How to return a promise with alertcontroller in ionic 4?

Question

I am trying to convert this ionic 3 code into ionic 4 but I don't know how the promise works on ionic 4. I tried looking into the documentations and I can't find any solutions to promises

async generateAlert(header, message, ok, notOk): Promise<boolean> {
    return new Promise((resolve, reject) => {
        let alert = await this.alertController.create({
            header: header,
            message: message,
            buttons: [
                {
                    text: notOk,
                    handler: _=> reject(false)
                },
                {
                    text: ok,
                    handler: _=> resolve(true)
                }
            ]
        })
        await alert.present();
    });
}

Answer 1

As you can see here:

The await operator is used to wait for a Promise.

So await is just another way to work with promises like you can see in the following example:

// Method that returns a promise
private resolveAfter2Seconds(x: number): Promise<number> { 
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

// Using await
private async f1(): Promise<void> {
  var x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

// Not using await
private f2(): void {
  resolveAfter2Seconds(10).then(x => {
    console.log(x); // 10
  });
}

In f1(){...} you can see how the app will wait for the promise to be resolved before executing the next line of code. That's why you can do something like

var x = await resolveAfter2Seconds(10);
console.log(x); // 10

without putting that console.log(x) in a .then(() => {...}) block.

In f2() since we don't use await, the app won't wait for the promise to be resolved before executing the next line of code, so we must use then to print the result in the console:

resolveAfter2Seconds(10).then(x => {
  console.log(x); // 10
});

---

That being said, if you just want to create a method that shows an alert and returns true/false when the user selects the ok/notOk buttons, you can do the following (which doesn't use await at all):

private generateAlert(header: string, message: string, ok: string, notOk: string): Promise<boolean> {
  return new Promise((resolve) => {
    // alertController.create(...) returns a promise!
    this.alertController
      .create({
        header: header,
        message: message,
        buttons: [
          {
            text: notOk,
            handler: () => resolve(false);
          },
          {
            text: ok,
            handler: () => resolve(true);
          }
        ]
      })
      .then(alert => {
        // Now we just need to present the alert
        alert.present();
      });
  });
}

Then you can use that method like this

private doSomething(): void {

  // ...

  this.generateAlert('Hello', 'Lorem ipsum', 'Ok', 'Cancel').then(selectedOk => {
    console.log(selectedOk);
  });

}