MinMax property drawer for Unity - Add a [MinMax] attribute to a property to draw a useful min/max setting slider.

MinMaxAttribute.cs

// NOTE DONT put in an editor folder

using UnityEngine;

public class MinMaxAttribute : PropertyAttribute
{
    public float MinLimit = 0;
    public float MaxLimit = 1;
    public bool ShowEditRange;
    public bool ShowDebugValues;

    public MinMaxAttribute(int min, int max)
    {
        MinLimit = min;
        MaxLimit = max;
    }
}

MinMaxDrawer.cs

// NOTE put in a Editor folder

using UnityEngine;
using UnityEditor;

[CustomPropertyDrawer ( typeof ( MinMaxAttribute ) )]
public class MinMaxDrawer : PropertyDrawer
{
    public override void OnGUI ( Rect position, SerializedProperty property, GUIContent label )
    {
        // cast the attribute to make life easier
        MinMaxAttribute minMax = attribute as MinMaxAttribute;

        // This only works on a vector2 and vector2Int! ignore on any other property type (we should probably draw an error message instead!)
        if ( property.propertyType == SerializedPropertyType.Vector2 )
        {
            // if we are flagged to draw in a special mode, lets modify the drawing rectangle to draw only one line at a time
            if ( minMax.ShowDebugValues || minMax.ShowEditRange )
            {
                position = new Rect ( position.x, position.y, position.width, EditorGUIUtility.singleLineHeight );
            }

            // pull out a bunch of helpful min/max values....
            float minValue = property.vector2Value.x; // the currently set minimum and maximum value
            float maxValue = property.vector2Value.y;
            float minLimit = minMax.MinLimit; // the limit for both min and max, min cant go lower than minLimit and maax cant top maxLimit
            float maxLimit = minMax.MaxLimit;

            // and ask unity to draw them all nice for us!
            EditorGUI.MinMaxSlider ( position, label, ref minValue, ref maxValue, minLimit, maxLimit );

            var vec = Vector2.zero; // save the results into the property!
            vec.x = minValue;
            vec.y = maxValue;

            property.vector2Value = vec;

            // Do we have a special mode flagged? time to draw lines!
            if ( minMax.ShowDebugValues || minMax.ShowEditRange )
            {
                bool isEditable = false;
                if ( minMax.ShowEditRange )
                {
                    isEditable = true;
                }

                if ( !isEditable )
                    GUI.enabled = false; // if were just in debug mode and not edit mode, make sure all the UI is read only!

                // move the draw rect on by one line
                position.y += EditorGUIUtility.singleLineHeight;

                float [] vals = new float [] { minLimit, minValue, maxValue, maxLimit }; // shove the values and limits into a vector4 and draw them all at once
                EditorGUI.MultiFloatField ( position, new GUIContent ( "Range" ), new GUIContent [] { new GUIContent ( "MinLimit" ), new GUIContent ( "MinVal" ), new GUIContent ( "MaxVal" ), new GUIContent ( "MaxLimit" ) }, vals );

                GUI.enabled = false; // the range part is always read only
                position.y += EditorGUIUtility.singleLineHeight;
                EditorGUI.FloatField ( position, "Selected Range", maxValue - minValue );
                GUI.enabled = true; // remember to make the UI editable again!

                if ( isEditable )
                {
                    property.vector2Value = new Vector2 ( vals [ 1 ], vals [ 2 ] ); // save off any change to the value~
                }
            }
        }
        else if ( property.propertyType == SerializedPropertyType.Vector2Int )
        {
            // if we are flagged to draw in a special mode, lets modify the drawing rectangle to draw only one line at a time
            if ( minMax.ShowDebugValues || minMax.ShowEditRange )
            {
                position = new Rect ( position.x, position.y, position.width, EditorGUIUtility.singleLineHeight );
            }

            // pull out a bunch of helpful min/max values....
            float minValue = property.vector2IntValue.x; // the currently set minimum and maximum value
            float maxValue = property.vector2IntValue.y;
            int minLimit = Mathf.RoundToInt ( minMax.MinLimit ); // the limit for both min and max, min cant go lower than minLimit and maax cant top maxLimit
            int maxLimit = Mathf.RoundToInt ( minMax.MaxLimit );

            // and ask unity to draw them all nice for us!
            EditorGUI.MinMaxSlider ( position, label, ref minValue, ref maxValue, minLimit, maxLimit );

            var vec = Vector2Int.zero; // save the results into the property!
            vec.x = Mathf.RoundToInt ( minValue );
            vec.y = Mathf.RoundToInt ( maxValue );

            property.vector2IntValue = vec;

            // Do we have a special mode flagged? time to draw lines!
            if ( minMax.ShowDebugValues || minMax.ShowEditRange )
            {
                bool isEditable = false;
                if ( minMax.ShowEditRange )
                {
                    isEditable = true;
                }

                if ( !isEditable )
                    GUI.enabled = false; // if were just in debug mode and not edit mode, make sure all the UI is read only!

                // move the draw rect on by one line
                position.y += EditorGUIUtility.singleLineHeight;

                float [] vals = new float [] { minLimit, minValue, maxValue, maxLimit }; // shove the values and limits into a vector4 and draw them all at once
                EditorGUI.MultiFloatField ( position, new GUIContent ( "Range" ), new GUIContent [] { new GUIContent ( "MinLimit" ), new GUIContent ( "MinVal" ), new GUIContent ( "MaxVal" ), new GUIContent ( "MaxLimit" ) }, vals );

                GUI.enabled = false; // the range part is always read only
                position.y += EditorGUIUtility.singleLineHeight;
                EditorGUI.FloatField ( position, "Selected Range", maxValue - minValue );
                GUI.enabled = true; // remember to make the UI editable again!

                if ( isEditable )
                {
                    property.vector2IntValue = new Vector2Int ( Mathf.RoundToInt ( vals [ 1 ] ), Mathf.RoundToInt ( vals [ 2 ] ) ); // save off any change to the value~
                }
            }
        }
    }

    // this method lets unity know how big to draw the property. We need to override this because it could end up meing more than one line big
    public override float GetPropertyHeight ( SerializedProperty property, GUIContent label )
    {
        MinMaxAttribute minMax = attribute as MinMaxAttribute;

        // by default just return the standard line height
        float size = EditorGUIUtility.singleLineHeight;

        // if we have a special mode, add two extra lines!
        if ( minMax.ShowEditRange || minMax.ShowDebugValues )
        {
            size += EditorGUIUtility.singleLineHeight * 2;
        }

        return size;
    }

}

VolumeRandomizer.cs

// This componnet Demos how to use MinMax attribute! Uses the ReadOnly attribute found here https://gist.github.com/LotteMakesStuff/c0a3b404524be57574ffa5f8270268ea
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

[ExecuteInEditMode]
public class VolumeRandomizer : MonoBehaviour {

    public AudioSource source;
    public float Size;
    [MinMax(0,1, ShowEditRange = true)]
    public Vector2 VolumeRange = new Vector2(0, 1);

	// Use this for initialization
	void Start () {
	}
	
	// Update is called once per frame
    [ExecuteInEditMode]
	void Update () {
    }
}

Minor formatting changes in MinMaxDrawer in Revision 6.