我认为更好的方法是合并两件事:

制作布局的位图，如下所示。 从位图绘制圆角图形，如下所示 在imageView上设置drawable。

这将处理其他解决方案无法解决的情况，例如内容有角落。

我认为它对gpu更友好，因为它显示了一个单层而不是两层。

唯一更好的方法是创建一个完全自定义的视图，但这需要大量代码，可能会花费大量时间。我认为我在这里提出的建议是两全其美的。

以下是如何做到这一点的一小段:

RoundedCornersDrawable.java

/**
 * shows a bitmap as if it had rounded corners. based on :
 * http://rahulswackyworld.blogspot.co.il/2013/04/android-drawables-with-rounded_7.html
 * easy alternative from support library: RoundedBitmapDrawableFactory.create( ...) ; 
 */
public class RoundedCornersDrawable extends BitmapDrawable {

    private final BitmapShader bitmapShader;
    private final Paint p;
    private final RectF rect;
    private final float borderRadius;

    public RoundedCornersDrawable(final Resources resources, final Bitmap bitmap, final float borderRadius) {
        super(resources, bitmap);
        bitmapShader = new BitmapShader(getBitmap(), Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);
        final Bitmap b = getBitmap();
        p = getPaint();
        p.setAntiAlias(true);
        p.setShader(bitmapShader);
        final int w = b.getWidth(), h = b.getHeight();
        rect = new RectF(0, 0, w, h);
        this.borderRadius = borderRadius < 0 ? 0.15f * Math.min(w, h) : borderRadius;
    }

    @Override
    public void draw(final Canvas canvas) {
        canvas.drawRoundRect(rect, borderRadius, borderRadius, p);
    }
}

CustomView.java

public class CustomView extends ImageView {
    private View mMainContainer;
    private boolean mIsDirty=false;

    // TODO for each change of views/content, set mIsDirty to true and call invalidate

    @Override
    protected void onDraw(final Canvas canvas) {
        if (mIsDirty) {
            mIsDirty = false;
            drawContent();
            return;
        }
        super.onDraw(canvas);
    }

    /**
     * draws the view's content to a bitmap. code based on :
     * http://nadavfima.com/android-snippet-inflate-a-layout-draw-to-a-bitmap/
     */
    public static Bitmap drawToBitmap(final View viewToDrawFrom, final int width, final int height) {
        // Create a new bitmap and a new canvas using that bitmap
        final Bitmap bmp = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
        final Canvas canvas = new Canvas(bmp);
        viewToDrawFrom.setDrawingCacheEnabled(true);
        // Supply measurements
        viewToDrawFrom.measure(MeasureSpec.makeMeasureSpec(canvas.getWidth(), MeasureSpec.EXACTLY),
                MeasureSpec.makeMeasureSpec(canvas.getHeight(), MeasureSpec.EXACTLY));
        // Apply the measures so the layout would resize before drawing.
        viewToDrawFrom.layout(0, 0, viewToDrawFrom.getMeasuredWidth(), viewToDrawFrom.getMeasuredHeight());
        // and now the bmp object will actually contain the requested layout
        canvas.drawBitmap(viewToDrawFrom.getDrawingCache(), 0, 0, new Paint());
        return bmp;
    }

    private void drawContent() {
        if (getMeasuredWidth() <= 0 || getMeasuredHeight() <= 0)
            return;
        final Bitmap bitmap = drawToBitmap(mMainContainer, getMeasuredWidth(), getMeasuredHeight());
        final RoundedCornersDrawable drawable = new RoundedCornersDrawable(getResources(), bitmap, 15);
        setImageDrawable(drawable);
    }

}

编辑:发现了一个不错的替代方案，基于“RoundKornersLayouts”库。有一个类将用于所有你想要扩展的布局类，四舍五入:

//based on https://github.com/JcMinarro/RoundKornerLayouts
class CanvasRounder(cornerRadius: Float, cornerStrokeColor: Int = 0, cornerStrokeWidth: Float = 0F) {
    private val path = android.graphics.Path()
    private lateinit var rectF: RectF
    private var strokePaint: Paint?
    var cornerRadius: Float = cornerRadius
        set(value) {
            field = value
            resetPath()
        }

    init {
        if (cornerStrokeWidth <= 0)
            strokePaint = null
        else {
            strokePaint = Paint()
            strokePaint!!.style = Paint.Style.STROKE
            strokePaint!!.isAntiAlias = true
            strokePaint!!.color = cornerStrokeColor
            strokePaint!!.strokeWidth = cornerStrokeWidth
        }
    }

    fun round(canvas: Canvas, drawFunction: (Canvas) -> Unit) {
        val save = canvas.save()
        canvas.clipPath(path)
        drawFunction(canvas)
        if (strokePaint != null)
            canvas.drawRoundRect(rectF, cornerRadius, cornerRadius, strokePaint)
        canvas.restoreToCount(save)
    }

    fun updateSize(currentWidth: Int, currentHeight: Int) {
        rectF = android.graphics.RectF(0f, 0f, currentWidth.toFloat(), currentHeight.toFloat())
        resetPath()
    }

    private fun resetPath() {
        path.reset()
        path.addRoundRect(rectF, cornerRadius, cornerRadius, Path.Direction.CW)
        path.close()
    }

}

然后，在每个自定义布局类中，添加类似于下面的代码:

class RoundedConstraintLayout : ConstraintLayout {
    private lateinit var canvasRounder: CanvasRounder

    constructor(context: Context) : super(context) {
        init(context, null, 0)
    }

    constructor(context: Context, attrs: AttributeSet) : super(context, attrs) {
        init(context, attrs, 0)
    }

    constructor(context: Context, attrs: AttributeSet, defStyle: Int) : super(context, attrs, defStyle) {
        init(context, attrs, defStyle)
    }

    private fun init(context: Context, attrs: AttributeSet?, defStyle: Int) {
        val array = context.obtainStyledAttributes(attrs, R.styleable.RoundedCornersView, 0, 0)
        val cornerRadius = array.getDimension(R.styleable.RoundedCornersView_corner_radius, 0f)
        val cornerStrokeColor = array.getColor(R.styleable.RoundedCornersView_corner_stroke_color, 0)
        val cornerStrokeWidth = array.getDimension(R.styleable.RoundedCornersView_corner_stroke_width, 0f)
        array.recycle()
        canvasRounder = CanvasRounder(cornerRadius,cornerStrokeColor,cornerStrokeWidth)
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN_MR2) {
            setLayerType(FrameLayout.LAYER_TYPE_SOFTWARE, null)
        }
    }

    override fun onSizeChanged(currentWidth: Int, currentHeight: Int, oldWidth: Int, oldheight: Int) {
        super.onSizeChanged(currentWidth, currentHeight, oldWidth, oldheight)
        canvasRounder.updateSize(currentWidth, currentHeight)
    }

    override fun draw(canvas: Canvas) = canvasRounder.round(canvas) { super.draw(canvas) }

    override fun dispatchDraw(canvas: Canvas) = canvasRounder.round(canvas) { super.dispatchDraw(canvas) }

}

如果你希望支持属性，可以像在库中写的那样使用:

<resources>
  <declare-styleable name="RoundedCornersView">
      <attr name="corner_radius" format="dimension"/>
      <attr name="corner_stroke_width" format="dimension"/>
      <attr name="corner_stroke_color" format="color"/>
  </declare-styleable>
</resources>

另一种选择，对于大多数用途来说可能更容易:使用MaterialCardView。它允许自定义圆角、笔画颜色和宽度以及仰角。

例子:

<FrameLayout
    xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:clipChildren="false" android:clipToPadding="false"
    tools:context=".MainActivity">

    <com.google.android.material.card.MaterialCardView
        android:layout_width="100dp" android:layout_height="100dp" android:layout_gravity="center"
        app:cardCornerRadius="8dp" app:cardElevation="8dp" app:strokeColor="#f00" app:strokeWidth="2dp">

        <ImageView
            android:layout_width="match_parent" android:layout_height="match_parent" android:background="#0f0"/>

    </com.google.android.material.card.MaterialCardView>

</FrameLayout>

结果是:

请注意，如果您使用它，在笔画的边缘有一个轻微的工件问题(在那里留下一些内容像素)。放大就能看到。我在这里报道过这个问题。

编辑:似乎是固定的，但不是在IDE。报告在这里。

我认为更好的方法是合并两件事:

制作布局的位图，如下所示。 从位图绘制圆角图形，如下所示 在imageView上设置drawable。

这将处理其他解决方案无法解决的情况，例如内容有角落。

我认为它对gpu更友好，因为它显示了一个单层而不是两层。

唯一更好的方法是创建一个完全自定义的视图，但这需要大量代码，可能会花费大量时间。我认为我在这里提出的建议是两全其美的。

以下是如何做到这一点的一小段:

RoundedCornersDrawable.java

/**
 * shows a bitmap as if it had rounded corners. based on :
 * http://rahulswackyworld.blogspot.co.il/2013/04/android-drawables-with-rounded_7.html
 * easy alternative from support library: RoundedBitmapDrawableFactory.create( ...) ; 
 */
public class RoundedCornersDrawable extends BitmapDrawable {

    private final BitmapShader bitmapShader;
    private final Paint p;
    private final RectF rect;
    private final float borderRadius;

    public RoundedCornersDrawable(final Resources resources, final Bitmap bitmap, final float borderRadius) {
        super(resources, bitmap);
        bitmapShader = new BitmapShader(getBitmap(), Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);
        final Bitmap b = getBitmap();
        p = getPaint();
        p.setAntiAlias(true);
        p.setShader(bitmapShader);
        final int w = b.getWidth(), h = b.getHeight();
        rect = new RectF(0, 0, w, h);
        this.borderRadius = borderRadius < 0 ? 0.15f * Math.min(w, h) : borderRadius;
    }

    @Override
    public void draw(final Canvas canvas) {
        canvas.drawRoundRect(rect, borderRadius, borderRadius, p);
    }
}

CustomView.java

public class CustomView extends ImageView {
    private View mMainContainer;
    private boolean mIsDirty=false;

    // TODO for each change of views/content, set mIsDirty to true and call invalidate

    @Override
    protected void onDraw(final Canvas canvas) {
        if (mIsDirty) {
            mIsDirty = false;
            drawContent();
            return;
        }
        super.onDraw(canvas);
    }

    /**
     * draws the view's content to a bitmap. code based on :
     * http://nadavfima.com/android-snippet-inflate-a-layout-draw-to-a-bitmap/
     */
    public static Bitmap drawToBitmap(final View viewToDrawFrom, final int width, final int height) {
        // Create a new bitmap and a new canvas using that bitmap
        final Bitmap bmp = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
        final Canvas canvas = new Canvas(bmp);
        viewToDrawFrom.setDrawingCacheEnabled(true);
        // Supply measurements
        viewToDrawFrom.measure(MeasureSpec.makeMeasureSpec(canvas.getWidth(), MeasureSpec.EXACTLY),
                MeasureSpec.makeMeasureSpec(canvas.getHeight(), MeasureSpec.EXACTLY));
        // Apply the measures so the layout would resize before drawing.
        viewToDrawFrom.layout(0, 0, viewToDrawFrom.getMeasuredWidth(), viewToDrawFrom.getMeasuredHeight());
        // and now the bmp object will actually contain the requested layout
        canvas.drawBitmap(viewToDrawFrom.getDrawingCache(), 0, 0, new Paint());
        return bmp;
    }

    private void drawContent() {
        if (getMeasuredWidth() <= 0 || getMeasuredHeight() <= 0)
            return;
        final Bitmap bitmap = drawToBitmap(mMainContainer, getMeasuredWidth(), getMeasuredHeight());
        final RoundedCornersDrawable drawable = new RoundedCornersDrawable(getResources(), bitmap, 15);
        setImageDrawable(drawable);
    }

}

编辑:发现了一个不错的替代方案，基于“RoundKornersLayouts”库。有一个类将用于所有你想要扩展的布局类，四舍五入:

//based on https://github.com/JcMinarro/RoundKornerLayouts
class CanvasRounder(cornerRadius: Float, cornerStrokeColor: Int = 0, cornerStrokeWidth: Float = 0F) {
    private val path = android.graphics.Path()
    private lateinit var rectF: RectF
    private var strokePaint: Paint?
    var cornerRadius: Float = cornerRadius
        set(value) {
            field = value
            resetPath()
        }

    init {
        if (cornerStrokeWidth <= 0)
            strokePaint = null
        else {
            strokePaint = Paint()
            strokePaint!!.style = Paint.Style.STROKE
            strokePaint!!.isAntiAlias = true
            strokePaint!!.color = cornerStrokeColor
            strokePaint!!.strokeWidth = cornerStrokeWidth
        }
    }

    fun round(canvas: Canvas, drawFunction: (Canvas) -> Unit) {
        val save = canvas.save()
        canvas.clipPath(path)
        drawFunction(canvas)
        if (strokePaint != null)
            canvas.drawRoundRect(rectF, cornerRadius, cornerRadius, strokePaint)
        canvas.restoreToCount(save)
    }

    fun updateSize(currentWidth: Int, currentHeight: Int) {
        rectF = android.graphics.RectF(0f, 0f, currentWidth.toFloat(), currentHeight.toFloat())
        resetPath()
    }

    private fun resetPath() {
        path.reset()
        path.addRoundRect(rectF, cornerRadius, cornerRadius, Path.Direction.CW)
        path.close()
    }

}

然后，在每个自定义布局类中，添加类似于下面的代码:

class RoundedConstraintLayout : ConstraintLayout {
    private lateinit var canvasRounder: CanvasRounder

    constructor(context: Context) : super(context) {
        init(context, null, 0)
    }

    constructor(context: Context, attrs: AttributeSet) : super(context, attrs) {
        init(context, attrs, 0)
    }

    constructor(context: Context, attrs: AttributeSet, defStyle: Int) : super(context, attrs, defStyle) {
        init(context, attrs, defStyle)
    }

    private fun init(context: Context, attrs: AttributeSet?, defStyle: Int) {
        val array = context.obtainStyledAttributes(attrs, R.styleable.RoundedCornersView, 0, 0)
        val cornerRadius = array.getDimension(R.styleable.RoundedCornersView_corner_radius, 0f)
        val cornerStrokeColor = array.getColor(R.styleable.RoundedCornersView_corner_stroke_color, 0)
        val cornerStrokeWidth = array.getDimension(R.styleable.RoundedCornersView_corner_stroke_width, 0f)
        array.recycle()
        canvasRounder = CanvasRounder(cornerRadius,cornerStrokeColor,cornerStrokeWidth)
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN_MR2) {
            setLayerType(FrameLayout.LAYER_TYPE_SOFTWARE, null)
        }
    }

    override fun onSizeChanged(currentWidth: Int, currentHeight: Int, oldWidth: Int, oldheight: Int) {
        super.onSizeChanged(currentWidth, currentHeight, oldWidth, oldheight)
        canvasRounder.updateSize(currentWidth, currentHeight)
    }

    override fun draw(canvas: Canvas) = canvasRounder.round(canvas) { super.draw(canvas) }

    override fun dispatchDraw(canvas: Canvas) = canvasRounder.round(canvas) { super.dispatchDraw(canvas) }

}

如果你希望支持属性，可以像在库中写的那样使用:

<resources>
  <declare-styleable name="RoundedCornersView">
      <attr name="corner_radius" format="dimension"/>
      <attr name="corner_stroke_width" format="dimension"/>
      <attr name="corner_stroke_color" format="color"/>
  </declare-styleable>
</resources>

另一种选择，对于大多数用途来说可能更容易:使用MaterialCardView。它允许自定义圆角、笔画颜色和宽度以及仰角。

例子:

<FrameLayout
    xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:clipChildren="false" android:clipToPadding="false"
    tools:context=".MainActivity">

    <com.google.android.material.card.MaterialCardView
        android:layout_width="100dp" android:layout_height="100dp" android:layout_gravity="center"
        app:cardCornerRadius="8dp" app:cardElevation="8dp" app:strokeColor="#f00" app:strokeWidth="2dp">

        <ImageView
            android:layout_width="match_parent" android:layout_height="match_parent" android:background="#0f0"/>

    </com.google.android.material.card.MaterialCardView>

</FrameLayout>

结果是:

请注意，如果您使用它，在笔画的边缘有一个轻微的工件问题(在那里留下一些内容像素)。放大就能看到。我在这里报道过这个问题。

编辑:似乎是固定的，但不是在IDE。报告在这里。

函数以编程方式设置角半径

static void setCornerRadius(GradientDrawable drawable, float topLeft,
        float topRight, float bottomRight, float bottomLeft) {
    drawable.setCornerRadii(new float[] { topLeft, topLeft, topRight, topRight,
            bottomRight, bottomRight, bottomLeft, bottomLeft });
}

static void setCornerRadius(GradientDrawable drawable, float radius) {
    drawable.setCornerRadius(radius);
}

使用

GradientDrawable gradientDrawable = new GradientDrawable();
gradientDrawable.setColor(Color.GREEN);
setCornerRadius(gradientDrawable, 20f);
//or setCornerRadius(gradientDrawable, 20f, 40f, 60f, 80f); 

view.setBackground(gradientDrawable);

 <shape xmlns:android="http://schemas.android.com/apk/res/android"
android:padding="@dimen/_10sdp"
android:shape="rectangle">

<solid android:color="@color/header" />

<corners
    android:bottomLeftRadius="@dimen/_5sdp"
    android:bottomRightRadius="@dimen/_5sdp"
    android:topLeftRadius="@dimen/_5sdp"
    android:topRightRadius="@dimen/_5sdp" />

如果你想要的只是一个简单的圆角矩形，长话短说。

float r=8;
ShapeDrawable shape = 
   new ShapeDrawable (new RoundRectShape(new float[] { r, r, r, r, r, r, r, r },null,null));
shape.getPaint().setColor(Color.RED);
view.setBackground(shape);

前两个浮动用于左上角(其余对按顺时针方向对应)。

要了解更多细节，请阅读这个答案

对于API 21+，使用剪辑视图

在API 21中，圆形轮廓剪辑被添加到View类中。更多信息请参见培训文档或参考资料。

这个内置的功能使圆角非常容易实现。它适用于任何视图或布局，并支持适当的剪辑。

下面是该怎么做:

创建一个可绘制的圆形形状，并将其设置为视图的背景: android:背景= " @drawable / round_outline” 在代码中剪辑轮廓:setClipToOutline(true)

文档曾经说过，你可以设置android:clipToOutline=“true”的XML，但这个错误现在终于解决了，文档现在正确地指出，你只能在代码中这样做。

它的样子:

关于ImageViews的特别说明

setClipToOutline()仅在视图的背景设置为可绘制的形状时才有效。如果此背景形状存在，视图将背景的轮廓作为剪切和阴影目的的边界。

这意味着如果你想圆角的ImageView与setClipToOutline()，你的图像必须来自android:src而不是android:background(因为背景是用于圆角形状)。如果你必须使用背景来设置你的图像而不是src，你可以使用这个嵌套视图的解决方案:

创建一个外部布局，将其背景设置为可绘制的形状 将布局包装在ImageView周围(没有填充) ImageView(包括布局中的任何其他内容)现在将被剪切到外部布局的圆形形状。