Vue GWT

# List Rendering

This page comes from the official Vue.js documentation and has been adapted for Vue GWT.

# The v-for directive

We can use the v-for directive to render a list of items based on an array. The v-for directive requires a special syntax in the form of Type item in items, where items is the source data array, item is an alias for the array element being iterated on and Type is item type:

# Basic Usage

<vue-gwt:import class="com.mypackage.Todo"/>
<ol>
    <li v-for="Todo todo in todos">
        {{ todo.getText() }}
    </li>
</ol>

@Component
public class SimpleTodoListComponent implements IsVueComponent, HasCreated {
    @Data
    @JsProperty
    List<Todo> todos = new LinkedList<>();
    
    @Override
    public void created() {
        this.todos.add(new Todo("Learn Java"));
        this.todos.add(new Todo("Learn Vue GWT"));
        this.todos.add(new Todo("Build something awesome"));
    }
}

Result:

Inside v-for blocks we have full access to parent scope properties. v-for also supports an optional second argument for the index of the current item.

<vue-gwt:import class="com.axellience.vuegwtexamples.client.examples.common.Todo"/>
<ul>
    <li v-for="(Todo todo, index)  in  todos">
        {{ parentMessage }} - {{ index }} -> {{ todo.getText() }}
    </li>
</ul>

@Component
public class VForWithIndexComponent implements IsVueComponent, HasCreated {
    @Data String parentMessage = "Message from parent";
    
    @Data
    @JsProperty
    List<Todo> todos = new LinkedList<>();

    @Override
    public void created() {
        this.todos.add(new Todo("Learn Java"));
        this.todos.add(new Todo("Learn Vue GWT"));
        this.todos.add(new Todo("Build something awesome"));
    }
}

Result:

You can also use of as the delimiter instead of in, so that it is closer to JavaScript's syntax for iterators:

<li v-for="Todo todo of todos">

WARNING

To avoid gotchas with Java Collection observation, you should read about Java Collection Observation.

# Template v-for

Similar to template v-if, you can also use a <template> tag with v-for to render a block of multiple elements. For example:

<vue-gwt:import class="com.mypackage.Todo"/>
<ul>
  <template v-for="Todo todo in todos">
    <li>{{ todo.getText() }}</li>
    <li class="divider"></li>
  </template>
</ul>

# Object v-for

You can also use v-for to iterate through the properties of an Object.

For this you have to cast to Object in your v-for. This tells Vue GWT that you are iterating on an Object and not an regular Collection.

<ul>
    <li v-for="Object value in (Object) myObject">
        {{ value }}
    </li>
</ul>

@Component
public class VForOnObjectComponent implements IsVueComponent, HasCreated {
    @Data JsObject<Object> myObject = new JsObject<>();

    @Override
    public void created() {
        this.myObject.set("myString", "Hello World");
        this.myObject.set("myInt", 12);
        this.myObject.set("myTodo", new Todo("I'm a Todo"));
    }
}

Result:

You can also provide a second argument for the key:

<ul>
    <li v-for="(Object value, key) in (Object) myObject">
        {{ key }}: {{ value }}
    </li>
</ul>

And another for the index:

<ul>
    <li v-for="(Object value, key, index) in (Object) myObject">
        {{ index }}. {{ key }}: {{ value }}
    </li>
</ul>

TIP

When iterating over an object, the order is based on the key enumeration order of Object.keys(), which is not guaranteed to be consistent across JavaScript engine implementations.

# Range v-for

v-for can also take an integer. In this case it will repeat the template that many times.

<div>
    <span v-for="int n in 5">{{ n }} </span>
</div>

# Components and v-for

You can directly use v-for on a custom component, like any normal element:

<vue-gwt:import class="com.mypackage.Item"/>
<my-component v-for="Item item in items" :key="item.getId()"></my-component>

In Vue.js 2.2.0+, when using v-for with a component, a key is now required.

However, this won't automatically pass any data to the component, because components have isolated scopes of their own. In order to pass the iterated data into the component, we should also use props:

<vue-gwt:import class="com.mypackage.Item"/>
<my-component
  v-for="Item item in items"
  v-bind:item="item"
  v-bind:key="item.getId()">
</my-component>

The reason for not automatically injecting item into the component is because that makes the component tightly coupled to how v-for works. Being explicit about where its data comes from makes the component reusable in other situations.

You can see our TodoList example.

# Using v-for with v-if

When they exist on the same node, v-for has a higher priority than v-if. That means the v-if will be run on each iteration of the loop separately. This is very useful when you want to render nodes for only some items, like below:

<vue-gwt:import class="com.mypackage.Todo"/>
<li v-for="Todo todo in todos" v-if="!todo.isComplete()">
  {{ todo }}
</li>

The above only renders the todos that are not complete.

If instead, your intent is to conditionally skip execution of the loop, you can place the v-if on a wrapper element (or <template> ). For example:

<vue-gwt:import class="com.mypackage.Todo"/>
<ul v-if="shouldRenderTodos">
  <li v-for="Todo todo in todos">
    {{ todo }}
  </li>
</ul>

# Using key

When Vue is updating a list of elements rendered with v-for, it by default uses an "in-place patch" strategy. If the order of the data items has changed, instead of moving the DOM elements to match the order of the items, Vue will simply patch each element in-place and make sure it reflects what should be rendered at that particular index. This is similar to the behavior of track-by="$index" in Vue 1.x.

This default mode is efficient, but only suitable when your list render output does not rely on child component state or temporary DOM state (e.g. form input values).

To give Vue a hint so that it can track each node's identity, and thus reuse and reorder existing elements, you need to provide a unique key attribute for each item. An ideal value for key would be the unique id of each item. This special attribute is a rough equivalent to track-by in 1.x, but it works like an attribute, so you need to use v-bind to bind it to dynamic values (using shorthand here):

<vue-gwt:import class="com.mypackage.Item"/>
<div v-for="Item item in items" :key="item.getId()">
  <!-- content -->
</div>

It is recommended to provide a key with v-for whenever possible, unless the iterated DOM content is simple, or you are intentionally relying on the default behavior for performance gains.

Since it's a generic mechanism for Vue to identify nodes, the key also has other uses that are not specifically tied to v-for, as we will see later in the guide.

# Displaying Filtered/Sorted Results

Sometimes we want to display a filtered or sorted version of an array without actually mutating or resetting the original data. In this case, you can create a computed property that returns the filtered or sorted array.

For example:

<div>
    <span v-for="Integer n in evenNumbers">{{ n }} </span>
</div>

@Component
public class EvenNumbersComponent implements IsVueComponent {
    @Data
    @JsProperty
    List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

    @Computed
    public List<Integer> getEvenNumbers() {
        return this.numbers.stream().filter(number -> number % 2 == 0).collect(Collectors.toList());
    }
}

In situations where computed properties are not feasible (e.g. inside nested v-for loops), you can just use a method:

...
<!-- The variable numbers comes from a v-for in the template -->
<span v-for="Integer n in getEven(numbers)">{{ n }} </span>
...

@Component
public class EvenNumbersComponent implements IsVueComponent {
    // No @Computed annotation
    public List<Integer> getEven(List<Integer> numbers) {
        return this.numbers.stream().filter(number -> number % 2 == 0).collect(Collectors.toList());
    }
}

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