如何正确使用SetTimeout（打字稿）

Question

I am working on an ionic-cordova app that uses Geolocation .

According to the app flow it is necessary to get an accurate position.

I wrote dedicated methods to achieve the location and I used setTimeout in them.

According to my logs it seems that I didn't use setTimeout correctly - or even worst I chose poorly and setTimeout is not what I need here.

This is my code:

  getFixedLocation() {
    let coordinates: Coordinates;
    let maxTries = 15;
    let isFixedLocation = false;

    this.forceFixLocation(10);

    do {
      console.log("Getting location, try #" + maxTries)
      this.geolocation.getCurrentPosition(GEOLOCATION_OPTIONS)
      .then((position) => {
        coordinates = position.coords;
      });

      if (coordinates.accuracy < 25 && coordinates.speed < 1) {
        isFixedLocation = true;
      } else {
        maxTries -= 1;
      }
    } while(maxTries > 0 && !isFixedLocation);

    if (isFixedLocation) {
      return coordinates;
    } else {
      return null;
    }
  }

  forceFixLocation(counter: number) {
    setTimeout(() => {
      if(counter > 0) {
        console.log("Forcing location #" + counter)        
        this.geolocation.getCurrentPosition(GEOLOCATION_OPTIONS)
        .then((position) => {
          console.log("Got position, accuracy: " + position.coords.accuracy)
        }); 
        this.forceFixLocation(counter - 1);
      }
    }, 3000);
  }

A look on the log reveals that my do-while loop is executed before the first iteration of forceFixLocation :

08-15 00:58:51.831 D/SystemWebChromeClient(19622): file:///android_asset/www/build/main.js: Line 56358 : Getting location, try #15
08-15 00:58:51.831 I/chromium(19622): [INFO:CONSOLE(56358)] "Getting location, try #15", source: file:///android_asset/www/build/main.js (56358)
08-15 00:58:51.831 D/GeolocationPlugin(19622): We are entering execute
08-15 00:58:51.841 D/SystemWebChromeClient(19622): file:///android_asset/www/build/main.js: Line 1362 : ERROR
08-15 00:58:51.841 I/chromium(19622): [INFO:CONSOLE(1362)] "ERROR", source: file:///android_asset/www/build/main.js (1362)
08-15 00:58:54.834 D/SystemWebChromeClient(19622): file:///android_asset/www/build/main.js: Line 56381 : Forcing location #10
08-15 00:58:54.844 I/chromium(19622): [INFO:CONSOLE(56381)] "Forcing location #10", source: file:///android_asset/www/build/main.js (56381)
08-15 00:58:54.844 D/GeolocationPlugin(19622): We are entering execute
08-15 00:58:56.936 D/SystemWebChromeClient(19622): file:///android_asset/www/build/main.js: Line 1362 : ERROR
08-15 00:58:56.946 I/chromium(19622): [INFO:CONSOLE(1362)] "ERROR", source: file:///android_asset/www/build/main.js (1362)
08-15 00:58:57.847 D/SystemWebChromeClient(19622): file:///android_asset/www/build/main.js: Line 56381 : Forcing location #9
08-15 00:58:57.847 I/chromium(19622): [INFO:CONSOLE(56381)] "Forcing location #9", source: file:///android_asset/www/build/main.js (56381)

What is the correct approach here?

Answer 1

The lambda function inside forceFixLocation will be called first time after 3 sec. When you calling this.forceFixLocation(10); from getFixedLocation - it immediate continues to next line and doing the logic of while. More than that, getFixedLocation may already return some value, but you will still get forceFixLocation called every 3 second (10 times).

Answer 2

You are missing a fundamental information here. And it's the JavaScript event loop!

You can NOT use do while because it will never (ever) end, and will hang your application, as the logs indicate.

It also looks like you are not waiting for the promise to resolve.. the then method a promise provides is just a function that might/will happen in the future. Your do/while hangs the app so it can't resolve to this promise...

I would suggest trying async/await as it offers a similar syntax to what you are expecting to get out of JavaScript.