Added sample for WithOnFitDelegate

docs/samples/Microsoft.ML.Samples/Dynamic/WithOnFitDelegate.cs

@@ -0,0 +1,75 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.Immutable;
+using System.Linq;
+using Microsoft.ML;
+using Microsoft.ML.Data;
+using Microsoft.ML.Transforms;
+using static Microsoft.ML.Transforms.NormalizingTransformer;
+
+namespace Samples.Dynamic
+{
+    public class WithOnFitDelegate
+    {
+        public static void Example()
+        {
+            // Create a new ML context, for ML.NET operations. It can be used for exception tracking and logging, 
+            // as well as the source of randomness.
+            var mlContext = new MLContext();
+            var samples = new List<DataPoint>()
+            {
+                new DataPoint(){ Features = new float[4] { 8, 1, 3, 0}, Label = true },
+                new DataPoint(){ Features = new float[4] { 6, 2, 2, 0}, Label = true },
+                new DataPoint(){ Features = new float[4] { 4, 0, 1, 0}, Label = false },
+                new DataPoint(){ Features = new float[4] { 2,-1,-1, 1}, Label = false }
+            };
+            // Convert training data to IDataView, the general data type used in ML.NET.
+            var data = mlContext.Data.LoadFromEnumerable(samples);
+
+            // Create a pipeline to normalize the features and train a binary classifier.
+            // We use WithOnFitDelegate for the intermediate binning normalization step,
+            // so that we can inspect the properties of the normalizer after fitting.
+            NormalizingTransformer binningTransformer = null;
+            var pipeline =
+                mlContext.Transforms.NormalizeBinning("Features", maximumBinCount: 3)
+                .WithOnFitDelegate(fittedTransformer => binningTransformer = fittedTransformer)
+                .Append(mlContext.BinaryClassification.Trainers.LbfgsLogisticRegression());
+
+            Console.WriteLine(binningTransformer == null);
+            // Expected Output:
+            //   True
+
+            var model = pipeline.Fit(data);
+
+            // During fitting binningTransformer will get assigned a new value
+            Console.WriteLine(binningTransformer == null);
+            // Expected Output:
+            //   False
+
+            // Inspect some of the properties of the binning transformer
+            var binningParam = binningTransformer.GetNormalizerModelParameters(0) as 
+                BinNormalizerModelParameters<ImmutableArray<float>>;
+
+            for (int i = 0; i < binningParam.UpperBounds.Length; i++)
+            {
+                var upperBounds = string.Join(", ", binningParam.UpperBounds[i]);
+                Console.WriteLine(
+                    $"Bin {i}: Density = {binningParam.Density[i]}, " +
+                    $"Upper-bounds = {upperBounds}");
+            }
+            // Expected output:
+            //   Bin 0: Density = 2, Upper-bounds = 3, 7, Infinity
+            //   Bin 1: Density = 2, Upper-bounds = -0.5, 1.5, Infinity
+            //   Bin 2: Density = 2, Upper-bounds = 0, 2.5, Infinity
+            //   Bin 3: Density = 1, Upper-bounds = 0.5, Infinity
+        }
+
+        private class DataPoint
+        {
+            [VectorType(4)]
+            public float[] Features { get; set; }
+            public bool Label { get; set; }
+        }
+    }
+}
+

src/Microsoft.ML.Data/DataLoadSave/EstimatorExtensions.cs

@@ -135,6 +135,13 @@ public SchemaShape GetOutputSchema(SchemaShape inputSchema)
         /// <see cref="IEstimator{TTransformer}.Fit(IDataView)"/> is called. Because <see cref="IEstimator{TTransformer}.Fit(IDataView)"/>
         /// may be called multiple times, this delegate may also be called multiple times.</param>
         /// <returns>A wrapping estimator that calls the indicated delegate whenever fit is called</returns>
+        /// <example>
+        /// <format type="text/markdown">
+        /// <![CDATA[
+        /// [!code-csharp[OnFit](~/../docs/samples/docs/samples/Microsoft.ML.Samples/Dynamic/WithOnFitDelegate.cs)]
+        /// ]]>
+        /// </format>
+        /// </example>
         public static IEstimator<TTransformer> WithOnFitDelegate<TTransformer>(this IEstimator<TTransformer> estimator, Action<TTransformer> onFit)
             where TTransformer : class, ITransformer
         {