📘
Scroll to the bottom of this page for an easy copy paste solution!

Step 1: Initialise SwymCallbacks

if(!window.SwymCallbacks){
    window.SwymCallbacks = [];
}
window.SwymCallbacks.push(this._initializeHeaderIcon.bind(this));

Step 2: Initialise the header icon

_initializeHeaderIcon(swat){
    this.button = this.querySelector('button');
    //All functional code here     
}

Step 3: Add an on-click event to the header icon that opens the Wishlist UI.

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Code overview:
swat.ui.open() : used to open the Wishlist UI. You may need to add your own function to close your wishlist modal in case it is a custom wishlist modal.

this.button.addEventListener('click', _openWishlistUi);

function _openWishlistUi(evt){
  
  // In case you have a custom wishlist page, modify the below code accordingly
  window.location = '/pages/wishlist'
  
  /* In case you need to use the default Swym Wishlist, 
  comment the above code (line 6) and uncomment the below code (line 11)  */
  
  // swat.ui.open();
}

Step 4 (optional): Initialize the header counter if required.

📘
Code overview:
getListsData() : Uses the swat.fetchLsts API to fetch a user's lists. The lists contains products that a user has wishlisted.

// HEADER COUNTER CODE 

this.counter = this.querySelector(".swym-wishlist-header-counter");

if(this.counter){
  const counter = this.counter; 
  
  // Fetch the lists 

  async function getListsData(){
    return new Promise(async (resolve, reject) => {
      swat.fetchLists({
        callbackFn: data => resolve(data), 
        onError: error => reject(error)
      })
    })
  }
  
  // Increase header count when product is added to the list.
          
  swat.evtLayer.addEventListener(swat.JSEvents.addedToWishlist, () => {
    const counterCount = +counter.innerHTML;
    counter.innerHTML =  counterCount+1; 
  })

  // Decrease header count when product is removed from the list.
  
  swat.evtLayer.addEventListener(swat.JSEvents.removedFromWishlist, () => {
    const counterCount = +counter.innerHTML;
    if(counterCount >= 0) counter.innerHTML =  counterCount - 1; 
  })
            
  // Fetch total number of products and update header counter.
   
  const listData = await getListsData();
  let products = [];
  for (const list of listData) {
    for (const product of list.listcontents) {
      products.push(product);
    }
  }
  this.counter.innerHTML = products.length;
}

Step 5: Add the HTML for the header icon

<swym-header-icon>
  <button id="swym-header-icon" aria-label="View Wishlist">
    {{ settings.swym_header_icon }}
  </button>
</swym-header-icon>

<swym-header-icon>
  <button id="swym-header-icon" aria-label="View Wishlist">
    {{ settings.swym_header_icon }}
    <span class="swym-wishlist-header-counter"></span>
  </button>
</swym-header-icon>

Step 6: Add styling for the header icon

<style>
  .swym-wishlist.swym-inject.header__icon { display: none }
  
  swym-header-icon #swym-header-icon {
    border: none; 
    width: auto; 
    height: 35px; 
    background: transparent; 
    display: flex; 
    align-items: end; 
    
    /* Update the color as per store requirements from the customisation settings */
    color: {{ settings.swym_header_icon_color }}; 
    
  }
</style>

Step 7: Add schema settings to customize the header icon from Shopify's theme customizer

{
    "name": "Custom Header Implementation",
    "settings": [
      {
        "type": "liquid",
        "id": "swym_header_icon",
        "label": "Swym Header icon",
        "default": "<svg height=\"24\" clip-rule=\"evenodd\" fill-rule=\"evenodd\" stroke-linejoin=\"round\" stroke-miterlimit=\"2\" viewBox=\"0 0 24 24\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\"><path fill=\"currentColor\" d=\"m7.234 3.004c-2.652 0-5.234 1.829-5.234 5.177 0 3.725 4.345 7.727 9.303 12.54.194.189.446.283.697.283s.503-.094.697-.283c4.977-4.831 9.303-8.814 9.303-12.54 0-3.353-2.58-5.168-5.229-5.168-1.836 0-3.646.866-4.771 2.554-1.13-1.696-2.935-2.563-4.766-2.563zm0 1.5c1.99.001 3.202 1.353 4.155 2.7.14.198.368.316.611.317.243 0 .471-.117.612-.314.955-1.339 2.19-2.694 4.159-2.694 1.796 0 3.729 1.148 3.729 3.668 0 2.671-2.881 5.673-8.5 11.127-5.454-5.285-8.5-8.389-8.5-11.127 0-1.125.389-2.069 1.124-2.727.673-.604 1.625-.95 2.61-.95z\" fill-rule=\"nonzero\"\/><\/svg>"
      },
      {
        "type": "color",
        "id": "swym_header_icon_color",
        "label": "Swym Header Icon color",
        "default": "#FFFFFF"
      }
    ]
}

Step 8: Add the header icon in the header section

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Keep in mind that the name of the header file may vary depending on the themes, so it's important to determine the name of the header file.

{% render 'swym-custom-header' %}

One Step Copy Paste - Final Code

🚧
You would still need to follow step 7 to add the schema settings in order to use Shopify's theme customizer to customize the header icon.

<swym-header-icon>
    <button id="swym-header-icon" aria-label="View Wishlist">
        {{ settings.swym_header_icon }}
        <span class="swym-wishlist-header-counter"></span>
    </button>
</swym-header-icon>

<style>
    .swym-wishlist.swym-inject.header__icon { display: none }
  
    swym-header-icon #swym-header-icon {
        border: none; 
        width: auto; 
        height: 35px; 
        background: transparent; 
        display: flex; 
        align-items: end; 
        /* Update the color as per store requirements from the customisation settings */
        color: {{ settings.swym_header_icon_color }}; 
    }
</style>

<script>
  if(!customElements.get('swym-header-icon')){
    class SwymHeaderIcon extends HTMLElement{
        constructor(){
          super();
          if (!window.SwymCallbacks) window.SwymCallbacks = [];
          window.SwymCallbacks.push(this._initializeSwymButton.bind(this));
        }

        _initializeSwymButton(swat){
          this.button = this.querySelector('button');
          this.button.addEventListener('click', _openWishlistUi);

          function _openWishlistUi(evt){
            swat.ui.open();
          }
          
          // HEADER COUNTER CODE 

          this.counter = this.querySelector(".swym-wishlist-header-counter");
          
          if(this.counter){
          
            // Fetch the lists 
          
            async function getListsData(){
              return new Promise(async (resolve, reject) => {
                swat.fetchLists({
                  callbackFn: data => resolve(data), 
                  onError: error => reject(error)
                })
              })
            }
            const listData = await getListsData();
            
            // Remove the duplicate occurences of products
            
            let products = {};
            for (const list of listData) {
              for (const product of list.listcontents) {
                  const epi = product.epi;
                  if (!products[epi]) {
                    products[epi] = product;
                  }
                }
              }
              this.counter.innerHTML = Object.values(products).length;
            }
        }
    }
    customElements.define('swym-header-icon', SwymHeaderIcon);
  }
</script>

Demo Store

Store Link
Password: demostore