## Tree

Display a set of data with hierarchies.

### Basic usage

Basic tree structure.

:::demo
```html
<el-tree :data="data" :props="defaultProps" @node-click="handleNodeClick"></el-tree>

<script>
  export default {
    data() {
      return {
        data: [{
          label: 'Level one 1',
          children: [{
            label: 'Level two 1-1',
            children: [{
              label: 'Level three 1-1-1'
            }]
          }]
        }, {
          label: 'Level one 2',
          children: [{
            label: 'Level two 2-1',
            children: [{
              label: 'Level three 2-1-1'
            }]
          }, {
            label: 'Level two 2-2',
            children: [{
              label: 'Level three 2-2-1'
            }]
          }]
        }, {
          label: 'Level one 3',
          children: [{
            label: 'Level two 3-1',
            children: [{
              label: 'Level three 3-1-1'
            }]
          }, {
            label: 'Level two 3-2',
            children: [{
              label: 'Level three 3-2-1'
            }]
          }]
        }],
        defaultProps: {
          children: 'children',
          label: 'label'
        }
      };
    },
    methods: {
      handleNodeClick(data) {
        console.log(data);
      }
    }
  };
</script>
```
:::

### Selectable

Used for node selection.

:::demo This example also shows how to load node data asynchronously.
```html
<el-tree
  :props="props"
  :load="loadNode"
  lazy
  show-checkbox
  @check-change="handleCheckChange">
</el-tree>

<script>
  export default {
    data() {
      return {
        props: {
          label: 'name',
          children: 'zones'
        },
        count: 1
      };
    },
    methods: {
      handleCheckChange(data, checked, indeterminate) {
        console.log(data, checked, indeterminate);
      },
      handleNodeClick(data) {
        console.log(data);
      },
      loadNode(node, resolve) {
        if (node.level === 0) {
          return resolve([{ name: 'Root1' }, { name: 'Root2' }]);
        }
        if (node.level > 3) return resolve([]);

        var hasChild;
        if (node.data.name === 'region1') {
          hasChild = true;
        } else if (node.data.name === 'region2') {
          hasChild = false;
        } else {
          hasChild = Math.random() > 0.5;
        }

        setTimeout(() => {
          var data;
          if (hasChild) {
            data = [{
              name: 'zone' + this.count++
            }, {
              name: 'zone' + this.count++
            }];
          } else {
            data = [];
          }

          resolve(data);
        }, 500);
      }
    }
  };
</script>
```
:::

### Custom leaf node in lazy mode

:::demo A node's data is not fetched until it is clicked, so the Tree cannot predict whether a node is a leaf node. That's why a drop-down button is added to each node, and if it is a leaf node, the drop-down button will disappear when clicked. That being said, you can also tell the Tree in advance whether the node is a leaf node, avoiding the render of the drop-down button before a leaf node.
```html
<el-tree
  :props="props"
  :load="loadNode"
  lazy
  show-checkbox>
</el-tree>

<script>
  export default {
    data() {
      return {
        props: {
          label: 'name',
          children: 'zones',
          isLeaf: 'leaf'
        },
      };
    },
    methods: {
      loadNode(node, resolve) {
        if (node.level === 0) {
          return resolve([{ name: 'region' }]);
        }
        if (node.level > 1) return resolve([]);

        setTimeout(() => {
          const data = [{
            name: 'leaf',
            leaf: true
          }, {
            name: 'zone'
          }];

          resolve(data);
        }, 500);
      }
    }
  };
</script>
```
:::

### Disabled checkbox

The checkbox of a node can be set as disabled.

:::demo In the example, 'disabled' property is declared in defaultProps, and some nodes are set as 'disabled:true'. The corresponding checkboxes are disabled and can't be clicked.
```html
<el-tree
  :data="data"
  :props="defaultProps"
  show-checkbox
  @check-change="handleCheckChange">
</el-tree>

<script>
  export default {
    data() {
      return {
        data: [{
          id: 1,
          label: 'Level one 1',
          children: [{
            id: 3,
            label: 'Level two 2-1',
            children: [{
              id: 4,
              label: 'Level three 3-1-1'
            }, {
              id: 5,
              label: 'Level three 3-1-2',
              disabled: true
            }]
          }, {
            id: 2,
            label: 'Level two 2-2',
            disabled: true,
            children: [{
              id: 6,
              label: 'Level three 3-2-1'
            }, {
              id: 7,
              label: 'Level three 3-2-2',
              disabled: true
            }]
          }]
        }],
        defaultProps: {
            children: 'children',
            label: 'label',
            disabled: 'disabled',
        },
      };
    }
  };
</script>
```
:::

### Default expanded and default checked
Tree nodes can be initially expanded or checked

:::demo Use `default-expanded-keys` and `default-checked-keys` to set initially expanded and initially checked nodes respectively. Note that for them to work, `node-key` is required. Its value is the name of a key in the data object, and the value of that key should be unique across the whole tree.
```html
<el-tree
  :data="data"
  show-checkbox
  node-key="id"
  :default-expanded-keys="[2, 3]"
  :default-checked-keys="[5]"
  :props="defaultProps">
</el-tree>

<script>
  export default {
    data() {
      return {
        data: [{
          id: 1,
          label: 'Level one 1',
          children: [{
            id: 4,
            label: 'Level two 1-1',
            children: [{
              id: 9,
              label: 'Level three 1-1-1'
            }, {
              id: 10,
              label: 'Level three 1-1-2'
            }]
          }]
        }, {
          id: 2,
          label: 'Level one 2',
          children: [{
            id: 5,
            label: 'Level two 2-1'
          }, {
            id: 6,
            label: 'Level two 2-2'
          }]
        }, {
          id: 3,
          label: 'Level one 3',
          children: [{
            id: 7,
            label: 'Level two 3-1'
          }, {
            id: 8,
            label: 'Level two 3-2'
          }]
        }],
        defaultProps: {
          children: 'children',
          label: 'label'
        }
      };
    }
  };
</script>
```
:::

### Checking tree nodes

:::demo This example shows how to get and set checked nodes. They both can be done in two approaches: node and key. If you are taking the key approach, `node-key` is required.
```html
<el-tree
  :data="data"
  show-checkbox
  default-expand-all
  node-key="id"
  ref="tree"
  highlight-current
  :props="defaultProps">
</el-tree>

<div class="buttons">
  <el-button @click="getCheckedNodes">get by node</el-button>
  <el-button @click="getCheckedKeys">get by key</el-button>
  <el-button @click="setCheckedNodes">set by node</el-button>
  <el-button @click="setCheckedKeys">set by key</el-button>
  <el-button @click="resetChecked">reset</el-button>
</div>

<script>
  export default {
    methods: {
      getCheckedNodes() {
        console.log(this.$refs.tree.getCheckedNodes());
      },
      getCheckedKeys() {
        console.log(this.$refs.tree.getCheckedKeys());
      },
      setCheckedNodes() {
        this.$refs.tree.setCheckedNodes([{
          id: 5,
          label: 'Level two 2-1'
        }, {
          id: 9,
          label: 'Level three 1-1-1'
        }]);
      },
      setCheckedKeys() {
        this.$refs.tree.setCheckedKeys([3]);
      },
      resetChecked() {
        this.$refs.tree.setCheckedKeys([]);
      }
    },

    data() {
      return {
        data: [{
          id: 1,
          label: 'Level one 1',
          children: [{
            id: 4,
            label: 'Level two 1-1',
            children: [{
              id: 9,
              label: 'Level three 1-1-1'
            }, {
              id: 10,
              label: 'Level three 1-1-2'
            }]
          }]
        }, {
          id: 2,
          label: 'Level one 2',
          children: [{
            id: 5,
            label: 'Level two 2-1'
          }, {
            id: 6,
            label: 'Level two 2-2'
          }]
        }, {
          id: 3,
          label: 'Level one 3',
          children: [{
            id: 7,
            label: 'Level two 3-1'
          }, {
            id: 8,
            label: 'Level two 3-2'
          }]
        }],
        defaultProps: {
          children: 'children',
          label: 'label'
        }
      };
    }
  };
</script>
```
:::

### Custom node content
The content of tree nodes can be customized, so you can add icons or buttons as you will

:::demo There are two ways to customize template for tree nodes: `render-content` and scoped slot. Use `render-content` to assign a render function that returns the content of tree nodes. See Vue's documentation for a detailed introduction of render functions. If you prefer scoped slot, you'll have access to `node` and `data` in the scope, standing for the TreeNode object and node data of the current node respectively. Note that the `render-content` demo can't run in jsfiddle because it doesn't support JSX syntax. In a real project, `render-content` will work if relevant dependencies are correctly configured.
```html
<div class="custom-tree-container">
  <div class="block">
    <p>Using render-content</p>
    <el-tree
      :data="data"
      show-checkbox
      node-key="id"
      default-expand-all
      :expand-on-click-node="false"
      :render-content="renderContent">
    </el-tree>
  </div>
  <div class="block">
    <p>Using scoped slot</p>
    <el-tree
      :data="data"
      show-checkbox
      node-key="id"
      default-expand-all
      :expand-on-click-node="false">
      <span class="custom-tree-node" slot-scope="{ node, data }">
        <span>{{ node.label }}</span>
        <span>
          <el-button
            type="text"
            size="mini"
            @click="() => append(data)">
            Append
          </el-button>
          <el-button
            type="text"
            size="mini"
            @click="() => remove(node, data)">
            Delete
          </el-button>
        </span>
      </span>
    </el-tree>
  </div>
</div>

<script>
  let id = 1000;

  export default {
    data() {
      const data = [{
        id: 1,
        label: 'Level one 1',
        children: [{
          id: 4,
          label: 'Level two 1-1',
          children: [{
            id: 9,
            label: 'Level three 1-1-1'
          }, {
            id: 10,
            label: 'Level three 1-1-2'
          }]
        }]
      }, {
        id: 2,
        label: 'Level one 2',
        children: [{
          id: 5,
          label: 'Level two 2-1'
        }, {
          id: 6,
          label: 'Level two 2-2'
        }]
      }, {
        id: 3,
        label: 'Level one 3',
        children: [{
          id: 7,
          label: 'Level two 3-1'
        }, {
          id: 8,
          label: 'Level two 3-2'
        }]
      }];
      return {
        data: JSON.parse(JSON.stringify(data)),
        data: JSON.parse(JSON.stringify(data))
      }
    },

    methods: {
      append(data) {
        const newChild = { id: id++, label: 'testtest', children: [] };
        if (!data.children) {
          this.$set(data, 'children', []);
        }
        data.children.push(newChild);
      },

      remove(node, data) {
        const parent = node.parent;
        const children = parent.data.children || parent.data;
        const index = children.findIndex(d => d.id === data.id);
        children.splice(index, 1);
      },

      renderContent(h, { node, data, store }) {
        return (
          <span class="custom-tree-node">
            <span>{node.label}</span>
            <span>
              <el-button size="mini" type="text" on-click={ () => this.append(data) }>Append</el-button>
              <el-button size="mini" type="text" on-click={ () => this.remove(node, data) }>Delete</el-button>
            </span>
          </span>);
      }
    }
  };
</script>

<style>
  .custom-tree-node {
    flex: 1;
    display: flex;
    align-items: center;
    justify-content: space-between;
    font-size: 14px;
    padding-right: 8px;
  }
</style>
```
:::

### Tree node filtering
Tree nodes can be filtered

:::demo Invoke the `filter` method of the Tree instance to filter tree nodes. Its parameter is the filtering keyword. Note that for it to work, `filter-node-method` is required, and its value is the filtering method.
```html
<el-input
  placeholder="Filter keyword"
  v-model="filterText">
</el-input>

<el-tree
  class="filter-tree"
  :data="data"
  :props="defaultProps"
  default-expand-all
  :filter-node-method="filterNode"
  ref="tree">
</el-tree>

<script>
  export default {
    watch: {
      filterText(val) {
        this.$refs.tree.filter(val);
      }
    },

    methods: {
      filterNode(value, data) {
        if (!value) return true;
        return data.label.indexOf(value) !== -1;
      }
    },

    data() {
      return {
        filterText: '',
        data: [{
          id: 1,
          label: 'Level one 1',
          children: [{
            id: 4,
            label: 'Level two 1-1',
            children: [{
              id: 9,
              label: 'Level three 1-1-1'
            }, {
              id: 10,
              label: 'Level three 1-1-2'
            }]
          }]
        }, {
          id: 2,
          label: 'Level one 2',
          children: [{
            id: 5,
            label: 'Level two 2-1'
          }, {
            id: 6,
            label: 'Level two 2-2'
          }]
        }, {
          id: 3,
          label: 'Level one 3',
          children: [{
            id: 7,
            label: 'Level two 3-1'
          }, {
            id: 8,
            label: 'Level two 3-2'
          }]
        }],
        defaultProps: {
          children: 'children',
          label: 'label'
        }
      };
    }
  };
</script>
```
:::

### Accordion

Only one node among the same level can be expanded at one time.

:::demo
```html
<el-tree
  :data="data"
  :props="defaultProps"
  accordion
  @node-click="handleNodeClick">
</el-tree>

<script>
  export default {
    data() {
      return {
        data: [{
          label: 'Level one 1',
          children: [{
            label: 'Level two 1-1',
            children: [{
              label: 'Level three 1-1-1'
            }]
          }]
        }, {
          label: 'Level one 2',
          children: [{
            label: 'Level two 2-1',
            children: [{
              label: 'Level three 2-1-1'
            }]
          }, {
            label: 'Level two 2-2',
            children: [{
              label: 'Level three 2-2-1'
            }]
          }]
        }, {
          label: 'Level one 3',
          children: [{
            label: 'Level two 3-1',
            children: [{
              label: 'Level three 3-1-1'
            }]
          }, {
            label: 'Level two 3-2',
            children: [{
              label: 'Level three 3-2-1'
            }]
          }]
        }],
        defaultProps: {
          children: 'children',
          label: 'label'
        }
      };
    },
    methods: {
      handleNodeClick(data) {
        console.log(data);
      }
    }
  };
</script>
```
:::

### Draggable

You can drag and drop Tree nodes by adding a `draggable` attribute.

:::demo
```html
<el-tree
  :data="data"
  node-key="id"
  default-expand-all
  @node-drag-start="handleDragStart"
  @node-drag-enter="handleDragEnter"
  @node-drag-leave="handleDragLeave"
  @node-drag-over="handleDragOver"
  @node-drag-end="handleDragEnd"
  @node-drop="handleDrop"
  draggable
  :allow-drop="allowDrop"
  :allow-drag="allowDrag">
</el-tree>

<script>
  export default {
    data() {
      return {
        data: [{
          label: 'Level one 1',
          children: [{
            label: 'Level two 1-1',
            children: [{
              label: 'Level three 1-1-1'
            }]
          }]
        }, {
          label: 'Level one 2',
          children: [{
            label: 'Level two 2-1',
            children: [{
              label: 'Level three 2-1-1'
            }]
          }, {
            label: 'Level two 2-2',
            children: [{
              label: 'Level three 2-2-1'
            }]
          }]
        }, {
          label: 'Level one 3',
          children: [{
            label: 'Level two 3-1',
            children: [{
              label: 'Level three 3-1-1'
            }]
          }, {
            label: 'Level two 3-2',
            children: [{
              label: 'Level three 3-2-1'
            }]
          }]
        }],
        defaultProps: {
          children: 'children',
          label: 'label'
        }
      };
    },
    methods: {
      handleDragStart(node, ev) {
        console.log('drag start', node);
      },
      handleDragEnter(draggingNode, dropNode, ev) {
        console.log('tree drag enter: ', dropNode.label);
      },
      handleDragLeave(draggingNode, dropNode, ev) {
        console.log('tree drag leave: ', dropNode.label);
      },
      handleDragOver(draggingNode, dropNode, ev) {
        console.log('tree drag over: ', dropNode.label);
      },
      handleDragEnd(draggingNode, dropNode, dropType, ev) {
        console.log('tree drag end: ', dropNode && dropNode.label, dropType);
      },
      handleDrop(draggingNode, dropNode, dropType, ev) {
        console.log('tree drop: ', dropNode.label, dropType);
      },
      allowDrop(draggingNode, dropNode, type) {
        if (dropNode.data.label === 'Level two 3-1') {
          return type !== 'inner';
        } else {
          return true;
        }
      },
      allowDrag(draggingNode) {
        return draggingNode.data.label.indexOf('Level three 3-1-1') === -1;
      }
    }
  };
</script>
```
:::

### Attributes
| Attribute             | Description                              | Type                        | Accepted Values | Default |
| --------------------- | ---------------------------------------- | --------------------------- | --------------- | ------- |
| data                  | tree data                                | array                       | —               | —       |
| empty-text            | text displayed when data is void         | string                      | —               | —       |
| node-key              | unique identity key name for nodes, its value should be unique across the whole tree | string                      | —               | —       |
| props                 | configuration options, see the following table | object                      | —               | —       |
| render-after-expand   | whether to render child nodes only after a parent node is expanded for the first time | boolean | — | true |
| load                  | method for loading subtree data, only works when `lazy` is true  | function(node, resolve)     | —               | —       |
| render-content        | render function for tree node            | Function(h, { node, data, store }        | —               | —       |
| highlight-current     | whether current node is highlighted      | boolean                     | —               | false   |
| default-expand-all    | whether to expand all nodes by default   | boolean                     | —               | false   |
| expand-on-click-node  | whether to expand or collapse node when clicking on the node, if false, then expand or collapse node only when clicking on the arrow icon. | boolean | — | true |
| check-on-click-node   | whether to check or uncheck node when clicking on the node, if false, the node can only be checked or unchecked by clicking on the checkbox. | boolean | — | false |
| auto-expand-parent    | whether to expand father node when a child node is expanded | boolean                     | —               | true    |
| default-expanded-keys | array of keys of initially expanded nodes | array                       | —               | —       |
| show-checkbox         | whether node is selectable               | boolean                     | —               | false   |
| check-strictly        | whether checked state of a node not affects its father and child nodes when `show-checkbox` is `true` | boolean                     | —               | false   |
| default-checked-keys  | array of keys of initially checked nodes | array                       | —               | —       |
| current-node-key      | key of initially selected node | string, number                       | —               | —       |
| filter-node-method    | this function will be executed on each node when use filter method. if return `false`, tree node will be hidden. | Function(value, data, node) | —               | —       |
| accordion             | whether only one node among the same level can be expanded at one time | boolean                     | —               | false   |
| indent                | horizontal indentation of nodes in adjacent levels in pixels | number                     | —    | 16 |
| icon-class            | custome tree node icon                                       | string                     | -    | -  |
| lazy                  | whether to lazy load leaf node, used with `load` attribute  | boolean                     | —    | false |
| draggable             | whether enable tree nodes drag and drop | boolean            | —    | false |
| allow-drag            | this function will be executed before dragging a node. If `false` is returned, the node can not be dragged | Function(node)  | —  | —  |
| allow-drop            | this function will be executed before the dragging node is dropped. If `false` is returned, the dragging node can not be dropped at the target node. `type` has three possible values: 'prev' (inserting the dragging node before the target node), 'inner' (inserting the dragging node to the target node) and 'next' (inserting the dragging node after the target node) | Function(draggingNode, dropNode, type)  | —    | —     |

### props
| Attribute | Description                              | Type   | Accepted Values | Default |
| --------- | ---------------------------------------- | ------ | --------------- | ------- |
| label     | specify which key of node object is used as the node's label | string, function(data, node) | —               | —       |
| children | specify which node object is used as the node's subtree | string | —               | —       |
| disabled | specify which key of node object represents if node's checkbox is disabled | boolean, function(data, node) | —    | —    |
| isLeaf | specify whether the node is a leaf node, only works when lazy load is enabled | boolean, function(data, node) | —    | —    |

### Method
`Tree` has the following method, which returns the currently selected array of nodes.
| Method          | Description                              | Parameters                               |
| --------------- | ---------------------------------------- | ---------------------------------------- |
| filter          | filter all tree nodes, filtered nodes will be hidden | Accept a parameter which will be used as first parameter for filter-node-method |
| updateKeyChildren | set new data to node, only works when `node-key` is assigned  | (key, data) Accept two parameters: 1. key of node 2. new data |
| getCheckedNodes | If the node can be selected (`show-checkbox` is `true`), it returns the currently selected array of nodes | (leafOnly, includeHalfChecked) Accept two boolean type parameters: 1. default value is `false`. If the parameter is `true`, it only returns the currently selected array of sub-nodes. 2. default value is `false`. If the parameter is `true`, the return value contains halfchecked nodes |
| setCheckedNodes | set certain nodes to be checked, only works when `node-key` is assigned | an array of nodes to be checked          |
| getCheckedKeys  | If the node can be selected (`show-checkbox` is `true`), it returns the currently selected array of node's keys | (leafOnly) Accept a boolean type parameter whose default value is `false`. If the parameter is `true`, it only returns the currently selected array of sub-nodes. |
| setCheckedKeys  | set certain nodes to be checked, only works when `node-key` is assigned | (keys, leafOnly) Accept two parameters: 1. an array of node's keys to be checked 2. a boolean type parameter whose default value is `false`. If the parameter is `true`, it only returns the currently selected array of sub-nodes. |
| setChecked      | set node to be checked or not, only works when `node-key` is assigned | (key/data, checked, deep) Accept three parameters: 1. node's key or data to be checked 2. a boolean typed parameter indicating checked or not. 3. a boolean typed parameter indicating deep or not. |
| getHalfCheckedNodes | If the node can be selected (`show-checkbox` is `true`), it returns the currently half selected array of nodes | - |
| getHalfCheckedKeys | If the node can be selected (`show-checkbox` is `true`), it returns the currently half selected array of node's keys | - |
| getCurrentKey   | return the highlight node's key (null if no node is highlighted) | — |
| getCurrentNode  | return the highlight node's data (null if no node is highlighted) | — |
| setCurrentKey   | set highlighted node by key, only works when `node-key` is assigned | (key) the node's key to be highlighted. If `null`, cancel the currently highlighted node |
| setCurrentNode  | set highlighted node, only works when `node-key` is assigned | (node) the node to be highlighted |
| getNode         | get node by data or key | (data) the node's data or key |
| remove          | remove a node, only works when node-key is assigned | (data) the node's data or node to be deleted |
| append          | append a child node to a given node in the tree | (data, parentNode) 1. child node's data to be appended 2. parent node's data, key or node |
| insertBefore    | insert a node before a given node in the tree | (data, refNode) 1. node's data to be inserted 2. reference node's data, key or node |
| insertAfter     | insert a node after a given node in the tree   | (data, refNode) 1. node's data to be inserted 2. reference node's data, key or node |

### Events
| Event Name     | Description                              | Parameters                               |
| -------------- | ---------------------------------------- | ---------------------------------------- |
| node-click     | triggers when a node is clicked          | three parameters: node object corresponding to the node clicked, `node` property of TreeNode, TreeNode itself |
| node-contextmenu     | triggers when a node is clicked by right button          | four parameters: event, node object corresponding to the node clicked, `node` property of TreeNode, TreeNode itself |
| check-change   | triggers when the selected state of the node changes | three parameters: node object corresponding to the node whose selected state is changed, whether the node is selected, whether node's subtree has selected nodes |
| check   | triggers after clicking the checkbox of a node | two parameters: node object corresponding to the node that is checked / unchecked, tree checked status object which has four props: checkedNodes, checkedKeys, halfCheckedNodes, halfCheckedKeys |
| current-change | triggers when current node changes       | two parameters: node object corresponding to the current node, `node` property of TreeNode |
| node-expand    | triggers when current node open          | three parameters: node object corresponding to the node opened, `node` property of TreeNode, TreeNode itself |
| node-collapse  | triggers when current node close         | three parameters: node object corresponding to the node closed, `node` property of TreeNode, TreeNode itself |
| node-drag-start | triggers when dragging starts  | two parameters: node object corresponding to the dragging node, event. |
| node-drag-enter | triggers when the dragging node enters another node  | three parameters: node object corresponding to the dragging node, node object corresponding to the entering node, event. |
| node-drag-leave | triggers when the dragging node leaves a node  | three parameters: node object corresponding to the dragging node, node object corresponding to the leaving node, event.  |
| node-drag-over | triggers when dragging over a node (like mouseover event) | three parameters: node object corresponding to the dragging node, node object corresponding to the dragging over node, event.  |
| node-drag-end  | triggers when dragging ends  | four parameters: node object corresponding to the dragging node, node object corresponding to the dragging end node (may be `undefined`), node drop type (before / after / inner), event. |
| node-drop  | triggers after the dragging node is dropped | four parameters: node object corresponding to the dragging node, node object corresponding to the dropped node, node drop type (before / after / inner), event. |

### Scoped Slot
| Name | Description |
|------|--------|
| — | Custom content for tree nodes. The scope parameter is { node, data } |
