为什么要使用Validators.compose（）？

问题描述

我有一个要通过多个验证器验证的字段。

使用模块驱动方法，代码看起来像这样：

I have a field I want to validate with multiple validators.
Using the Module Driven approach the code looks likes this:

this.exampleForm = this.fb.group({
    date_start : ['', Validators.compose([
                          Validators.required, 
                          Validators.pattern("[0-9]{2}-[0-9]{2}-[0-9]{4}")
                      ])
                 ]
})

但是我也可以不用 Validators.compose（）这样写：

But I can also write this withouth Validators.compose() like:

this.exampleForm = this.fb.group({
    date_start : ['', [
                          Validators.required, 
                          Validators.pattern("[0-9]{2}-[0-9]{2}-[0-9]{4}")
                      ]
                 ]
})

而且效果很好。我个人更喜欢第二版（无需 compose ），更少的代码和更好的可读性。这就引出了一个问题，我为什么要使用Validators.compose（）？

And it works just fine. Personally I prefer the 2nd version (without compose), less code and better readability. And this begs the question, why should I use Validators.compose()?

推荐答案

创建 new FormControl / FormGroup / FormArray （AbstractControl）- coerceToValidator 被调用。

When we create new FormControl/FormGroup/FormArray(AbstractControl) - coerceToValidator is called.

function coerceToValidator(
    validatorOrOpts?: ValidatorFn | ValidatorFn[] | AbstractControlOptions | null): ValidatorFn|
    null {
  const validator =
      (isOptionsObj(validatorOrOpts) ? (validatorOrOpts as AbstractControlOptions).validators :
                                       validatorOrOpts) as ValidatorFn |
      ValidatorFn[] | null;

  return Array.isArray(validator) ? composeValidators(validator) : validator || null;
}

export function composeValidators(validators: Array<Validator|Function>): ValidatorFn|null {
  return validators != null ? Validators.compose(validators.map(normalizeValidator)) : null;
}

因此，在将验证器传递给AbstractControl之前，无需编写验证器。

So there is no need to compose validators before we pass it to an AbstractControl.

6/13/16添加了 feat（forms）：如果是数组，则自动组成验证器fns ， Validators.compose是为了向后兼容。

6/13/16 was added feat(forms): compose validator fns automatically if arrays from now on, Validators.compose is there for backward compatibility.

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