EditorGUILayout Fields which properly handles multi-object editing

EditorGUILayoutExtensions.cs

using System;
using UnityEngine;
using UnityEditor;

public static class EditorGUILayoutExtensions
{
    /// <summary>
    /// Add a EditorGUILayout.ToggleLeft which properly handles multi-object editing
    /// </summary>
    public static void ToggleLeft(this SerializedProperty prop, GUIContent label, params GUILayoutOption[] options)
    {
        EditorGUI.BeginChangeCheck();
        EditorGUI.showMixedValue = prop.hasMultipleDifferentValues;
        var newValue = EditorGUILayout.ToggleLeft(label, prop.boolValue, options);
        EditorGUI.showMixedValue = false;
        if (EditorGUI.EndChangeCheck())
            prop.boolValue = newValue;
    }

    /// <summary>
    /// Create a EditorGUILayout.Slider which properly handles multi-object editing
    /// We apply the 'convIn' conversion to the SerializedProperty value before exposing it as a Slider.
    /// We apply the 'convOut' conversion to the Slider value to store it back to the SerializedProperty.
    /// </summary>
    /// <param name="prop">The value the slider shows. This determines the position of the draggable thumb.</param>
    /// <param name="label">Label in front of the slider.</param>
    /// <param name="leftValue">The value at the left end of the slider.</param>
    /// <param name="rightValue">The value at the right end of the slider.</param>
    /// <param name="convIn">Conversion function applied on the SerializedProperty to get the Slider value</param>
    /// <param name="convOut">Conversion function applied on the Slider value to get the SerializedProperty</param>
    public static void FloatSlider(
        this SerializedProperty prop,
        GUIContent label,
        float leftValue, float rightValue,
        Func<float, float> convIn,
        Func<float, float> convOut,
        params GUILayoutOption[] options)
    {
        var floatValue = convIn(prop.floatValue);
        EditorGUI.BeginChangeCheck();
        {
            EditorGUI.showMixedValue = prop.hasMultipleDifferentValues;
            {
                floatValue = EditorGUILayout.Slider(label, floatValue, leftValue, rightValue, options);
            }
            EditorGUI.showMixedValue = false;
        }
        if (EditorGUI.EndChangeCheck())
            prop.floatValue = convOut(floatValue);
    }
}

UsageExampleEditor.cs

using UnityEditor;
using UnityEngine;

[CustomEditor(typeof(UsageExample))]
[CanEditMultipleObjects]
public class UsageExampleEditor : Editor
{
    SerializedProperty propBool;
    SerializedProperty propFloat;

    void OnEnable()
    {
        propBool = serializedObject.FindProperty("MyBoolProp");
        propFloat = serializedObject.FindProperty("MyFloatProp");
    }

    public override void OnInspectorGUI()
    {
        serializedObject.Update();

        ///////////////////////////////////////////////
        // HOW TO PROPERLY EXPOSE A TOGGLE LEFT
        ///////////////////////////////////////////////

        // /!\ THIS CODE DOESN'T PROPERLY HANDLE MULTI-OBJECT EDITING /!\
        propBool.boolValue = EditorGUILayout.ToggleLeft("My Bool", propBool.boolValue);

        // THIS IS HOW YOU PROPERLY HANDLE MULTI-OBJECT EDITING
        EditorGUI.BeginChangeCheck();
        EditorGUI.showMixedValue = propBool.hasMultipleDifferentValues;
        var newValue = EditorGUILayout.ToggleLeft("My Bool", propBool.boolValue);
        EditorGUI.showMixedValue = false;
        if (EditorGUI.EndChangeCheck())
            propBool.boolValue = newValue;

        // OR USE THIS EXTENSION METHOD
        propBool.ToggleLeft(new GUIContent("My Bool"));


        //////////////////////////////////////////////////////////////
        // HOW TO PROPERLY EXPOSE A FLOAT SLIDER WITH SOME CONVERSION
        // for example we want to expose 1/value as a slider
        //////////////////////////////////////////////////////////////

        // /!\ THIS CODE DOESN'T PROPERLY HANDLE MULTI-OBJECT EDITING /!\
        var exposedFloatValue = 1f / propFloat.floatValue;
        exposedFloatValue = EditorGUILayout.Slider("My Float", exposedFloatValue, 0.1f, 1.0f);
        propFloat.floatValue = 1f / exposedFloatValue;

        // USE THIS NICE EXTENSION METHOD INSTEAD :)
        propFloat.FloatSlider(
            new GUIContent("My Float"),
            0.1f, 1.0f,
            (v) => 1f / v,      // conversion SerializedProperty => Slider
            (v) => 1f / v);     // conversion Slider => SerializedProperty

        serializedObject.ApplyModifiedProperties();
    }
}