如何在Elm中设置元素的焦点？

Question

如何在Elm中设置Html元素的焦点？我试图在元素上设置自动聚焦属性，它只在页面加载上设置焦点。

Answer 1

我最近花了不少时间来探索这一点。不幸的是，我认为使用现有的elmhtml库是不可能的。然而，我想出了一个利用css动画触发事件并将其嵌入纯js的方法。

下面是我在Elm中使用脚本节点和样式节点的技巧。在我看来这很难看。

import Html exposing (div, button, text, input, node)
import Html.Events exposing (onClick)
import Html.Attributes exposing (type', class)
import StartApp.Simple

main =
  StartApp.Simple.start { model = model, view = view, update = update }

model = []

view address model =
  -- View now starts with a <style> and <script> (hacky)
  (node "style" [] [ Html.text style ]) ::
  (node "script" [] [Html.text script ]) ::
  (button [ onClick address AddInput ] [ text "Add Input" ]) ::
  model |>
  div []    

type Action = AddInput 

update action model =
  case action of
    AddInput -> (Html.p [] [input [type' "text", class "focus"] []]) :: model

-- Use pure string css (hacky)

style = """
.focus {
  animation-name: set-focus;
  animation-duration: 0.001s;
  -webkit-animation-name: set-focus;
  -webkit-animation-duration: 0.001s;
}
@-webkit-keyframes set-focus {
    0%   {color: #fff}
}
@keyframes set-focus {
    0%   {color: #fff}
}
"""

-- Cheating by embedding pure javascript... (hacky)

script = """
var insertListener = function(event){
 if (event.animationName == "set-focus") {
   event.target.focus();
 }               
}
document.addEventListener("animationstart", insertListener, false); // standard + firefox
document.addEventListener("MSAnimationStart", insertListener, false); // IE
document.addEventListener("webkitAnimationStart", insertListener, false); // Chrome + Safari
"""

Answer 2

一个解决方法是使用变异观察员。在主HTML页面或Elm代码的主视图中插入此JavaScript：

js prettyprint-override">var observer = new MutationObserver(function(mutations) {
  mutations.forEach(function(mutation) {
    handleAutofocus(mutation.addedNodes);
  });
});
var target = document.querySelector('body > div');
var config = { childList: true, subtree: true };
observer.observe(target, config);

function handleAutofocus(nodeList) {
  for (var i = 0; i < nodeList.length; i++) {
    var node = nodeList[i];
    if (node instanceof Element && node.hasAttribute('data-autofocus')) {
      node.focus();
      break;
    } else {
      handleAutofocus(node.childNodes);
    }
  }
}

然后通过包含Html创建超文本标记语言元素。ttributes.attribute数据自动对焦。

Answer 3

elm lang/dom包中的focus函数用于通过任务设置焦点（不使用任何端口或JavaScript）。

在内部，它使用请求动画框架来确保在它试图找到要关注的DOM节点之前呈现任何新的DOM更新。

使用示例：

type Msg
    = FocusOn String
    | FocusResult (Result Dom.Error ())

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        FocusOn id ->
            ( model, Dom.focus id |> Task.attempt FocusResult )

        FocusResult result ->
            -- handle success or failure here
            case result of
                Err (Dom.NotFound id) ->
                    -- unable to find dom 'id'
                Ok () ->
                    -- successfully focus the dom

艾莉的完整例子