Download Switch Dlc [NEW]
Chieko Topalian
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:
Unlike if-then and if-then-else statements, the switch statement can have a number of possible execution paths. A switch works with the byte, short, char, and int primitive data types. It also works with enumerated types (discussed in Enum Types), the String class, and a few special classes that wrap certain primitive types: Character, Byte, Short, and Integer (discussed in Numbers and Strings).
The body of a switch statement is known as a switch block. A statement in the switch block can be labeled with one or more case or default labels. The switch statement evaluates its expression, then executes all statements that follow the matching case label.
Deciding whether to use if-then-else statements or a switch statement is based on readability and the expression that the statement is testing. An if-then-else statement can test expressions based on ranges of values or conditions, whereas a switch statement tests expressions based only on a single integer, enumerated value, or String object.
Another point of interest is the break statement. Each break statement terminates the enclosing switch statement. Control flow continues with the first statement following the switch block. The break statements are necessary because without them, statements in switch blocks fall through: All statements after the matching case label are executed in sequence, regardless of the expression of subsequent case labels, until a break statement is encountered. The program SwitchDemoFallThrough shows statements in a switch block that fall through. The program displays the month corresponding to the integer month and the months that follow in the year:
Technically, the final break is not required because flow falls out of the switch statement. Using a break is recommended so that modifying the code is easier and less error prone. The default section handles all values that are not explicitly handled by one of the case sections.
In Java SE 7 and later, you can use a String object in the switch statement's expression. The following code example, StringSwitchDemo, displays the number of the month based on the value of the String named month:
The String in the switch expression is compared with the expressions associated with each case label as if the String.equals method were being used. In order for the StringSwitchDemo example to accept any month regardless of case, month is converted to lowercase (with the toLowerCase method), and all the strings associated with the case labels are in lowercase.
Note: This example checks if the expression in the switch statement is null. Ensure that the expression in any switch statement is not null to prevent a NullPointerException from being thrown.
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
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.
Guards evaluate an arbitrary boolean expression after matching.This allows you to add further constraints onwhether a case body should execute.When the guard clause evaluates to false, execution proceeds to the next case ratherthan exiting the entire switch.
By switching to edit mode, users can modify settings and export a new file. Switch lets you specify a new file format; a new video or audio codec; trim, scale or crop the video; add metadata (Pro only), and much more. Export to Apple ProRes*, Windows Media, H264, MPEG-2 Video, MP4, MPEG-2 program streams and transport streams.
Optionally a new branch could be created with either -c, -C,automatically from a remote branch of same name (see --guess), ordetach the working tree from any branch with --detach, along withswitching.
You can use the @-N syntax to refer to the N-th lastbranch/commit switched to using "git switch" or "git checkout"operation. You may also specify - which is synonymous to @-1.This is often used to switch quickly between two branches, or to undoa branch switch by mistake.
Proceed even if the index or the working tree differs fromHEAD. Both the index and working tree are restored to matchthe switching target. If --recurse-submodules is specified,submodule content is also restored to match the switchingtarget. This is used to throw away local changes.
If you have local modifications to one or more files that aredifferent between the current branch and the branch to whichyou are switching, the command refuses to switch branches inorder to preserve your modifications in context. However,with this option, a three-way merge between the currentbranch, your working tree contents, and the new branch isdone, and you will be on the new branch.
When you run git checkout or git switch and only have oneremote, it may implicitly fall back on checking out andtracking e.g. origin/. This stops working as soonas you have more than one remote with a reference. This setting allows for setting the name of apreferred remote that should always win when it comes todisambiguation. The typical use-case is to set this toorigin.
Currently this is used by git-switch[1] andgit-checkout[1] when git checkout or git switch will checkout the branch on another remote,and by git-worktree[1] when git worktree add refers to aremote branch. This setting might be used for other checkout-likecommands or functionality in the future.
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