Add preserveWatchOutput option to build option and report starting compilation and file changes detected status

Author: sheetalkamat

src/compiler/tsbuild.ts

@@ -53,6 +53,7 @@ namespace ts {
         /*@internal*/ clean?: boolean;
         /*@internal*/ watch?: boolean;
         /*@internal*/ help?: boolean;
+        preserveWatchOutput?: boolean;
     }
 
     enum BuildResultFlags {
@@ -464,6 +465,12 @@ namespace ts {
             host.reportSolutionBuilderStatus(createCompilerDiagnostic(message, ...args));
         }
 
+        function reportWatchStatus(message: DiagnosticMessage, ...args: string[]) {
+            if (hostWithWatch.onWatchStatusChange) {
+                hostWithWatch.onWatchStatusChange(createCompilerDiagnostic(message, ...args), host.getNewLine(), { preserveWatchOutput: context.options.preserveWatchOutput });
+            }
+        }
+
         function startWatching() {
             const graph = getGlobalDependencyGraph()!;
             if (!graph.buildQueue) {
@@ -499,6 +506,7 @@ namespace ts {
             }
 
             function invalidateProjectAndScheduleBuilds(resolved: ResolvedConfigFileName) {
+                reportWatchStatus(Diagnostics.File_change_detected_Starting_incremental_compilation);
                 invalidateProject(resolved);
                 if (!hostWithWatch.setTimeout) {
                     return;
@@ -1037,6 +1045,7 @@ namespace ts {
         }
 
         function buildAllProjects(): ExitStatus {
+            if (context.options.watch) { reportWatchStatus(Diagnostics.Starting_compilation_in_watch_mode); }
             const graph = getGlobalDependencyGraph();
             if (graph === undefined) return ExitStatus.DiagnosticsPresent_OutputsSkipped;
 

src/tsc/tsc.ts

@@ -211,7 +211,13 @@ namespace ts {
                 category: Diagnostics.Command_line_Options,
                 description: Diagnostics.Watch_input_files,
                 type: "boolean"
-            }
+            },
+            {
+                name: "preserveWatchOutput",
+                type: "boolean",
+                category: Diagnostics.Command_line_Options,
+                description: Diagnostics.Whether_to_keep_outdated_console_output_in_watch_mode_instead_of_clearing_the_screen,
+            },
         ];
         let buildOptionNameMap: OptionNameMap | undefined;
         const returnBuildOptionNameMap = () => (buildOptionNameMap || (buildOptionNameMap = createOptionNameMap(buildOpts)));