Making a triangle shape using XML definitions?

In this post I describe how to do it. And here is the XML defining triangle:

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android" >
    <item>
        <rotate
            android:fromDegrees="45"
            android:toDegrees="45"
            android:pivotX="-40%"
            android:pivotY="87%" >
            <shape
                android:shape="rectangle" >
                <stroke android:color="@color/transparent" android:width="10dp"/>
                <solid
                    android:color="@color/your_color_here" />
            </shape>
        </rotate>
    </item>
</layer-list>

Refer to my post if something is unclear or you need explanation how it is built. It is rotated an cutout rectangle :) it is very smart and well working solution.

EDIT: to create an arrow pointing like --> use:

...
android:fromDegrees="45"
android:toDegrees="45"
android:pivotX="13%"
android:pivotY="-40%" >
...

And to create an arrow pointing like <-- use:

android:fromDegrees="45"
android:toDegrees="45"
android:pivotX="87%"
android:pivotY="140%" >

<TextView 
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="▼"/>

You can get here more options.

You can use vector to make triangle like this

ic_triangle_right.xml

<vector xmlns:android="http://schemas.android.com/apk/res/android"
        android:width="24dp"
        android:height="24dp"
        android:viewportWidth="24.0"
        android:viewportHeight="24.0">
    <path
        android:pathData="M0,12l0,12 11.5,-5.7c6.3,-3.2 11.5,-6 11.5,-6.3 0,-0.3 -5.2,-3.1 -11.5,-6.3l-11.5,-5.7 0,12z"
        android:strokeColor="#00000000"
        android:fillColor="#000000"/>
</vector>

Then use it like

<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    app:srcCompat="@drawable/ic_triangle_right"
    />

For change the color and direction, use android:tint and android:rotation

<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    app:srcCompat="@drawable/ic_triangle_right"
    android:rotation="180" // change direction
    android:tint="#00f" // change color
    />

Result

For change the shape of vector, you can change the width/height of vector. Example change width to 10dp

<vector 
        android:width="10dp"
        android:height="24dp"
        >
       ...
</vector>