(feat) add --preserveWatchOutput option to svelte-check (#1715)

Context:
I am trying to add svelte to our application, which has previously been
using tsc to typecheck our codebase, (and esbuild to build).

tsc and svelte-check were both annoying when run from a process manager,
such as overmind, which runs multiple cli tools/watchers etc in the same
terminal. This is because they by default clear the screen periodically,
which blows away useful information from other tools.

tsc supports --preserveWatchOutput, so I added the same flag to
svelte-check.

packages/svelte-check/README.md

@@ -55,6 +55,7 @@ Usage:
 | `--workspace <path>`                                            | Path to your workspace. All subdirectories except node_modules and those listed in `--ignore` are checked                                                                                                                                                                                                        |
 | `--output <human\|human-verbose\|machine>`                      |
 | `--watch`                                                       | Will not exit after one pass but keep watching files for changes and rerun diagnostics                                                                                                                                                                                                                           |
+| `--preserveWatchOutput`                                         | Do not clear the screen in watch mode                                                                                                                                                                                                                                                                            |
 | `--tsconfig <path>`                                             | Pass a path to a tsconfig or jsconfig file. The path can be relative to the workspace path or absolute. Doing this means that only files matched by the files/include/exclude pattern of the config file are diagnosed. It also means that errors from TypeScript and JavaScript files are reported.             |
 | `--ignore <path1,path2>`                                        | Files/folders to ignore - relative to workspace root, comma-separated, inside quotes. Example: `--ignore "dist,build"`. When used in conjunction with `--tsconfig`, this will only have effect on the files watched, not on the files that are diagnosed, which is then determined by the `tsconfig.json`        |
 | `--fail-on-warnings`                                            | Will also exit with error code when there are warnings                                                                                                                                                                                                                                                           |

packages/svelte-check/src/index.ts

@@ -163,6 +163,7 @@ function instantiateWriter(opts: SvelteCheckCliOptions): Writer {
             process.stdout,
             opts.outputFormat === 'human-verbose',
             opts.watch,
+            !opts.preserveWatchOutput,
             filter
         );
     } else {

packages/svelte-check/src/options.ts

@@ -6,6 +6,7 @@ export interface SvelteCheckCliOptions {
     workspaceUri: URI;
     outputFormat: OutputFormat;
     watch: boolean;
+    preserveWatchOutput: boolean;
     tsconfig?: string;
     filePathsToIgnore: string[];
     failOnWarnings: boolean;
@@ -34,6 +35,7 @@ export function parseOptions(cb: (opts: SvelteCheckCliOptions) => any) {
             'Will not exit after one pass but keep watching files for changes and rerun diagnostics',
             false
         )
+        .option('--preserveWatchOutput', 'Do not clear the screen in watch mode', false)
         .option(
             '--tsconfig',
             'Pass a path to a tsconfig or jsconfig file. The path can be relative to the workspace path or absolute. Doing this means that only files matched by the files/include/exclude pattern of the config file are diagnosed. It also means that errors from TypeScript and JavaScript files are reported.'
@@ -72,6 +74,7 @@ export function parseOptions(cb: (opts: SvelteCheckCliOptions) => any) {
                 workspaceUri,
                 outputFormat: getOutputFormat(opts),
                 watch: !!opts.watch,
+                preserveWatchOutput: !!opts.preserveWatchOutput,
                 tsconfig: getTsconfig(opts, workspaceUri.fsPath),
                 filePathsToIgnore: getFilepathsToIgnore(opts),
                 failOnWarnings: !!opts['fail-on-warnings'],

packages/svelte-check/src/writers.ts

@@ -24,11 +24,12 @@ export class HumanFriendlyWriter implements Writer {
         private stream: Writable,
         private isVerbose = true,
         private isWatchMode = false,
+        private clearScreen = true,
         private diagnosticFilter: DiagnosticFilter = DEFAULT_FILTER
     ) {}
 
     start(workspaceDir: string) {
-        if (process.stdout.isTTY && this.isWatchMode) {
+        if (process.stdout.isTTY && this.isWatchMode && this.clearScreen) {
             // Clear screen
             const blank = '\n'.repeat(process.stdout.rows);
             this.stream.write(blank);