harmony 鸿蒙if/else: Conditional Rendering

2023-06-24
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if/else: Conditional Rendering

ArkTS provides conditional rendering. Use the if, else, and else if statements to enable your application to display different content based on the condition or state.

NOTE

Since API version 9, this API is supported in ArkTS widgets.

Rules of Use

The if, else, and else if statements are supported.
The conditional statements following if and else if can use state variables.
Use of the conditional statements within a container component is allowed for building different child components.
Conditional statements are “transparent” when it comes to the parent-child relationship of components. Rules about permissible child components must be followed when there is one or more if statements between the parent and child components.
The build function inside each branch must follow the special rules for build functions. Each of such build functions must create one or more components. An empty build function that creates no components will result in a syntax error.
Some container components impose restrictions on the type or number of child components. When conditional statements are used in such components, these restrictions also apply to the components created in conditional statements. For example, when a conditional statement is used in the <Grid> container component, whose child components can only be <GridItem>, only the <GridItem> component can be used in the conditional statement.

Update Mechanism

A conditional statement updates whenever a state variable used inside the if condition or the else if condition changes. Specifically:

The conditional statement re-evaluates the conditions. If the evaluation of the conditions changes, steps 2 and 3 are performed. Otherwise, no follow-up operation is required.
The framework removes all child components that have been built.
The framework executes the build function of the branch again to add the generated child component to its parent component. If an applicable else branch is missing, no new build function will be executed.

A condition can include Typescript expressions. As for any expression inside build functions, such an expression must not change any application state.

Application Scenarios

Using if for Conditional Rendering

@Entry
@Component
struct ViewA {
  @State count: number = 0;

  build() {
    Column() {
      Text(`count=${this.count}`)

      if (this.count > 0) {
        Text(`count is positive`)
          .fontColor(Color.Green)
      }

      Button('increase count')
        .onClick(() => {
          this.count++;
        })

      Button('decrease count')
        .onClick(() => {
          this.count--;
        })
    }
  }
}

Each branch of the if statement includes a build function. Each of such build functions must create one or more components. On initial render, if will execute a build function and add the generated child component to its parent component.

if updates whenever a state variable used inside the if condition or the else if condition changes and re-evaluates the conditions. If the evaluation of the conditions changes, it means that another branch of if needs to be built. In this case, the ArkUI framework will:

Remove all previously rendered components (of the earlier branch).
Execute the build function of the branch and add the generated child component to its parent component.

In the preceding example, if count increases from 0 to 1, then, if updates, the condition count > 0 is re-evaluated, and the evaluation result changes from false to true. Therefore, the positive branch build function will be executed, which creates a <Text> component and adds it to the <Column> parent component. If count changes back to 0 later, then, the <Text> component will be removed from the <Column> component. Since there is no else branch, no new build function will be executed.

if … else … and Child Component State

This example involves if… else… and a child component with an \@State decorated variable.

@Component
struct CounterView {
  @State counter: number = 0;
  label: string = 'unknown';

  build() {
    Row() {
      Text(`${this.label}`)
      Button(`counter ${this.counter} +1`)
        .onClick(() => {
          this.counter += 1;
        })
    }
  }
}

@Entry
@Component
struct MainView {
  @State toggle: boolean = true;

  build() {
    Column() {
      if (this.toggle) {
        CounterView({ label: 'CounterView #positive' })
      } else {
        CounterView({ label: 'CounterView #negative' })
      }
      Button(`toggle ${this.toggle}`)
        .onClick(() => {
          this.toggle = !this.toggle;
        })
    }
  }
}

On first render, the CounterView (label: ‘CounterView #positive’) child component is created. This child component carries the \@State decorated variable counter. When the CounterView.counter state variable is updated, the CounterView (label: ‘CounterView #positive’) child component is re-rendered, with its state variable value preserved. When the value of the MainView.toggle state variable changes to false, the if statement inside the MainView parent component gets updated, and subsequently the CounterView (label: ‘CounterView #positive’) child component will be removed. At the same time, a new CounterView (label: ‘CounterView #negative’) child component will be created. Its own counter state variable is set to the initial value 0.

NOTE

CounterView (label: ‘CounterView #positive’) and CounterView (label: ‘CounterView #negative’) are two distinct instances of the same custom component. When the if branch changes, there is no updating of an existing child component and no preservation of state.

The following example shows the required modifications if the value of counter be preserved when the if condition changes:

@Component
struct CounterView {
  @Link counter: number;
  label: string = 'unknown';

  build() {
    Row() {
      Text(`${this.label}`)
      Button(`counter ${this.counter} +1`)
        .onClick(() => {
          this.counter += 1;
        })
    }
  }
}

@Entry
@Component
struct MainView {
  @State toggle: boolean = true;
  @State counter: number = 0;

  build() {
    Column() {
      if (this.toggle) {
        CounterView({ counter: $counter, label: 'CounterView #positive' })
      } else {
        CounterView({ counter: $counter, label: 'CounterView #negative' })
      }
      Button(`toggle ${this.toggle}`)
        .onClick(() => {
          this.toggle = !this.toggle;
        })
    }
  }
}

Here, the \@State decorated variable counter is owned by the parent component. Therefore, it is not destroyed when a CounterView component instance is removed. The CounterView component refers to the state by an \@Link decorator. This technique is sometimes referred to as “pushing up the state in the component tree.” The state must be moved from a child to its parent (or parent of parent) to avoid losing it when the conditional content (or repeated content) is destroyed.

Nested if Statements

The nesting of if statements makes no difference to the rule about the parent component.

@Entry
@Component
struct CompA {
  @State toggle: boolean = false;
  @State toggleColor: boolean = false;

  build() {
    Column() {
      Text('Before')
        .fontSize(15)
      if (this.toggle) {
        Text('Top True, positive 1 top')
          .backgroundColor('#aaffaa').fontSize(20)
        // Inner if statement
        if (this.toggleColor) {
          Text('Top True, Nested True, positive COLOR  Nested ')
            .backgroundColor('#00aaaa').fontSize(15)
        } else {
          Text('Top True, Nested False, Negative COLOR  Nested ')
            .backgroundColor('#aaaaff').fontSize(15)
        }
      } else {
        Text('Top false, negative top level').fontSize(20)
          .backgroundColor('#ffaaaa')
        if (this.toggleColor) {
          Text('positive COLOR  Nested ')
            .backgroundColor('#00aaaa').fontSize(15)
        } else {
          Text('Negative COLOR  Nested ')
            .backgroundColor('#aaaaff').fontSize(15)
        }
      }
      Text('After')
        .fontSize(15)
      Button('Toggle Outer')
        .onClick(() => {
          this.toggle = !this.toggle;
        })
      Button('Toggle Inner')
        .onClick(() => {
          this.toggleColor = !this.toggleColor;
        })
    }
  }
}