I have an application, which contains a logger. The logger already exists and is ready to be used, so it can be passed directly to the constructor.
class App {
private readonly logger: Logger;
public constructor(logger: Logger) {
this.logger = logger;
}
public run() {
// do stuff
}
}
function main() {
const logger = new Logger();
const app = new App(logger);
app.run();
return 0;
}
main();
Now I want to add configuration, so I can modify its behavior without having to rebuild the application (e.g., send requests to a certain URL without hardcoding the URL). I don't want my constructors to throw, though. I also want to be able to reload at runtime, so thats another restriction leading to a 2-phase init.
class App {
private readonly logger: Logger;
+ private readonly configManager: ConfigManager;
- public constructor(logger: Logger) {
+ public constructor(logger: Logger, configsDir: string) {
this.logger = logger;
+ this.configManager = new ConfigManager(configsDir);
}
+ public loadConfigs(configsDir: string) {
+ // It needs a separate initialization step, because reading files
+ // might fail and I don't want my constructors to throw
+ return this.configManager.loadConfigs();
+ }
public run() {
// do stuff
}
}
function main() {
const logger = new Logger();
- const app = new App(logger);
+ const app = new App(logger, "./run/configs");
+ if (!app.loadConfigs()) return 1;
app.run();
return 0;
}
main();
I have a widget/module/part that will make use of the configuration. Unfortunately, because the configuration uses a 2-phase init, it cannot go in the constructor, and must wait for the second init. And because the init is not in the constructor, it cannot be readonly. Additionally, because Typescript cannot be certain that the variable is initialized, it must either be declared as T | undefined or asserted to be non-null with the ! operator.
class App {
private readonly logger: Logger;
private readonly configManager: ConfigManager;
+ private widget!: Widget; // The exclamation mark asserts its definition
public constructor(logger: Logger, configsDir: string) {
this.logger = logger;
this.configManager = new ConfigManager(configsDir);
+ // Can't init the widget here, because the config has not loaded yet
+ // this.widget = new Widget(this.configManager.getWidgetSettings());
}
+ public loadWidget() {
+ // Because the init is not in the constructor, the variable cannot
+ // be `readonly`.
+ this.widget = new Widget(this.configManager.getWidgetSettings());
+ }
public loadConfigs(configsDir: string) {
// It needs a separate initialization step, because reading files
// might fail and I don't want my constructors to throw
return this.configManager.loadConfigs();
}
public run() {
// do stuff
}
}
function main() {
const logger = new Logger();
const app = new App(logger, "./run/configs");
if (!app.loadConfigs()) return 1;
+ app.loadWidget();
app.run();
return 0;
}
main();
Here I've gone with the assert route, since it looks nicer than throwing a bunch of if (!this.configManager) throw new Error()s around. Then I need to pray that I remember to load everything and in the correct order.
What was originally a simple and straightforward "make the app run itself" turned into "pass everything in the constructor and run" and is now further degrading into a maze.
Is there a better design I can use to avoid all of the following:
- throwing constructors
- 2-phase initialization
- repetitive definition checks
- asserting
In any case, I will definitely catch mistakes since it crashes, but ideally I can avoid them entirely. I can't figure out a way to avoid all those at the same time, but maybe there's a less convoluted design I haven't thought of.
Result<TOk, TErr>objects, with static "constructor methods" and private true constructors.