Detect --watch mode reliably

[s-panferov]

Hi @sokra. I'm making https://github.com/s-panferov/awesome-typescript-loader and I need some way to detect if webpack is working in --watch mode or not (without plugins). I need this because I need to know if I should kill a spawned child_process or not.

Right now I'm doing something like:

function isWatching(compiler) {
    return compiler.options.watch || compiler.outputFileSystem.constructor.name === 'MemoryFileSystem';
}

But in doesn't seems to work in the latest webpack@beta. Can you please advise some reliable solution?

Right now the only way I see is to use a compiler plugin:

export const WatchModeSymbol = Symbol('WatchMode');

export class DetectWatchPlugin {
    apply(compiler) {
        compiler.plugin("run", function(params, callback) {
            compiler[WatchModeSymbol] = false;
            callback();
        });

        compiler.plugin("watch-run", function(params, callback) {
            compiler[WatchModeSymbol] = true;
            callback();
        });
    }
}

[eeue56]

I also need this for the elm-webpack-loader

[TheLarkInn]

@sokra Maybe we can pass this in with a small plugin that leverages: 'normal-module-loader' hook from Compilation.

@s-panferov I'm going to test something and see if it works. There is a hook found in NormalModule.js that does the following:

	compilation.applyPlugins("normal-module-loader", loaderContext, this);
	if(options.loader)
		for(var key in options.loader)
			loaderContext[key] = options.loader[key];

This adds information to the loaderContext. If @sokra agree's it's an okay direction maybe we can add a small plugin that detects whether compiler.watch or not. and set a loaderContext.watch = true/false

[TheLarkInn]

Further research shows that it could be passed through to loaderContext in this manner:

		function apply(compiler) {
			compiler.plugin('compilation', function(compilation) {
				compilation.plugin('normal-module-loader', function(loaderContext, nmf) {
					loaderContext.watch = compiler.options.watch;
				});
			});
		}

But again, I'll yield to @sokra to ensure this is an okay practice to use this hook. Otherwise we could try and implement it into core in NormalModule

[s-panferov]

@TheLarkInn the main point is how to make this without any plugins because they require more configuration from a user side.

[s-panferov]

Also please note that you can't rely only on compiler.options.watch, because webpack-dev-server doesn't set it while watching.

[TheLarkInn]

I'm saying internal implementation from core, IE new feature

[ZebraFlesh]

This would be a nice new feature. Typical webpack usage seems to be running webpack in watch mode when doing active development. Being able to easily detect this would allow customization of your webpack config. For example, if in watch mode then set config.devtool = "source-map"; or something similar.

[PanayotCankov]

I am using something like:

export class WatchStateLoggerPlugin {
    apply(compiler) {
        compiler.plugin("watch-run", function(compiler, callback) {
            isWatching = true;
        }
    }
}

Hope it sounds reasonable.

[alexander-akait]

Closing due to inactivity. Please test with latest version and feel free to new create if still have problems/regressions. Thanks!

[s-panferov]

Please reopen. This is a feature request and it is NOT solved.

[alexander-akait]

Pr welcome

[TheAifam5]

@s-panferov you can detect the "watch" mode just by subscribing to "watchRun" and "watchClose" compiler hooks - so where is the problem?

[s-panferov]

@TheAifam5 I just want to be able to understand this from a loader, without requiring user to install a compiler plugin just for this.

[renatoagds]

@evilebottnawi and @TheLarkInn looks like this still open, I would like to help with this, can you guys point out where can I start?

[alexander-akait]

@renatoagds
Looks need add isWatch (maybe better name) with false/true (false by default) to Compiler https://github.com/webpack/webpack/blob/master/lib/Compiler.js and set true here https://github.com/webpack/webpack/blob/master/lib/Watching.js#L40

[renatoagds]

@evilebottnawi pretty thanks! I'll work on this one.

[renatoagds]

@evilebottnawi added the flag at #8253. can you take a look? 👍