ENH: support preloading fields for setExpr* utilities (#2014)

- can use either command-line option "-load-fields" or dictionary
  entry "readFields" to specify field names to be preloaded.

  Essentially the same functionality as with a readFields function
  object but with a lot less typing.

- tutorial examples provided by Ryan Danks <ryan.danks@rwdi.com>
  illustrate using setExpr* utilities to calculate a quantity
  as a post-processing step.

applications/utilities/preProcessing/setExprBoundaryFields/readFields.H

@@ -0,0 +1,190 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2021 OpenCFD Ltd.
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+Class
+    Foam::readFieldsHandler
+
+Description
+    A simple field-loader, as per the readFields function object
+
+\*---------------------------------------------------------------------------*/
+
+#ifndef readFieldsHander_H
+#define readFieldsHander_H
+
+#include "fvMesh.H"
+#include "volFields.H"
+#include "surfaceFields.H"
+#include "messageStream.H"
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+namespace Foam
+{
+
+/*---------------------------------------------------------------------------*\
+                      Class readFieldsHandler Declaration
+\*---------------------------------------------------------------------------*/
+
+class readFieldsHandler
+{
+    // Private Data
+
+        //- Mesh reference
+        fvMesh& mesh_;
+
+        //- Output logging (verbosity)
+        bool log;
+
+
+    // Private Member Functions
+
+        //- Attempt load from io, store on database if successful
+        template<class FieldType>
+        bool loadAndStore(const IOobject& io)
+        {
+            if (FieldType::typeName == io.headerClassName())
+            {
+                // Store field on mesh database
+                Log << "    Reading " << io.name()
+                    << " (" << FieldType::typeName << ')' << endl;
+
+                mesh_.objectRegistry::store(new FieldType(io, mesh_));
+                return true;
+            }
+
+            return false;
+        }
+
+        //- Forward to loadAndStore for supported types
+        template<class Type>
+        bool loadField(const IOobject& io)
+        {
+            typedef GeometricField<Type, fvPatchField, volMesh> VolFieldType;
+            typedef typename VolFieldType::Internal IntVolFieldType;
+            typedef GeometricField<Type, fvsPatchField, surfaceMesh>
+                SurfaceFieldType;
+
+            return
+            (
+                loadAndStore<VolFieldType>(io)
+             || loadAndStore<IntVolFieldType>(io)
+             || loadAndStore<SurfaceFieldType>(io)
+            );
+        }
+
+
+        //- Load all fields
+        label loadFields(const UList<word>& fieldSet_)
+        {
+            label nLoaded = 0;
+
+            for (const word& fieldName : fieldSet_)
+            {
+                // Already loaded?
+                const auto* ptr = mesh_.cfindObject<regIOobject>(fieldName);
+
+                if (ptr)
+                {
+                    ++nLoaded;
+                    DebugInfo
+                        << "readFields : "
+                        << ptr->name() << " (" << ptr->type()
+                            << ") already in database" << endl;
+                    continue;
+                }
+
+                // Load field as necessary
+                IOobject io
+                (
+                    fieldName,
+                    mesh_.time().timeName(),
+                    mesh_,
+                    IOobject::MUST_READ,
+                    IOobject::NO_WRITE
+                );
+
+                const bool ok =
+                (
+                    io.typeHeaderOk<regIOobject>(false) // Preload header info
+                 && !io.headerClassName().empty()       // Extra safety
+                 &&
+                    (
+                        loadField<scalar>(io)
+                     || loadField<vector>(io)
+                     || loadField<sphericalTensor>(io)
+                     || loadField<symmTensor>(io)
+                     || loadField<tensor>(io)
+                    )
+                );
+
+                if (ok)
+                {
+                    ++nLoaded;
+                }
+                else
+                {
+                    DebugInfo
+                        << "readFields : failed to load " << fieldName
+                        << endl;
+                }
+        }
+
+        return nLoaded;
+    }
+
+
+public:
+
+    static const bool debug = false;
+
+
+    // Constructors
+
+        //- Construct
+        explicit readFieldsHandler(fvMesh& mesh, bool verbose=true)
+        :
+            mesh_(mesh),
+            log(verbose)
+        {}
+
+
+    // Member Functions
+
+        bool execute(const UList<word>& fieldNames)
+        {
+            loadFields(fieldNames);
+            return true;
+        }
+};
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+} // End namespace Foam
+
+#endif
+
+// ************************************************************************* //

applications/utilities/preProcessing/setExprBoundaryFields/setExprBoundaryFields.C

@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2019 OpenCFD Ltd.
+    Copyright (C) 2019-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -44,10 +44,10 @@ Note
 #include "pointMesh.H"
 #include "volFields.H"
 #include "surfaceFields.H"
-#include "surfaceFields.H"
 #include "pointFields.H"
 #include "patchExprDriver.H"
 #include "timeSelector.H"
+#include "readFields.H"
 
 using namespace Foam;
 
@@ -67,20 +67,25 @@ int main(int argc, char *argv[])
         "file",
         "Alternative dictionary for setExprBoundaryFieldsDict"
     );
-
     argList::addBoolOption
     (
         "cache-fields",
         "Cache fields between calls",
         true // Advanced
     );
+    argList::addOption
+    (
+        "load-fields",
+        "wordList",
+        "Specify field or fields to preload. Eg, 'T' or '(p T U)'",
+        true // Advanced
+    );
     argList::addBoolOption
     (
         "backup",
         "Preserve sub-entry as .backup",
         true // Advanced
     );
-
     argList::addBoolOption
     (
         "dry-run",
@@ -128,12 +133,29 @@ int main(int argc, char *argv[])
 
         mesh.readUpdate();
 
+        // preload fields specified on command-line
+        if (timei == 0)
+        {
+            wordList preloadFields;
+            args.readListIfPresent("load-fields", preloadFields);
+            readFieldsHandler(mesh).execute(preloadFields);
+        }
+        // preload fields specified in dictionary
+        {
+            wordList preloadFields;
+            setExprDict.readIfPresent("readFields", preloadFields);
+            readFieldsHandler(mesh).execute(preloadFields);
+        }
+
         for (const entry& dEntry : setExprDict)
         {
             if (!dEntry.isDict())
             {
-                Info<< "Ignoring non-dictionary entry "
-                    << dEntry.keyword() << nl;
+                if (dEntry.keyword() != "readFields")
+                {
+                    Info<< "Ignoring non-dictionary entry "
+                        << dEntry.keyword() << nl;
+                }
                 continue;
             }
 
@@ -193,8 +215,8 @@ int main(int argc, char *argv[])
 
             for (const dictionary& currDict : exprDicts)
             {
-                const word targetName = currDict.get<word>("target");
-                const word patchName = currDict.get<word>("patch");
+                const word patchName(currDict.get<word>("patch"));
+                const word targetName(currDict.get<word>("target"));
 
                 dictionary& patchDict = boundaryFieldDict.subDict(patchName);