# List Rendering

# wx:for

In a component, by binding an array using the wx:for control property, you can use the data of the array items to re-render this component.

The subscript variable name of the current item of the default array defaults to index, and the variable name of the current item of the array defaults to item.

<view wx:for="{{array}}">
  {{index}}: {{item.message}}
</view>

Page({
  data: {
    array: [{
      message: 'foo',
    }, {
      message: 'bar'
    }]
  }
})

You can use wx:for-item to specify the variable name of the array’s current element.

You can use wx:for-index to specify the variable name of the array’s current subscript:

<view wx:for="{{array}}" wx:for-index="idx" wx:for-item="itemName">
  {{idx}}: {{itemName.message}}
</view>

wx:for can also be embedded. Below is the Chinese multiplication table:

<view wx:for="{{[1, 2, 3, 4, 5, 6, 7, 8, 9]}}" wx:for-item="i">
  <view wx:for="{{[1, 2, 3, 4, 5, 6, 7, 8, 9]}}" wx:for-item="j">
    <view wx:if="{{i <= j}}">
      {{i}} * {{j}} = {{i * j}}
    </view>
  </view>
</view>

# block wx:for

Similar to block wx:if, you can use wx:for on a <block/> tag to render a structural block containing multiple nodes. For example:

<block wx:for="{{[1, 2, 3]}}">
  <view> {{index}}: </view>
  <view> {{item}} </view>
</block>

# wx:key

If the positions of list items dynamically change or new items are added to the list and you want the items in the list to retain their features and statuses (such as the content input in input and the selection status of switch), you must use wx:key to specify the unique identifiers of the items in the list.

The wx:key is provided in two formats:

1. String: Represents a property of an item in the for loop array. The value of this property must be a unique string or number in the list and cannot dynamically change.
2. Reserved keyword *this: Represents the item itself in the for loop. This expresses that the item itself is a unique string or number. For example:

When a data change triggers re-rending at the rendering layer, the components that have keys are corrected. The framework ensures that they are reordered, rather than recreated. This ensures the components retain their statuses and improves the efficiency of list rendering.

If wx:key, is not provided, a warning is reported. If you know that this list is static or the order is not important, you can ignore the warning.

Sample code:

Preview with Developer Tool

<switch wx:for="{{objectArray}}" wx:key="unique" style="display: block;"> {{item.id}} </switch>
<button bindtap="switch"> Switch </button>
<button bindtap="addToFront"> Add to the front </button>

<switch wx:for="{{numberArray}}" wx:key="*this" style="display: block;"> {{item}} </switch>
<button bindtap="addNumberToFront"> Add to the front </button>

Page({
  data: {
    objectArray: [
      {id: 5, unique: 'unique_5'},
      {id: 4, unique: 'unique_4'},
      {id: 3, unique: 'unique_3'},
      {id: 2, unique: 'unique_2'},
      {id: 1, unique: 'unique_1'},
      {id: 0, unique: 'unique_0'},
    ],
    numberArray: [1, 2, 3, 4]
  },
  switch: function(e) {
    const length = this.data.objectArray.length
    for (let i = 0; i < length; ++i) {
      const x = Math.floor(Math.random() * length)
      const y = Math.floor(Math.random() * length)
      const temp = this.data.objectArray[x]
      this.data.objectArray[x] = this.data.objectArray[y]
      this.data.objectArray[y] = temp
    }
    this.setData({
      objectArray: this.data.objectArray
    })
  },
  addToFront: function(e) {
    const length = this.data.objectArray.length
    this.data.objectArray = [{id: length, unique: 'unique_' + length}].concat(this.data.objectArray)
    this.setData({
      objectArray: this.data.objectArray
    })
  },
  addNumberToFront: function(e){
    this.data.numberArray = [ this.data.numberArray.length + 1 ].concat(this.data.numberArray)
    this.setData({
      numberArray: this.data.numberArray
    })
  }
})

Note:

When the wx:for value is a string, the string is parsed into a string array

<view wx:for="array">
  {{item}}
</view>

equivalent to

<view wx:for="{{['a','r','r','a','y']}}">
  {{item}}
</view>

Note: If there are spaces between curly brackets and quotes, the content is ultimately parsed into a string

<view wx:for="{{[1,2,3]}} ">
  {{item}}
</view>

equivalent to

<view wx:for="{{[1,2,3] + ' '}}" >
  {{item}}
</view>