2,
            bounds.bottom - mInsetPercentage * bounds.bottom * 2
        )
    }

    override fun draw(canvas: Canvas) {
        canvas.withTranslation(
            (bounds.width() - mRectCurrent.right) / 2,
            (bounds.height() - mRectCurrent.bottom) / 2
        ) {
            canvas.drawRect(mRectCurrent, mPaint)
        }
    }

    override fun setAlpha(alpha: Int) {
        mPaint.alpha = alpha
    }

    override fun setColorFilter(colorFilter: ColorFilter?) {
        mPaint.colorFilter = colorFilter
    }

    override fun getOpacity() = PixelFormat.TRANSLUCENT

    override fun start() {
        if (isRunning) return
        mRunning = true
        mStartTime = SystemClock.uptimeMillis()
        invalidateSelf()
        mTimeAnimator.duration = mAnimDuration
        mTimeAnimator.start()
    }

    override fun stop() {
        if (!isRunning) return
        mTimeAnimator.cancel()
        mRunning = false
        invalidateSelf()
    }

    override fun isRunning() = mRunning

    override fun onTimeUpdate(animation: TimeAnimator, totalTime: Long, deltaTime: Long) {
        val progress = totalTime.toFloat() / animation.duration
        mRectCurrent.right = mRectMin.right + (bounds.right - mRectMin.right) * progress
        mRectCurrent.bottom = mRectMin.bottom + (bounds.bottom - mRectMin.bottom) * progress
        if (progress >= 1F) {
            stop()
        } else {
            invalidateSelf()
        }
    }

    fun setColor(color: Int) {
        mPaint.color = color
        invalidateSelf()
    }
}

The custom drawable would be created as follows:

imageButton.background = getBackground(context)

private fun getBackground(view: View): Drawable {
    val context = view.context

    // Create the inset drawable that is inset 12.5% on each side.
    // This will be the default state.
    val red = ContextCompat.getColor(context, android.R.color.holo_red_light)
    val colorDrawable = ColorDrawable(red)
    val insetDrawable = InsetDrawable(colorDrawable, 0.125f)

    // Get the expanding background drawable that is the pressed state drawable.
    val expandingStates = intArrayOf(
        android.R.attr.state_pressed,
        android.R.attr.state_accelerated
    )
    val mExpandingBackground = ExpandingBackgroundDrawable(
        expandingStates,
        ContextCompat.getColor(context, R.color.accent),
        0.125f,
        context.resources.getInteger(android.R.integer.config_shortAnimTime).toLong()
    )

    return StateListDrawable().apply {
        addState(expandingStates, mExpandingBackground)
        addState(StateSet.WILD_CARD, insetDrawable)
    }
}

While the state of the ImageButton is "pressed", the background will start as the size of the InsetDrawable and expand to the size of the outer square. When the "pressed" state is removed, the drawable will disappear.

Answer 2

在您的 xml 代码中尝试

android:foreground="?selectableItemBackground"