2020-04-15 03:15:30 +00:00
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# 联合(Union)
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2020-04-16 07:09:09 +00:00
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联合的定义和接口非常像,**但不允许定义字段**。这两个类型的实现原理也差不多,对于`Async-graphql`来说,联合类型是接口类型的子集。
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下面把接口定义的例子做一个小小的修改,去掉字段的定义。
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```rust
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use async_graphql::*;
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struct Circle {
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radius: f32,
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}
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2020-09-18 00:52:13 +00:00
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#[Object]
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2020-04-16 07:09:09 +00:00
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impl Circle {
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async fn area(&self) -> f32 {
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std::f32::consts::PI * self.radius * self.radius
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}
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async fn scale(&self, s: f32) -> Shape {
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Circle { radius: self.radius * s }.into()
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}
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}
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struct Square {
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width: f32,
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}
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2020-09-18 00:52:13 +00:00
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#[Object]
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2020-04-16 07:09:09 +00:00
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impl Square {
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async fn area(&self) -> f32 {
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self.width * self.width
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}
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async fn scale(&self, s: f32) -> Shape {
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Square { width: self.width * s }.into()
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}
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}
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2020-09-18 00:52:13 +00:00
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#[derive(Union)]
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2020-05-11 04:39:43 +00:00
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enum Shape {
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Circle(Circle),
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Square(Square),
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}
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```
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2020-10-01 03:23:24 +00:00
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## 展平嵌套联合
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GraphQL的有个限制是`Union`类型内不能包含其它联合类型。 所有成员必须为`Object`。
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位置支持嵌套`Union`,我们可以用`#graphql(flatten)`,是它们合并到上级`Union`类型。
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```rust
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#[derive(async_graphql::Union)]
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pub enum TopLevelUnion {
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A(A),
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// 除非我们使用`flatten`属性,否则将无法编译
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#[graphql(flatten)]
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B(B),
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}
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#[derive(async_graphql::SimpleObject)]
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pub struct A {
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// ...
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}
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#[derive(async_graphql::Union)]
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pub enum B {
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C(C),
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D(D),
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}
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#[derive(async_graphql::SimpleObject)]
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pub struct C {
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// ...
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}
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#[derive(async_graphql::SimpleObject)]
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pub struct D {
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// ...
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}
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```
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上面的示例将顶级`Union`转换为以下等效形式:
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```rust
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#[derive(async_graphql::Union)]
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pub enum TopLevelUnion {
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A(A),
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C(C),
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D(D),
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}
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```
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