How To Create A Custom Ring shaped drawable android

Question

How can a achieve a curve like this:

Answer 1

The easiest solution would be to use a VectorDrawable. Create a new drawable

custom_ring.xml

<vector xmlns:android="http://schemas.android.com/apk/res/android"
    android:width="100dp"
    android:height="100dp"
    android:viewportHeight="700"
    android:viewportWidth="700">
    <path
        android:pathData="M0,0Q350,150,700,0L700,200Q400,300,0,200"
        android:strokeColor="@color/colorPrimary"
        android:strokeWidth="1"
        android:fillColor="@color/colorYellow"/>    
</vector>

And then add it as the background for the required view

<FrameLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@drawable/custom_ring" />

Details on VectorDrawables

VectorDrawables are easy enough to understand and it is possible to get simple shapes created within Android Studio itself. For more complex shapes you'd have to resort to other tools to generate SVG files which can later be converted to VectorDrawables in AS.

For details refer to this post to get an idea on how to work with VectorDrawables.

I'll try to give a line by line explanation for the custom_ring.xml file I have used. It is correct to the best of my knowledge though I am open to suggestions and corrections.

Height and Width

Vector drawables are immune to scaling as far as I have observed. The only condition is that the aspect ratio needs to be maintained (I could be wrong here).

When familiarising myself with drawables for the first time, I used to wonder why height and width were required fields. I used to change the values to different values and never observed any change in the preview. It took me longer than truly necessary to realise that this value is required to give the correct dimensions to the view which contains it. For example, if you have an ImageView and set its height and width to wrap_content the ImageView will assume a height and width equal to the value set in the Vector height and width property respectively.

Viewport height and width

I cannot explain better than this image

Setting the viewport as I have in the post makes it possible to actually draw (almost like you'd do with Logo) on a coordinate plane with it's coordinates ranging from (0,0) in the top left corner to (700,700) at the bottom right.

The path

Stroke width: Specifies the width of the outline.

Fill color: Fills the area between the first and last point in the path data with color.

Path data: Probably the most important element and least understood. Please read the post I had linked above. It gives a pretty good explanation.

M0,0 (Moveto instruction) moves the cursor to the coordinate 0,0 without drawing.

Q350,150,700,0 creates a quadratic curve from the current cursor location (which we got by (M0,0)) to (700,0) which is the last 2 parameters of the Q instruction. The first 2 parameters of the Q instruction (350,150) dictate the shape and size of the curve. For example,

<path
    android:pathData="M0,0Q350,750,700,0"
    android:strokeColor="#FF0000"
    android:strokeWidth="10"/>

would generate this curve

while

<path
    android:pathData="M0,0Q50,750,700,0"
    android:strokeColor="#FF0000"
    android:strokeWidth="10"/>

would render the curve like this. Notice the change caused by changing Q350,700,700,0 to Q50,750,700,0

Changing the 2nd parameter will define the amplitude of the curve.

<path
    android:pathData="M0,0Q350,350,700,0"
    android:strokeColor="#FF0000"
    android:strokeWidth="10"/>

will give

L350,350 (Lineto instruction) would draw a line from the current cursor position to the coordinates (350,350)

<path
    android:pathData="M0,0L350,350"
    android:strokeColor="#FF0000"
    android:strokeWidth="10"/>

will draw the below line

That's about all the info you need to figure out how I've written the path data for the curve in the question.