省道构造函数后冒号

这段代码来自扑动画廊，我正在尝试理解和适应它。我就会知道这个语法是什么意思:

class DemoItem<T> {
DemoItem({
this.valueName,
this.hintName,
this.valueSurname,
this.hintSurname,
this.builder,
this.valueToString


}) : textController = new TextEditingController(text: valueToString(valueName));

特别是我会知道 构造函数后面的冒号是什么意思，如果有一个方法来定义另一个文本编辑控制器，除了一个已经定义。

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最佳答案

The part after : is called "initializer list. It is a ,-separated list of expressions that can access constructor parameters and can assign to instance fields, even final instance fields. This is handy to initialize final fields with calculated values.

The initializer list is also used to call other constructors like : ..., super('foo').

Since about Dart version 1.24 the initializer list also supports assert(...) which is handy to check parameter values.

The initializer list can't read from this because the super constructors need to be completed before access to this is valid, but it can assign to this.xxx.

Pointing out as mentioned in the comments by user693336:

This also means the initializer list is executed before the constructor body. Also the initializer lists of all superclasses are executed before any of the constructor bodies are executed.

Example (copied from https://github.com/dart-lang/language/issues/1394):

class C {
final int x;
final int y;
C(this.x) : y = x + 1;
}
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To elaborate on other answers and to complete the syntax, it is also possible to have a real body for the constructor along with initializer code

NonNegativePoint(this.x, this.y) : assert(x >= 0), assert(y >= 0) {
print('I just made a NonNegativePoint: ($x, $y)');
}

^ Here the assertions happen before the execution of the body

Another use case is to assign values to final fields before body executes

final num x;
final num y;


Point.fromJson(Map<String, num> json) : x = json['x'], y = json['y'] {
print('In Point.fromJson(): ($x, $y)');
}

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