如何使用安卓画布绘制一个矩形只有左上角和右上角圆？

You can draw that piece by piece using drawLine() and drawArc() functions from the Canvas.

I would draw two rectangles:

canvas.drawRect(new RectF(0, 110, 100, 290), paint);
canvas.drawRoundRect(new RectF(0, 100, 100, 200), 6, 6, paint);

Or something like that, you just overlap them so that the upper corners will be round. Preferably you should write a method for this

Use a path. It has the advantage of working for APIs less than 21 (Arc is also limited thusly, which is why I quad). Which is a problem because not everybody has Lollipop yet. You can however specify a RectF and set the values with that and use arc back to API 1, but then you wouldn't get to use a static (without declaring a new object to build the object).

Drawing a rounded rect:

    path.moveTo(right, top + ry);
path.rQuadTo(0, -ry, -rx, -ry);
path.rLineTo(-(width - (2 * rx)), 0);
path.rQuadTo(-rx, 0, -rx, ry);
path.rLineTo(0, (height - (2 * ry)));
path.rQuadTo(0, ry, rx, ry);
path.rLineTo((width - (2 * rx)), 0);
path.rQuadTo(rx, 0, rx, -ry);
path.rLineTo(0, -(height - (2 * ry)));
path.close();

As a full function:

static public Path RoundedRect(float left, float top, float right, float bottom, float rx, float ry, boolean conformToOriginalPost) {
Path path = new Path();
if (rx < 0) rx = 0;
if (ry < 0) ry = 0;
float width = right - left;
float height = bottom - top;
if (rx > width/2) rx = width/2;
if (ry > height/2) ry = height/2;
float widthMinusCorners = (width - (2 * rx));
float heightMinusCorners = (height - (2 * ry));


path.moveTo(right, top + ry);
path.rQuadTo(0, -ry, -rx, -ry);//top-right corner
path.rLineTo(-widthMinusCorners, 0);
path.rQuadTo(-rx, 0, -rx, ry); //top-left corner
path.rLineTo(0, heightMinusCorners);


if (conformToOriginalPost) {
path.rLineTo(0, ry);
path.rLineTo(width, 0);
path.rLineTo(0, -ry);
}
else {
path.rQuadTo(0, ry, rx, ry);//bottom-left corner
path.rLineTo(widthMinusCorners, 0);
path.rQuadTo(rx, 0, rx, -ry); //bottom-right corner
}


path.rLineTo(0, -heightMinusCorners);


path.close();//Given close, last lineto can be removed.


return path;
}

You'd want to line all the way to those corner bits, rather than quad across them. This is what setting true to conformToOriginalPost does. Just line to the control point there.

If you want to do that all but don't care about pre-Lollipop stuff, and urgently insist that if your rx and ry are high enough, it should draw a circle.

@TargetApi(Build.VERSION_CODES.LOLLIPOP)
static public Path RoundedRect(float left, float top, float right, float bottom, float rx, float ry, boolean conformToOriginalPost) {
Path path = new Path();
if (rx < 0) rx = 0;
if (ry < 0) ry = 0;
float width = right - left;
float height = bottom - top;
if (rx > width/2) rx = width/2;
if (ry > height/2) ry = height/2;
float widthMinusCorners = (width - (2 * rx));
float heightMinusCorners = (height - (2 * ry));


path.moveTo(right, top + ry);
path.arcTo(right - 2*rx, top, right, top + 2*ry, 0, -90, false); //top-right-corner
path.rLineTo(-widthMinusCorners, 0);
path.arcTo(left, top, left + 2*rx, top + 2*ry, 270, -90, false);//top-left corner.
path.rLineTo(0, heightMinusCorners);
if (conformToOriginalPost) {
path.rLineTo(0, ry);
path.rLineTo(width, 0);
path.rLineTo(0, -ry);
}
else {
path.arcTo(left, bottom - 2 * ry, left + 2 * rx, bottom, 180, -90, false); //bottom-left corner
path.rLineTo(widthMinusCorners, 0);
path.arcTo(right - 2 * rx, bottom - 2 * ry, right, bottom, 90, -90, false); //bottom-right corner
}


path.rLineTo(0, -heightMinusCorners);


path.close();//Given close, last lineto can be removed.
return path;
}

conformToOriginalPost actually draws a rounded rect without the bottom two bits rounded.

draw round rect with left rounded corners

  private void drawRoundRect(float left, float top, float right, float bottom, Paint paint, Canvas canvas) {
Path path = new Path();
path.moveTo(left, top);
path.lineTo(right, top);
path.lineTo(right, bottom);
path.lineTo(left + radius, bottom);
path.quadTo(left, bottom, left, bottom - radius);
path.lineTo(left, top + radius);
path.quadTo(left, top, left + radius, top);
canvas.drawPath(path, onlinePaint);
}

I changed this answer so you can set which corner you want to be round and which one you want to be sharp. also works on pre-lolipop

only top-right and botton-right corners are rounded

 Path path = RoundedRect(0, 0, fwidth , fheight , 5,5,
false, true, true, false);
canvas.drawPath(path,myPaint);

RoundRect:

    public static Path RoundedRect(
float left, float top, float right, float bottom, float rx, float ry,
boolean tl, boolean tr, boolean br, boolean bl
){
Path path = new Path();
if (rx < 0) rx = 0;
if (ry < 0) ry = 0;
float width = right - left;
float height = bottom - top;
if (rx > width / 2) rx = width / 2;
if (ry > height / 2) ry = height / 2;
float widthMinusCorners = (width - (2 * rx));
float heightMinusCorners = (height - (2 * ry));


path.moveTo(right, top + ry);
if (tr)
path.rQuadTo(0, -ry, -rx, -ry);//top-right corner
else{
path.rLineTo(0, -ry);
path.rLineTo(-rx,0);
}
path.rLineTo(-widthMinusCorners, 0);
if (tl)
path.rQuadTo(-rx, 0, -rx, ry); //top-left corner
else{
path.rLineTo(-rx, 0);
path.rLineTo(0,ry);
}
path.rLineTo(0, heightMinusCorners);


if (bl)
path.rQuadTo(0, ry, rx, ry);//bottom-left corner
else{
path.rLineTo(0, ry);
path.rLineTo(rx,0);
}


path.rLineTo(widthMinusCorners, 0);
if (br)
path.rQuadTo(rx, 0, rx, -ry); //bottom-right corner
else{
path.rLineTo(rx,0);
path.rLineTo(0, -ry);
}


path.rLineTo(0, -heightMinusCorners);


path.close();//Given close, last lineto can be removed.


return path;
}

I achieved this by following the below steps.

These are the pre-requisites for the rounded rectangle to look neat

• The radius of the edges have to be equal to the (height of the rectangle / 2). This is because if its any different value then the place where the curve meets straight line of the rectangle will not be

Next is the steps to draw the rounded rectangle.

• First we draw 2 circles on the left and right side, with the radius = height of rectange / 2

• Then we draw a rectangle between these circles to get the desired rounded rectangle.

I am posting the code below

private void drawRoundedRect(Canvas canvas, float left, float top, float right, float bottom) {
float radius = getHeight() / 2;
canvas.drawCircle(radius, radius, radius, mainPaint);
canvas.drawCircle(right - radius, radius, radius, mainPaint);
canvas.drawRect(left + radius, top, right - radius, bottom, mainPaint);
}

Now this results in a really nice rounded rectangle like the one shown below

public static Path composeRoundedRectPath(RectF rect, float topLeftDiameter, float topRightDiameter,float bottomRightDiameter, float bottomLeftDiameter){
Path path = new Path();
topLeftDiameter = topLeftDiameter < 0 ? 0 : topLeftDiameter;
topRightDiameter = topRightDiameter < 0 ? 0 : topRightDiameter;
bottomLeftDiameter = bottomLeftDiameter < 0 ? 0 : bottomLeftDiameter;
bottomRightDiameter = bottomRightDiameter < 0 ? 0 : bottomRightDiameter;


path.moveTo(rect.left + topLeftDiameter/2 ,rect.top);
path.lineTo(rect.right - topRightDiameter/2,rect.top);
path.quadTo(rect.right, rect.top, rect.right, rect.top + topRightDiameter/2);
path.lineTo(rect.right ,rect.bottom - bottomRightDiameter/2);
path.quadTo(rect.right ,rect.bottom, rect.right - bottomRightDiameter/2, rect.bottom);
path.lineTo(rect.left + bottomLeftDiameter/2,rect.bottom);
path.quadTo(rect.left,rect.bottom,rect.left, rect.bottom - bottomLeftDiameter/2);
path.lineTo(rect.left,rect.top + topLeftDiameter/2);
path.quadTo(rect.left,rect.top, rect.left + topLeftDiameter/2, rect.top);
path.close();


return path;
}

One simple and efficient way to draw a solid side is to use clipping - rect clipping is essentially free, and a lot less code to write than a custom Path.

If I want a 300x300 rect, with the top left and right rounded by 50 pixels, you can do:

canvas.save();
canvas.clipRect(0, 0, 300, 300);
canvas.drawRoundRect(new RectF(0, 0, 300, 350), 50, 50, paint);
canvas.restore();

This approach will only work for rounding on 2 or 3 adjacent corners, so it's a little less configurable than a Path based approach, but using round rects is more efficient, since drawRoundRect() is fully hardware accelerated (that is, tessellated into triangles) while drawPath() always falls back to software rendering (software-draw a path bitmap, and upload that to be cached on the GPU).

Not a huge performance issue for small infrequent drawing, but if you're animating paths, the cost of software draw can make your frame times longer, and increase your chance to drop frames. The path mask also costs memory.

If you do want to go with a Path-based approach, I'd recommend using GradientDrawable to simplify the lines of code (assuming you don't need to set a custom shader, e.g. to draw a Bitmap).

mGradient.setBounds(0, 0, 300, 300);
mGradient.setCornerRadii(new int[] {50,50, 50,50, 0,0, 0,0});

With GradientDrawable#setCornerRadii(), you can set any corner to be any roundedness, and reasonably animate between states.

Simple helper function written in Kotlin.

private fun Canvas.drawTopRoundRect(rect: RectF, paint: Paint, radius: Float) {
// Step 1. Draw rect with rounded corners.
drawRoundRect(rect, radius, radius, paint)


// Step 2. Draw simple rect with reduced height,
// so it wont cover top rounded corners.
drawRect(
rect.left,
rect.top + radius,
rect.right,
rect.bottom,
paint
)
}

Usage:

canvas.drawTopRoundRect(rect, paint, radius)

Use PaintDrawable could be better:

    val topLeftRadius = 10
val topRightRadius = 10
val bottomLeftRadius = 0
val bottomRightRadius = 0
val rect = Rect(0, 0, 100, 100)
val paintDrawable = PaintDrawable(Color.RED)
val outter = floatArrayOf(topLeftRadius, topLeftRadius, topRightRadius, topRightRadius,
bottomLeftRadius, bottomLeftRadius, bottomRightRadius, bottomRightRadius)
paintDrawable.setCornerRadii(outter)
paintDrawable.bounds = rect
paintDrawable.draw(canvas)

For API 21 and above the Path class added a new method addRoundRect() which you can use it like this.

corners = new float[]{
80, 80,        // Top left radius in px
80, 80,        // Top right radius in px
0, 0,          // Bottom right radius in px
0, 0           // Bottom left radius in px
};


final Path path = new Path();
path.addRoundRect(rect, corners, Path.Direction.CW);
canvas.drawPath(path, mPaint);

in Kotlin

val corners = floatArrayOf(
80f, 80f,   // Top left radius in px
80f, 80f,   // Top right radius in px
0f, 0f,     // Bottom right radius in px
0f, 0f      // Bottom left radius in px
)


val path = Path()
path.addRoundRect(rect, corners, Path.Direction.CW)
canvas.drawPath(path, mPaint)

This is an old question, however I wanted to add my solution because it uses the native SDK without lots of custom code or hacky drawing. This solution is supported back to API 1.

The way to do this properly is to create a path (as mentioned in other answers) however the previous answers seem to overlook the addRoundedRect function call that takes radii for each corner.

Variables

private val path = Path()
private val paint = Paint()

Setup Paint

paint.color = Color.RED
paint.style = Paint.Style.FILL

Update Path with Size Changes

Call this somewhere that isn't onDraw, such as onMeasure for a view or onBoundChange for a drawable. If it doesn't change (like this example) you could put this code where you set up your paint.

val radii = floatArrayOf(
25f, 25f, //Top left corner
25f, 25f, //Top right corner
0f, 0f,   //Bottom right corner
0f, 0f,   //Bottom left corner
)


path.reset() //Clears the previously set path
path.addRoundedRect(0f, 0f, 100f, 100f, radii, Path.Direction.CW)

This code creates a 100x100 rounded rect with the top corners rounded with a 25 radius.

Draw Path

Call this in onDraw for a view or draw for a drawable.

canvas.drawPath(path, paint)

you can easily achieve this by using Path:

val radiusArr = floatArrayOf(
15f, 15f,
15f, 15f,
0f, 0f,
0f, 0f
)
val myPath = Path()
myPath.addRoundRect(
RectF(0f, 0f, 400f, 400f),
radiusArr,
Path.Direction.CW
)


canvas.drawPath(myPath, paint)

A Path#arcTo() version to draw the rounded side if the radius is half of the height.

fun getPathOfRoundedRectF(
rect: RectF,
topLeftRadius: Float = 0f,
topRightRadius: Float = 0f,
bottomRightRadius: Float = 0f,
bottomLeftRadius: Float = 0f
): Path {
val tlRadius = topLeftRadius.coerceAtLeast(0f)
val trRadius = topRightRadius.coerceAtLeast(0f)
val brRadius = bottomRightRadius.coerceAtLeast(0f)
val blRadius = bottomLeftRadius.coerceAtLeast(0f)


with(Path()) {
moveTo(rect.left + tlRadius, rect.top)


//setup top border
lineTo(rect.right - trRadius, rect.top)


//setup top-right corner
arcTo(
RectF(
rect.right - trRadius * 2f,
rect.top,
rect.right,
rect.top + trRadius * 2f
), -90f, 90f
)


//setup right border
lineTo(rect.right, rect.bottom - trRadius)


//setup bottom-right corner
arcTo(
RectF(
rect.right - brRadius * 2f,
rect.bottom - brRadius * 2f,
rect.right,
rect.bottom
), 0f, 90f
)


//setup bottom border
lineTo(rect.left + blRadius, rect.bottom)


//setup bottom-left corner
arcTo(
RectF(
rect.left,
rect.bottom - blRadius * 2f,
rect.left + blRadius * 2f,
rect.bottom
), 90f, 90f
)


//setup left border
lineTo(rect.left, rect.top + tlRadius)


//setup top-left corner
arcTo(
RectF(
rect.left,
rect.top,
rect.left + tlRadius * 2f,
rect.top + tlRadius * 2f
),
180f,
90f
)


close()


return this
}
}

Here is my answer to the above question. Here, I have created Kotlin extension function which uses Path along with the quadTo function which can be used in lower-level APIs also.

fun Canvas.drawRoundRectPath(
rectF: RectF,
radius: Float,
roundTopLeft: Boolean,
roundTopRight: Boolean,
roundBottomLeft: Boolean,
roundBottomRight: Boolean,
paint: Paint) {


val path = Path()


//Move path cursor to start point
if (roundBottomLeft) {
path.moveTo(rectF.left, rectF.bottom - radius)
} else {
path.moveTo(rectF.left, rectF.bottom)
}


// drawing line and rounding top left curve
if (roundTopLeft) {
path.lineTo(rectF.left, rectF.top + radius)
path.quadTo(rectF.left, rectF.top, rectF.left + radius, rectF.top)
} else {
path.lineTo(rectF.left, rectF.top)
}


// drawing line an rounding top right curve
if (roundTopRight) {
path.lineTo(rectF.right - radius, rectF.top)
path.quadTo(rectF.right, rectF.top, rectF.right, rectF.top + radius)
} else {
path.lineTo(rectF.right, rectF.top)
}


// drawing line an rounding bottom right curve
if (roundBottomRight) {
path.lineTo(rectF.right, rectF.bottom - radius)
path.quadTo(rectF.right, rectF.bottom, rectF.right - radius, rectF.bottom)
} else {
path.lineTo(rectF.right, rectF.bottom)
}


// drawing line an rounding bottom left curve
if (roundBottomLeft) {
path.lineTo(rectF.left + radius, rectF.bottom)
path.quadTo(rectF.left, rectF.bottom, rectF.left, rectF.bottom - radius)
} else {
path.lineTo(rectF.left, rectF.bottom)
}
path.close()


drawPath(path, paint)
}

We can call the function with canvas object and pass the RectF with the dimension on which we want to apply the curve.

Also, we can pass the boolean for the corners which we want to round. This answer can further be customized to accept radius for individual corners.

Maybe the following code can help you

        Paint p = new Paint();
p.setColor(color);


float[] corners = new float[]{
15, 15,        // Top, left in px
15, 15,        // Top, right in px
15, 15,          // Bottom, right in px
15, 15           // Bottom,left in px
};


final Path path = new Path();
path.addRoundRect(rect, corners, Path.Direction.CW);
// Draw
canvas.drawPath(path, p);