Nut Download Switch
Marquetta Marteney
The switch statement evaluates an expression, matching the expression's value against a series of case clauses, and executes statements after the first case clause with a matching value, until a break statement is encountered. The default clause of a switch statement will be jumped to if no case matches the expression's value.
A case clause used to match against expression. If the value of expression matches the value of any caseExpressionN, execution starts from the first statement after that case clause until either the end of the switch statement or the first encountered break.
A switch statement first evaluates its expression. It then looks for the first case clause whose expression evaluates to the same value as the result of the input expression (using the strict equality comparison) and transfers control to that clause, executing all statements following that clause.
If no matching case clause is found, the program looks for the optional default clause, and if found, transfers control to that clause, executing statements following that clause. If no default clause is found, the program continues execution at the statement following the end of switch. By convention, the default clause is the last clause, but it does not need to be so. A switch statement may only have one default clause; multiple default clauses will result in a SyntaxError.
You can use the break statement within a switch statement's body to break out early, often when all statements between two case clauses have been executed. Execution will continue at the first statement following switch.
In the appropriate context, other control-flow statements also have the effect of breaking out of the switch statement. For example, if the switch statement is contained in a function, then a return statement terminates the execution of the function body and therefore the switch statement. If the switch statement is contained in a loop, then a continue statement stops the switch statement and jumps to the next iteration of the loop.
This example will output the error "Uncaught SyntaxError: Identifier 'message' has already been declared", because the first const message = 'hello'; conflicts with the second const message = 'hi'; declaration, even when they're within their own separate case clauses. Ultimately, this is due to both const declarations being within the same block scope created by the switch body.
In the following example, if expr evaluates to Bananas, the program matches the value with case case 'Bananas' and executes the associated statement. When break is encountered, the program breaks out of switch and executes the statement following switch. If break were omitted, the statement for the case 'Cherries' would also be executed.
This example will output the error \"Uncaught SyntaxError: Identifier 'message' has already been declared\", because the first const message = 'hello'; conflicts with the second const message = 'hi'; declaration, even when they're within their own separate case clauses. Ultimately, this is due to both const declarations being within the same block scope created by the switch body.
The if, if-else and switch statements select statements to execute from many possible paths based on the value of an expression. The if statement executes a statement only if a provided Boolean expression evaluates to true. The if-else statement allows you to choose which of the two code paths to follow based on a Boolean expression. The switch statement selects a statement list to execute based on a pattern match with an expression.
The preceding example also demonstrates the default case. The default case specifies statements to execute when a match expression doesn't match any other case pattern. If a match expression doesn't match any case pattern and there's no default case, control falls through a switch statement.
A switch statement executes the statement list in the first switch section whose case pattern matches a match expression and whose case guard, if present, evaluates to true. A switch statement evaluates case patterns in text order from top to bottom. The compiler generates an error when a switch statement contains an unreachable case. That is a case that is already handled by an upper case or whose pattern is impossible to match.
The default case can appear in any place within a switch statement. Regardless of its position, the default case is evaluated only if all other case patterns aren't matched or the goto default; statement is executed in one of the switch sections.
Within a switch statement, control can't fall through from one switch section to the next. As the examples in this section show, typically you use the break statement at the end of each switch section to pass control out of a switch statement. You can also use the return and throw statements to pass control out of a switch statement. To imitate the fall-through behavior and pass control to other switch section, you can use the goto statement.
A case pattern may be not expressive enough to specify the condition for the execution of the switch section. In such a case, you can use a case guard. That is an additional condition that must be satisfied together with a matched pattern. A case guard must be a Boolean expression. You specify a case guard after the when keyword that follows a pattern, as the following example shows:
The switch itself does not maintain any state. Instead, when the state ofthe switch changes, the widget calls the onChanged callback. Most widgetsthat use a switch will listen for the onChanged callback and rebuild theswitch with a new value to update the visual appearance of the switch.
If the onChanged callback is null, then the switch will be disabled (itwill not respond to input). A disabled switch's thumb and track are renderedin shades of grey by default. The default appearance of a disabled switchcan be overridden with inactiveThumbColor and inactiveTrackColor.
This example shows a toggleable Switch. When the thumb slides to the otherside of the track, the switch is toggled between on/off. link To create a local project with this code sample, run:
flutter create --sample=material.Switch.1 mysample
This example shows how to customize Switch using MaterialStatePropertyswitch properties. link To create a local project with this code sample, run:
flutter create --sample=material.Switch.2 mysample
Numeric comparison is performed if SwitchValue and the case value are both numbers or numeric strings. Each case value is considered separately and does not affect the type of comparison used for other case values. [v1.1.36+]: If either expression is a lone quoted string, the comparison is non-numeric. For example, switch v:="00" matches case "00": or case 0: but not case "0":.
The first statement of each case may be below Case or on the same line, following the colon. Each case implicitly ends at the next Case/Default or the closing brace. Unlike the switch statement found in some other languages, there is no implicit fall-through and Break is not used (except to break out of an enclosing loop).
A switch expression produces a value based on the expressionbody of whichever case matches. You can use a switch expression wherever Dart allows expressions,except at the start of an expression statement. For example:
Enums and sealed types are particularly useful forswitches because, even without a default case, their possible values are known and fully enumerable. Use the sealed modifier on a class to enableexhaustiveness checking when switching over subtypes of that class:
If anyone were to add a new subclass of Shape, this switch expression would be incomplete. Exhaustiveness checking would inform you of the missing subtype.This allows you to use Dart in a somewhat functional algebraic datatype style.
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