Created
September 7, 2022 08:59
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CopyWith that works for "resetting" nullable fields
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| class Foo { | |
| int bar; | |
| int? baz; | |
| Foo(this.bar, this.baz); | |
| // The "normal" copyWith does not allow to "reset" nullable | |
| // value to `null`. | |
| // Passing `null` means "keep the value" | |
| Foo copyWith({ | |
| int? bar, | |
| int? baz, | |
| }) => Foo( | |
| bar ?? this.bar, | |
| // This means we can never "reset" baz to `null` | |
| baz ?? this.baz, | |
| ); | |
| // Using callback for the copy means we get more options | |
| // - dont pass the function (`null`) => keep the old value | |
| // - pass the function (non `null`) => use the returned value | |
| // - can be non-null | |
| // - can be null | |
| Foo copyWithFunc({ | |
| int Function(int barOld)? bar, | |
| // null function => no change; null return from function => reset to null | |
| int? Function(int? bazOld)? baz, | |
| }) => Foo( | |
| bar == null ? this.bar : bar(this.bar), | |
| // can now be reset to null | |
| baz == null ? this.baz : baz(this.baz), | |
| ); | |
| } | |
| void main() { | |
| final foo = Foo(0,0); | |
| // 1) change value | |
| final one = foo.copyWithFunc(baz: (_) => 1); | |
| print(one.baz); | |
| // 2) keep value | |
| final old = foo.copyWithFunc(baz: null); | |
| print(old.baz); | |
| // 3) reset to null | |
| final nul = foo.copyWithFunc(baz: (_) => null); | |
| print(nul.baz); | |
| // For nested objects we could even do the following | |
| // foo.copyWithFunc(nested: (old) => old.copyWithFunc(baz: (_) => newBaz)); | |
| } |
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