mdb-ui-kit/scss/mixins/_forms.scss

@mixin bmd-disabled() {
  fieldset[disabled][disabled] &,
  &.disabled,
  &:disabled,
  &[disabled] {
    @content
  }
}

@mixin bmd-selection-color() {
  .radio label,
  .radio-inline,
  .checkbox label,
  .checkbox-inline,
  .switch label {

    // override bootstrap focus and keep all the standard color (could be multiple radios in the form group)
    //color: $bmd-label-color;

    &,
    .is-focused & {
      // form-group focus could change multiple checkboxes/radios, disable that change by using the same color as non-form-group is-focused
      color: $bmd-label-color;

      // on focus just darken the specific labels, do not turn them to the brand-primary
      @include hover-focus-active() {
        //&:hover,
        //&:focus {
        color: $bmd-label-color-inner-focus;
      }

      // correct the above focus color for disabled items
      label:has(input[type=radio][disabled]), // css 4 which is unlikely to work for a while, but no other pure css way.
      label:has(input[type=checkbox][disabled]), // css 4
      fieldset[disabled] & {
        &,
        &:hover,
        &:focus {
          color: $bmd-label-color;
        }
      }
    }
  }

  // Style for disabled inputs  OLD, use color approach with opacity built in, see radios
  //fieldset[disabled] &,
  //fieldset[disabled] & input[type=checkbox],
  //input[type=checkbox][disabled]:not(:checked) ~ .checkbox-decorator .check::before,
  //input[type=checkbox][disabled]:not(:checked) ~ .checkbox-decorator .check,
  //input[type=checkbox][disabled] + .bmd-radio-outer-circle {
  //  opacity: 0.5;
  //}
}

@mixin bmd-radio-color($color) {

  ~ .bmd-radio-inner-circle {
    background-color: $color;
  }

  ~ .bmd-radio-outer-circle {
    border-color: $color;
  }
}

@mixin bmd-form-color($label-color, $label-color-focus, $border-color) {

  [class^='bmd-label'],
  [class*=' bmd-label'] {
    color: $label-color;
  }

  // override BS and keep the border-color normal/grey so that overlaid focus animation draws attention
  .form-control {

    // underline animation color on focus
    $underline-background-image:          linear-gradient(to top, $label-color-focus 2px, fade-out($label-color-focus, 1) 2px), linear-gradient(to top, $input-border-color 1px, fade-out($input-border-color, 1) 1px);
    $underline-background-image-invalid:  linear-gradient(to top, $bmd-invalid-underline 2px, fade-out($bmd-invalid-underline, 1) 2px), linear-gradient(to top, $input-border-color 1px, fade-out($input-border-color, 1) 1px);
    $underline-background-image-readonly: linear-gradient(to top, $bmd-readonly-underline 1px, fade-out($bmd-readonly-underline, 1) 1px), linear-gradient(to top, $input-border-color 1px, fade-out($input-border-color, 1) 1px);
    $underline-background-image-disabled: linear-gradient(to right, $input-border-color 0%, $input-border-color 30%, transparent 30%, transparent 100%);

    // bg image is always there, we just need to reveal it
    &,
    .is-focused & {
      background-image: $underline-background-image;
    }

    &:invalid {
      background-image: $underline-background-image-invalid;
    }

    &:read-only {
      background-image: $underline-background-image-readonly;
    }

    @include bmd-disabled() {
      background-image: $underline-background-image-disabled;
      background-repeat: repeat-x;
      background-size: 3px 1px;
    }

    // allow underline focus image and validation images to coexist
    &.form-control-success {
      &,
      .is-focused & {
        background-image: $underline-background-image, $form-icon-success;
      }
    }
    &.form-control-warning {
      &,
      .is-focused & {
        background-image: $underline-background-image, $form-icon-warning;
      }
    }
    &.form-control-danger {
      &,
      .is-focused & {
        background-image: $underline-background-image, $form-icon-danger;
      }
    }
  }

  .is-focused, // may or may not be a form-group or bmd-form-group
  &.is-focused {
    // on focus set borders and labels to the validation color

    // Use the BS provided mixin for the bulk of the color
    @include form-control-validation($label-color);

    [class^='bmd-label'],
    [class*=' bmd-label'] {
      color: $label-color-focus;
    }

    .bmd-label-placeholder {
      color: $label-color; // keep the placeholder color
    }

    // Set the border and box shadow on specific inputs to match
    .form-control {
      border-color: $border-color;
    }

    // Set validation states also for addons
    //.input-group-addon {
    //  border-color: $border-color;
    //}

    .bmd-help {
      color: $bmd-label-color-inner-focus;
    }
  }
}

// must be broken out for reuse - webkit selector breaks firefox
@mixin bmd-label-static($label-top, $static-font-size) {
  top: $label-top;
  left: 0;
  // must repeat because the previous (more generic) selectors
  font-size: $static-font-size;
}

@mixin bmd-form-size-variant($font-size, $label-top-margin, $variant-padding-y, $variant-line-height, $form-group-context: null) {

  $variant-input-height: (($font-size * $variant-line-height) + ($variant-padding-y * 2));
  $static-font-size: ($bmd-bmd-label-static-size-ratio * $font-size);
  $help-font-size: ($bmd-help-size-ratio * $font-size);

  $label-static-top: $label-top-margin;
  $label-placeholder-top: $label-top-margin + $static-font-size + $variant-padding-y;

  //@debug "font-size: #{$font-size} static-font-size: #{$static-font-size} help-font-size: #{$help-font-size} form-group-context: #{$form-group-context} ";

  //Label height: 72dp
  //Padding above label text: 16dp
  //Padding between label and input text: 8dp
  //Padding below input text (including divider): 16dp
  //Padding below text divider: 8dp

  @if $form-group-context {
    // Create a space at the top of the bmd-form-group for the label.
    //  The label is absolutely positioned, so we use top padding to make space.  This padding extends over the label down to the top of the input (padding).
    padding-top: ($label-top-margin + $static-font-size);
    // note: bottom-margin of this is determined by $spacer.  @see _spacer.scss
    //margin-bottom: (1.5 * $help-font-size);
  }

  // TODO: remove this when known stable.  https://github.com/FezVrasta/bootstrap-material-design/issues/849
  //@else {
  //
  //  // for radios and checkboxes without a form-group, add some extra vertical spacing to pad down so that
  //  //  any help text above is not encroached upon, or so that it appears more evenly spaced vs form-groups
  //  .radio,
  //  label.radio-inline,
  //  .checkbox,
  //  label.checkbox-inline,
  //  .switch {
  //    padding-top: $spacer-y;
  //  }
  //}

  // Set all line-heights preferably to 1 so that we can space out everything manually without additional added space
  //  from the default line-height of 1.5
  .form-control,
  label,
  input::placeholder {
    line-height: $variant-line-height;
  }

  .radio label,
  label.radio-inline,
  .checkbox label,
  label.checkbox-inline,
  .switch label {
    line-height: $line-height-base; // keep the same line height for radios and checkboxes
  }

  // Note: this may be inside or outside a form-group, may be .bmd-form-group.bmd-form-group-sm or .bmd-form-group.bmd-form-group-lg
  input::placeholder {
    font-size: $font-size;
  }

  // generic labels used anywhere in the form
  .checkbox label,
  .radio label,
  label {
    font-size: $font-size;
  }

  // floating/placeholder default (no focus)
  .bmd-label-floating,
  .bmd-label-placeholder {
    //@debug "top: #{$label-as-placeholder-top}";
    top: $label-placeholder-top; // place the floating label to look like a placeholder with input padding
  }

  // floating focused/filled will look like static
  &.is-focused,
  .is-focused,
  &.is-filled,
  .is-filled {
    .bmd-label-floating {
      @include bmd-label-static($label-static-top, $static-font-size);
    }
  }

  // static
  .bmd-label-static {
    @include bmd-label-static($label-static-top, $static-font-size);
  }
  // #559 Fix for webkit/chrome autofill - rule must be separate because it breaks firefox otherwise #731
  //input:-webkit-autofill ~ .bmd-label-floating {  FIXME: confirm that the autofill js generation of change event makes this unnecessary
  //  @include bmd-label-static($label-top, $static-font-size, $static-line-height);
  //}

  .bmd-help {
    margin-top: 0; // allow the input margin to set-off the top of the help-block
    font-size: $help-font-size;
  }

  // validation icon placement
  .form-control {
    &.form-control-success,
    &.form-control-warning,
    &.form-control-danger {
      $icon-bg-size: ($variant-input-height * .5) ($variant-input-height * .5);
      background-size: $bmd-form-control-bg-size, $icon-bg-size;

      &,
      &:focus,
      .bmd-form-group.is-focused & {
        padding-right: ($input-padding-x * 3);
        background-repeat: $bmd-form-control-bg-repeat-y, no-repeat;
        background-position: $bmd-form-control-bg-position, center right ($variant-input-height * .25);
      }

      &:focus,
      .bmd-form-group.is-focused & {
        background-size: $bmd-form-control-bg-size-active, $icon-bg-size;
      }
    }
  }
}