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View Layer

Last updated: 2024-11-21 18:34:19
    The view layer of the framework is written using WXML and WXSS and presented by components.
    WXML (WeiXin Markup language) is used to describe the page structure;
    WXS (WeiXin Script) is a scripting language for Mini Programs. Combined with WXML, it can be used to construct the page structure;
    WXSS (WeiXin Style Sheet) is used to describe the page style.
    Components are the building blocks of a view.

    1. WXML

    WXML (WeiXin Markup Language) is a markup language for framework design. It can be used to build the page structure when combined with base components and event system.
    For more information on WXML syntax, see WXML syntax.
    The following examples show what WXML can do:

    1.1 Data Binding

    <!-- wxml -->
    <view> {{message}} </view>
    // page.js
    Page({
    data: {
    message: 'Hello MINA!'
    }
    })

    1.2 List Rendering

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

    1.3 Condition Rendering

    <!--wxml-->
    <view wx:if="{{view == 'WEBVIEW'}}"> WEBVIEW </view>
    <view wx:elif="{{view == 'APP'}}"> APP </view>
    <view wx:else="{{view == 'MINA'}}"> MINA </view>
    // page.js
    Page({
    data: {
    view: 'MINA'
    }
    })

    1.4 Template

    <!--wxml-->
    <template name="staffName">
    <view>
    FirstName: {{firstName}}, LastName: {{lastName}}
    </view>
    </template>
    
    <template is="staffName" data="{{...staffA}}"></template>
    <template is="staffName" data="{{...staffB}}"></template>
    <template is="staffName" data="{{...staffC}}"></template>
    // page.js
    Page({
    data: {
    staffA: {firstName: 'Hulk', lastName: 'Hu'},
    staffB: {firstName: 'Shang', lastName: 'You'},
    staffC: {firstName: 'Gideon', lastName: 'Lin'}
    }
    })
    

    2. WXSS

    WXSS (WeiXin Style Sheets) is a set of style languages that describe WXML component styles,
    and is used to determine how WXML components are displayed.
    WXSS has most of the features of CSS to cater for the needs of front-end developers, but incorporates some new features and modifications to adapt to the development of Weixin Mini Programs.
    Compared with CSS, WXSS offers the following additional features:
    Dimension unit
    Style import;
    Dimension Unit.
    Note:
    For iOS 8 and below, if you use flexbox layout, you need to add display: -webkit-flex property.

    2.1 Dimension Unit

    rpx (responsive pixel): Adaptable to the screen width. The specified screen width is 750 rpx. If the screen width on iPhone6 is 375 px (750 physical pixels), then 750 rpx = 375 px = 750 physical pixels, i.e. 1 rpx = 0.5 px = 1 physical pixel.
    Device
    rpx-to-px conversion (screen width/750)
    px-to-rpx conversion (750/screen width)
    iPhone5
    1rpx = 0.42px
    1px = 2.34rpx
    iPhone6
    1rpx = 0.5px
    1px = 2rpx
    iPhone6 Plus
    1rpx = 0.552px
    1px = 1.81rpx
    Note:
    Tip: Designers can use iPhone6 as a standard in Visual Design when developing Weixin Mini Programs.
    Note: It is inevitable to have some glitches on small screens. Please try to avoid this during development.

    2.2 Style Import

    You can use the @import statement to import external style sheet. The @import statement should be followed by the relative path of the external style sheet and ends with ;.
    /** common.wxss **/
    .small-p {
    padding:5px;
    }
    /** app.wxss **/
    @import "common.wxss";
    .middle-p {
    padding:15px;
    }

    2.3 Inline Style

    You can use "style" and "class" properties for framework components to control the component style.
    style: Static styles are all written to "class". "style" receives dynamic styles and parses them at runtime. It is not recommended to write static styles to "style" to avoid affecting the rendering speed.
    /** common.wxss **/
    .small-p {
    padding:5px;
    }
    class: Used to specify style rules. Its value is a collection of class selector names (style class names) in the style rules. The style class names do not contain . and are separated with spaces.
    <view class="normal_view" />

    2.4 Selectors

    The following selectors are supported:
    Selector
    Example
    Description
    .class
    .intro
    Selects all components with class="intro"
    #id
    #firstname
    Selects the component with id="firstname"
    element
    view
    Selects all "view" components
    element, element
    view, checkbox
    Selects all "checkbox" components and the "view" components of all documents
    ::after
    view::after
    Inserts content after the "view" component
    ::before
    view::before
    Inserts content before the "view" component

    2.5 Global and Local Styles

    A global style is defined in app.wxss and applies to every page, while a local one is defined in the page's wxss file and only applies to the current page (overrides the same selector in app.wxss).

    3. WXS

    WXS (WeiXin Script) is a script segment inlined in WXML. WXS can be used to inline a small number of processing scripts in the template to enrich the data preprocessing capability of the template. In addition, WXS can also be used to write simple WXS event response functions.
    Syntactically, WXS is similar to JavaScript with a few limitations. For a complete overview of the WXS syntax, see WXS - Introduction.
    Here are some simple examples of using WXS.
    Page Rendering
    <!--wxml-->
    <wxs module="m1">
    var msg = "hello world";
    
    module.exports.message = msg;
    </wxs>
    
    <view> {{m1.message}} </view>
    Page output:
    hello world
    Data Processing
    // page.js
    Page({
    data: {
    array: [1, 2, 3, 4, 5, 1, 2, 3, 4]
    }
    })
    <!--wxml-->
    <!-- The following getMax function takes an array and returns the value of the largest element in the array -->
    <wxs module="m1">
    var getMax = function(array) {
    var max = undefined;
    for (var i = 0; i < array.length; ++i) {
    max = max === undefined ?
    array[i] :
    (max >= array[i] ? max : array[i]);
    }
    return max;
    }
    
    module.exports.getMax = getMax;
    </wxs>
    
    <!-- Calls the getMax function in WXS with the "array" in page.js as the parameter -->
    <view> {{m1.getMax(array)}} </view>
    Page output:
    5

    4. Event System

    4.1 What Is an Event?

    An event is a channel through which the view layer communicants with the logical layer;
    An event can pass user's action on to the logical layer for processing;
    An event can be bound to a component. When the event is triggered, the corresponding event handler is executed at the logic layer;
    An event object can have additional information such as ID, dataset, touches.

    4.2 How to Use an Event

    Bind an event handler to the component. For example, if the event is bindtap, the event handler can be found in the page when a user taps the component.
    <view id="tapTest" data-hi="Weixin" bindtap="tapName"> Click me! </view>
    Write the event handler in the Page definition with "event" as the parameter.
    Page({
    tapName: function(event) {
    console.log(event)
    }
    })
    The log content is as follows:
    {
    "type":"tap",
    "timeStamp":895,
    "target": {
    "id": "tapTest",
    "dataset": {
    "hi":"Weixin"
    }
    },
    "currentTarget": {
    "id": "tapTest",
    "dataset": {
    "hi":"Weixin"
    }
    },
    "detail": {
    "x":53,
    "y":14
    },
    "touches":[{
    "identifier":0,
    "pageX":53,
    "pageY":14,
    "clientX":53,
    "clientY":14
    }],
    "changedTouches":[{
    "identifier":0,
    "pageX":53,
    "pageY":14,
    "clientX":53,
    "clientY":14
    }]
    }

    4.3 Respond to Events with WXS Function

    4.3.1 Background

    The effect of frequent user interactions on mini programs is laggy, for example, if there are two elements A and B on a page, and the user makes a touchmove gesture on A, and asks B to follow the gesture, movable-view is a typical example. The response to a touchmove event is as follows:
    The touchmove event is thrown from the rendering layer (Webview) to the logical layer (App Service);
    The logical layer (App Service) handles the touchmove event and then changes the position of B through setData.
    A touchmove response requires two communications between the logic layer and the rendering layer and one rendering, which is time-consuming. In addition, the setData rendering will also block the execution of other scripts, resulting in a delay in the animation process of the whole user interaction.

    4.3.2 Solution

    The basic idea of this solution is to reduce the number of communications and let the events respond in the rendering layer (Webview). The framework of the mini program is divided into the rendering layer (Webview) and the logic layer (App Service), the purpose of this layering is to control, the developer's code can only run in the logic layer (App Service), and this idea must allow the developer's code to run in the rendering layer (Webview).
    WXS functions used to respond to mini program events, can only respond to the built-in component events, does not support custom component events.
    WXS function in addition to the pure logic of the operation, but also through the encapsulated ComponentDescriptor instance to access and set the component class and style, for interactive animation, set the style and class enough.
    WXS function accepts two parameters, the first is event, in the original event based on the event.instance object, the second parameter is ownerInstance, and event.instance is a ComponentDescriptor object. The specific use is as follows:
    WXS functions for binding and registering event handling in a component
    <wxs module="wxs" src="./test.wxs"></wxs>
    <view id="tapTest" data-hi="Weixin" bindtap="{{wxs.tapName}}"> Click me! </view>
    **Note: Bound WXS functions must be enclosed in {{}}**
    The test.wxs file implements the tapName function.
    function tapName(event, ownerInstance) {
    console.log('tap Weixin', JSON.stringify(event))
    }
    module.exports = {
    tapName: tapName
    }
    ownerInstance contains methods to set the style and class of the component.
    An example of a WXS function is shown below:=
    var wxsFunction = function(event, ownerInstance) {
    var instance = ownerInstance.selectComponent('.classSelector') // Returns an instance of the component
    instance.setStyle({
    "font-size": "14px" // Support for rpx
    })
    instance.getDataset()
    instance.setClass(className)
    // ...
    return false // does not bubble up, which is equivalent to calling both stopPropagation and preventDefault
    }
    The input parameter event is the ComponentDescriptor instance of the component that triggered the event, which is the ComponentDescriptor instance of the component that triggered the event, and the ownerInstance is the instance of the page if the component that triggered the event is inside the page.
    The ownerInstance represents the ComponentDescriptor instance of the component that triggered the event. If the component that triggered the event is inside a page, the ownerInstance represents the page instance.
    The ComponentDescriptor currently supports the following APIs.
    Method
    Parameter
    Description
    selectComponent
    selector objects
    Returns the ComponentDescriptor instance of the component
    selectAllComponents
    Array of selector objects
    Returns an array of ComponentDescriptor instances of the component
    setStyle
    Object/string
    Set the style of the component, supports rpx, the style set has higher priority than the style defined inside the component wxml. Cannot set the style of the topmost page
    addClass/removeClass/hasClass
    string
    Set the class of the component, the class priority is higher than the class defined inside the component wxml. Cannot set the class of the top level page
    getDataset
    -
    Return the dataset of the current component/page
    callMethod
    (funcName:string, args:object)
    Call the function defined in the logic layer (App Service) of the current component/page. funcName is the name of the function, args is the parameter of the function
    getState
    -
    Returns an object, use this method when there are local variables that need to be stored for subsequent use
    triggerEvent
    (eventName, detail)
    Same as the triggerEvent of the component

    4.3.3 Usage

    WXML defines the event:
    <wxs module="test" src="./test.wxs"></wxs>
    <view
    change:prop="{{test.propObserver}}"
    prop="{{propValue}}"
    bindtouchmove="{{test.touchmove}}"
    class="movable">
    </view>
    The change:prop above (with the change: prefix in front of the property) triggers the WXS function when the prop property is set, and the value must be enclosed in {{}}. Similarly the observer property inside properties defined by Component is triggered after a setData({propValue: newValue})call.
    Note:
    WXS functions must be enclosed in {{}}. The WXS function is triggered when the value of the prop is set, not just when the value is changed, so the WxsPropObserver function is called once when the page is initialised.
    The WXS file test.wxs defines and exports event handlers and functions triggered by property changes:
    module.exports = {
    touchmove: function(event, instance) {
    console.log('log event', JSON.stringify(event))
    },
    propObserver: function(newValue, oldValue, ownerInstance, instance) {
    console.log('prop observer', newValue, oldValue)
    }
    }

    4.4Events

    4.4.1 Event Types

    Events are divided into bubbling events and non-bubbling ones.
    Bubbling event: When an event is triggered on a component, this event is propagated to the component's parent node;
    Non-bubbling event: When an event is triggered on a component, this event is not propagated to the component's parent node.
    WXML bubbling events:
    Type
    Trigger Condition
    touchstart
    Finger touch starts
    touchmove
    Finger moves after touch
    touchcancel
    Finger touch is interrupted by call reminder, pop-up window, etc.
    touchend
    Finger touch ends
    tap
    The finger leaves the screen after touch
    longpress
    The finger leaves the screen after it taps and holds on the screen for more than 350 ms. If an event callback function is specified and this event is triggered, the tap event is not triggered.
    longtap
    The finger leaves the screen after it taps and holds on the screen for more than 350 ms (it is recommended to use longpress event instead).
    transitionend
    Triggered when a WXSS transition or wx.createAnimation animation ends
    animationstart
    Triggered when a WXSS animation starts
    animationiteration
    Triggered after an iteration of a WXSS animation ends
    animationend
    Triggered when a WXSS animation completes
    touchforcechange
    Triggered when the screen is tapped again on an iPhone supporting 3D Touch.
    Note:
    Unless otherwise specified, the custom events of components other than those listed above are non-bubbling events, such as the submit event of [form]((form), input event of [input]((input), and scroll event of [scroll-view](scroll-view). For more information, see Component Overview.

    4.4.2 Event Binding and Bubbling

    Event bindings are written similarly to component properties, e.g.:
    <view bindtap="handleTap">
    Click here!
    </view>
    If the user clicks on this view, the page's handleTap is called.
    The event binding function can be a data binding, e.g.:
    <view bindtap="{{ handlerName }}">
    Click here!
    </view>
    In this case, the page's this.data.handlerName must be a string specifying the name of the event handler; if it's an empty string, the binding is disabled (you can use this feature to temporarily disable some events).
    In most components and custom components, bind can be immediately followed by a colon, and the meaning remains the same, e.g. bind:tap

    4.4.3 Binding and stopping event bubbling

    In addition to bind, events can also be bound with catch. Unlike bind, catch prevents events from bubbling up.
    For example, in the following example, clicking on the inner view calls handleTap3 and handleTap2 successively (because the tap event bubbles up to the middle view, which prevents the tap event from bubbling up to the parent node), and clicking on the middle view triggers handleTap2, which triggers handleTap2, which triggers handleTap2, which triggers handleTap2, which triggers handleTap2, which triggers handleTap2. Click outer view will trigger handleTap1.
    <view id="outer" bindtap="handleTap1">
    outer view
    <view id="middle" catchtap="handleTap2">
    middle view
    <view id="inner" bindtap="handleTap3">
    inner view
    </view>
    </view>
    </view>

    4.4.4 Mutually exclusive event binding

    As of Base Library version 1.5.44, you can use mut-bind to bind events in addition to bind and catch. When a mut-bind is triggered, if the event bubbles to another node, the mut-bind bindings on the other node will not be triggered, but the bind and catch bindings will still be triggered.
    In other words, all mut-binds are "mutually exclusive" and only one of them will be triggered. At the same time, it doesn't affect the binding effect of bind and catch at all.
    For example, in the example below, clicking inner view will call handleTap3 and handleTap2, and clicking middle view will call handleTap2 and handleTap1.
    <view id="outer" mut-bind:tap="handleTap1">
    outer view
    <view id="middle" bindtap="handleTap2">
    middle view
    <view id="inner" mut-bind:tap="handleTap3">
    inner view
    </view>
    </view>
    </view>

    4.4.5 Event Capturing

    touch-related events support capturing. In event capturing, which precedes bubbling, events arrive at nodes in an order reverse of that in which they do during bubbling. To listen to events during capturing, use capture-bind or capture-catch as the keyword. The latter interrupts the capturing and cancels bubbling.
    In the code below, tap "inner view" to call handleTap2, handleTap4, handleTap3, and handleTap1 in turn.
    <view id="outer" bind:touchstart="handleTap1" capture-bind:touchstart="handleTap2">
    outer view
    <view id="inner" bind:touchstart="handleTap3" capture-bind:touchstart="handleTap4">
    inner view
    </view>
    </view>
    If the first capture-bind in the above code is changed to capture-catch, only handleTap2 is triggered.
    <view id="outer" bind:touchstart="handleTap1" capture-catch:touchstart="handleTap2">
    outer view
    <view id="inner" bind:touchstart="handleTap3" capture-bind:touchstart="handleTap4">
    inner view
    </view>
    </view>

    4.4.6 Event Objects

    Unless otherwise specified, when the component triggers an event, the handler bound to the event at logic layer receives an event object.
    BaseEvent object properties:
    Property
    Type
    Description
    type
    String
    Event Type
    timeStamp
    Integer
    Timestamp when the event is generated
    target
    Object
    A collection of property values​for the component triggering the event
    Object
    A collection of property values​for the current component
    mark
    Object
    Event flag data
    CustomEvent object properties (inherit from BaseEvent):
    Property
    Type
    Description
    detail
    Object
    Additional information
    TouchEvent object properties (inherit from BaseEvent):
    Property
    Type
    Description
    touches
    Array
    Touch event. An array of information of touch points staying in the screen.
    changedTouches
    Array
    Touch event. An array of information of touch points involving changes in the screen
    Special events: The touch events in canvas are non-bubbling events and thus have no currentTarget.

    4.4.7 type

    Represents the type of an event.

    4.4.8 timeStamp

    Number of milliseconds from the moment when the page is opened until the event is triggered.

    4.4.9 target

    The source component triggering the event.
    Property
    Type
    Description
    id
    String
    ID of the event source component
    dataset
    Object
    A collection of custom properties starting with data- on the event source component

    4.4.10 currentTarget

    The current component bound to the event.
    Property
    Type
    Description
    id
    String
    Current component ID
    dataset
    Object
    A collection of custom properties starting with data- on the current component
    Note:
    You can refer to the above example for "target" and "currentTarget". When you tap "inner view", the event objects "target" and "currentTarget" received by handleTap3 are both "inner", but the "target" and "currentTarget" received by handleTap2 are "inner" and "middle", respectively.

    4.4.11 dataset

    You can add some custom data to the component node so as to obtain the custom node data from the event for logical processing.
    In WXML, these custom data begins with data- and multiple words are joined with hyphen -. In the code, a hyphenation is converted to camel case, and the uppercase characters are automatically converted to lowercase ones. For example:
    data-element-type is converted to event.currentTarget.dataset.elementType;
    data-elementType is converted to event.currentTarget.dataset.elementtype.
    Example:
    <view data-alpha-beta="1" data-alphaBeta="2" bindtap="bindViewTap"> DataSet Test </view>
    Page({
    bindViewTap:function(event){
    event.currentTarget.dataset.alphaBeta === 1 // - will convert to camel case
    event.currentTarget.dataset.alphaBeta === 2 // uppercase will be converted to lowercase
    }
    })

    4.4.12 touches

    "touches" is an array, in which each element is a Touch object ("touches" in the canvas touch event is a CanvasTouch array). Touch points staying in the screen.
    Touch objects
    Property
    Type
    Description
    identifier
    Number
    Touch point identifier
    pageX, pageY
    Number
    Distance from the upper left corner of the document. The top left corner is the origin, the horizontal line is X-axis, and the vertical line is Y-axis.
    clientX, clientY
    Number
    Distance from the upper left corner of the page display area (the area on the screen without navigation pane). The horizontal line is X-axis and the vertical line is Y-axis.
    CanvasTouch objects
    Property
    Type
    Description
    identifier
    Number
    Touch point identifier
    x, y
    Number
    Distance from the upper left corner of Canvas. Canvas's upper left corner is the origin, the horizontal line is X-axis, and the vertical line is Y-axis.

    4.4.13 changedTouches

    Indicates the touch points involving changes (in the same format as "touches"), including touchstart (appearance of touch point), touchmove (movement of touch point), and touchend/touchcancel (disappearance of touch point).

    4.4.14 detail

    The data carried by the custom event. For example, the submission event of a form component carries user's input, and the error event of media carries the error message. For more information, see the event definitions in Component Overview.
    Similar to pageX and page Y, the x and y in detail of the tap event represent the distance from the upper left corner of the document.

    5. Basic Components

    The framework provides developers with a set of basic components that can be combined to achieve quick development. For more information, see Component Overview.
    What is component?
    A component is the basic building block of a view layer;
    A component includes a start tag, end tag, property (modifies the component), and content (between the two tags).
    <tagname property="value">
    Content goes here ...
    </tagname>
    Note:
    All components and properties are in lowercase and joined with a hyphen -
    Property Types
    Type
    Description
    Comment
    Boolean
    Boolean value
    If a component has this property, the property value is always considered true regardless of what the actual value is. The property value is false only when the component has no this property. If the property value is a variable, the variable is converted to a Boolean value.
    Number
    Number
    1, 2.5
    String
    String
    "string"
    Array
    Array
    [ 1, "string" ]
    Object
    Object
    { key: value }
    EventHandler
    Event handler
    "handlerName" is the event handler name defined in Page
    Any
    Any property
    -
    Common Properties
    All components have the following properties:
    Property
    Type
    Description
    Comment
    id
    String
    A unique identifier of a component
    Remains unique across the entire page.
    class
    String
    Component style class
    The style class defined in the WXSS
    style
    String
    Component inline style
    This is an inline style that can be dynamically set
    hidden
    Boolean
    Whether to display the component
    All components are displayed by default
    data-*
    Any
    Custom property
    When an event is triggered on the component, it is sent to the event handler
    bind* / catch*
    EventHandler
    Component event
    See Event
    Special Properties
    Almost all components have their own custom properties. You can modify the features or style of the component. See the definition of each Component Overview.

    6. Getting Node Information on Interface

    6.1 WXML Node Information

    The API for querying node information is used to get information of a node such as its properties, style, and position in the interface.
    This API is mostly used to query the current position and scroll position of a node in the interface.
    Sample code:
    const query = wx.createSelectorQuery()
    query.select('#the-id').boundingClientRect(function(res){
    res.top // Coordinates of the upper boundary of the #the-id node (relative to the display area)
    })
    query.selectViewport().scrollOffset(function(res){
    res.scrollTop // the vertical scroll position of the display area
    })
    query.exec()
    In the above example, #the-id is a node selector, which is slightly different from the CSS selector. For more information, see SelectorQuery.select.
    In custom components or pages that contain custom components, it is recommended to use this.createSelectorQuery, instead of wx.createSelectorQuery, to ensure nodes are selected within the correct area.

    6.2 WXML Node Layout Intersection

    The APIs for node layout intersection is used to listen to the layout intersection between two or more component nodes. This set of APIs are often used to determine whether certain nodes are visible to users and the percentage of the visible area.
    The set of APIs involve the following concepts:
    Reference node: The reference node for listening, whose layout area is taken as the reference area. If there are multiple reference nodes, the intersection of their layout areas is taken as the reference area. The page display area can also be used as a reference area;
    Target node: The target to be listened to. Only one target node is allowed by default (you can listen to multiple nodes at the same time using the selectAll option);
    Intersection area: The intersection area between the layout area of the target node and the reference area;
    Intersection percentage: The percentage of the intersection area in the reference area
    Threshold: If the intersection percentage reaches the threshold, the callback function of the listener is triggered. Multiple thresholds can be set;
    In the following sample code, a callback function is triggered each time the target node (specified using the selector .target-class) enters or exits the page display area.
    Sample code:
    Page({
    onLoad: function(){
    wx.createIntersectionObserver().relativeToViewport().observe('.target-class', (res) => {
    res.id // target node id
    res.dataset // target node dataset
    res.intersectionRatio // the proportion of the intersection area to the layout area of the target node
    res.intersectionRect // The intersection region
    res.intersectionRect.left // the coordinates of the left boundary of the intersection region
    res.intersectionRect.top // the upper boundary of the intersection region
    res.intersectionRect.width // width of the intersection region
    res.intersectionRect.height // height of the intersection region
    })
    }
    })
    The following sample code can be in the target node (specified with the selector .target-class) and the reference node (specified with the selector .relative-class) in the page display area of the intersection or separation, and the degree of intersection or separation to the extent of the layout of the target node of the area of the 20% and 50%, trigger the callback function.
    Sample code:
    Page({
    onLoad: function(){
    wx.createIntersectionObserver(this, {
    thresholds: [0.2, 0.5]
    }).relativeTo('.relative-class').relativeToViewport().observe('.target-class', (res) => {
    res.intersectionRatio // the proportion of the intersection area to the layout area of the target node
    res.intersectionRect // The intersection region
    res.intersectionRect.left // the coordinates of the left boundary of the intersection region
    res.intersectionRect.top // the upper boundary of the intersection region
    res.intersectionRect.width // width of the intersection region
    res.intersectionRect.height // height of the intersection region
    })
    }
    })
    Note:
    The intersection area with the page display area does not accurately represent the area visible to users, because the area involved in the calculation is the "layout area", which may be cropped or hidden by, for example, a node whose ancestor node has the overflow style "hidden", or obscured by, for example, a node with fixed position during drawing.
    In custom components or pages that contain custom components, it is recommended to use this.createIntersectionObserver, instead of wx.createIntersectionObserver, to ensure nodes are selected within the correct area.

    7. Respond to Changes in Display Area

    7.1 Display Area Size

    The display area refers to the area in Mini Program interface where you can arrange elements freely. By default, the size of the display area remains unchanged after the page initialization. But you can change the default size by either of the following two ways.
    Enable Screen Rotation on a Phone
    To allow the Mini Program pages to support screen rotation, set "pageOrientation": "auto" in the window segment of app.json, or configure "pageOrientation": "auto" in the page json file.
    The following example shows how to enable screen rotation in a single page json file.
    Code example:
    {
    "pageOrientation": "auto"
    }
    If the above declaration is added to the page, the page rotates and the display area is resized as the screen rotates.pageOrientation can be set to landscape, which locks the screen orientation to "Landscape".
    Enable Screen Rotation on an iPad
    To allow the Mini Program to support screen rotation, add "resizable": true to app.json.
    Code example:
    {
    "resizable": true
    }
    If the above declaration is added to the Mini Program, the Mini Program page rotates and the display area is resized as the screen rotates.
    Note:
    You cannot enable or disable screen rotation for a single page.

    7.2 Media Query

    Sometimes, the page layout varies with the size of the display area. You can use "media query" to solve most of the problems.
    Code example:
    .my-class {
    width: 40px;
    }
    
    @media (min-width: 480px) {
    /* Style rules that only work on screens 480px or wider */
    .my-class {
    width: 200px;
    }
    }

    7.3 Screen Rotation Event

    Sometimes, subtle layout changes cannot be controlled by using "media query" alone. In this case, js can be used.
    You can use selectorQuery.selectViewport to read the size of the display area of the page in js.
    The onResize of the page can be used to listen to the page resizing event. For custom components, you can use the "resize lifecycle" function to listen to the event. The callback function returns the size of the display area.
    Code example:
    Page({
    onResize(res) {
    res.size.windowWidth // The width of the new display area.
    res.size.windowHeight // the height of the new display area
    }
    })
    Component({
    pageLifetimes: {
    resize(res) {
    res.size.windowWidth // The width of the new display area.
    res.size.windowHeight // the height of the new display area
    }
    }
    })
    In addition, you can also use wx.onWindowResize to listen to the event (not recommended).

    8. Animation

    8.1 Common User Interface Animations

    In Mini Programs, you can use CSS transitions and CSS animations to create simple UI animations.
    During the animation creation, you can use bindtransitionend, bindanimationstart, bindanimationiteration, and bindanimationend to listen to animation events.
    Event
    Description
    transitionend
    A CSS transition ends or a step of wx.createAnimation ends
    animationstart
    CSS animation starts
    animationiteration
    A step of CSS animation ends
    animationend
    A CSS animation ends
    Note:
    These events are not bubbling events and need to be bound to the node that is being animated to take effect.
    It is also possible to use the wx.createAnimation interface to dynamically create simple animation effects (the keyframe animation interface described below is recommended in the new version of the mini program base library instead).

    8.2 Advanced Animations

    The above animations may not be applicable to complicated scenarios.
    You can dynamically adjust the "style" property of a node by responding to events with WXS. Changing the value of "style" continuously can produce animation effects. And this method can be used to dynamically generate animations based on users' touch events.
    Animation effects can also be achieved by using setData to change the user interface continuously. This can change the interface arbitrarily, but usually produces a large delay or stutter, and even causes the Mini Program to freeze. In this case, you can improve performance by changing the setData of the page to the setData in Custom Components. The following example shows how to generate a stopwatch animation using setData.
    
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