Flutter 学习之旅(四十七) 关键字 async syn

相对于java dart 还有一些非常重要的关键字,他们各有各特点,但是如果你不理解他们的用法,看代码的话肯定是一点也看不懂的,先来说一下async 这个我们之前看到过的

async await

应用场景 异步 返回类型 Future

Future<int> test_async() async {
  Future<int>  future = Future.delayed(Duration(seconds: 2),()=>1);
  return future;
}

如果我们想要结果的话则需要配合await 使用

Future<int> test_async() async {
  int  future =await Future.delayed(Duration(seconds: 2),()=>1);
  printString(future);
  return future;
}
}

sync* yield

应用场景 同步 返回类型Iterable<T>

void test(){

  test_sync([4,7,12,15,22]).forEach((element) {printString(element);});

}
Iterable<int> test_sync(List<int> array) sync* {
  for (int i = 0; i < array.length; i++) {
    yield array[i];
  }
}

结果

I/flutter ( 7129): tian.shm =4
I/flutter ( 7129): tian.shm =7
I/flutter ( 7129): tian.shm =12
I/flutter ( 7129): tian.shm =15
I/flutter ( 7129): tian.shm =22

sync* yield*

应用场景 同步 返回类型Iterable<T> ,相对于async* yield ,yield* 后面必须是一个Iterable<T> ,yield 则必须是T ,

test() {
  test_yield().forEach((element) {printString(element);});
}

Iterable<int> test_yield() sync*{
  yield* test_sync([4,7,12,15,22]).map((e){
    return e*e;
  });
}

Iterable<int> test_sync(List<int> array) sync* {
  for (int i = 0; i < array.length; i++) {
    yield array[i];
  }
}

结果

I/flutter ( 7129): tian.shm =4
I/flutter ( 7129): tian.shm =7
I/flutter ( 7129): tian.shm =12
I/flutter ( 7129): tian.shm =15
I/flutter ( 7129): tian.shm =22

async* yield

应用场景 异步 返回类型 Stream<T> yield 后面必须是 T

test() {
  test_async([2, 15,42,53,64,73,77,99]).interval(Duration(seconds: 1)).listen((event) {printString('listen1:$event');});
  
}
Stream<int> test_async(List<int> array) async*{
  for(int i=0;i<array.length;i++){
    yield array[i];
  }
}

结果

I/flutter ( 7129): tian.shm =listen1:2
I/flutter ( 7129): tian.shm =listen1:15
I/flutter ( 7129): tian.shm =listen1:42
I/flutter ( 7129): tian.shm =listen1:53
I/flutter ( 7129): tian.shm =listen1:64
I/flutter ( 7129): tian.shm =listen1:73
I/flutter ( 7129): tian.shm =listen1:77
I/flutter ( 7129): tian.shm =listen1:99

async* yield*

应用场景 异步 返回类型 Stream<T> yield* 后面必须是Stream<T>

test() {
  test_async_1().listen((event) {printString('listen2:$event');});
}
Stream<int> test_async(List<int> array) async*{
  for(int i=0;i<array.length;i++){
    yield array[i];
  }
}
Stream<int> test_async_1() async*{
  yield* test_async([2, 15,42,53,64,73,77,99]).map((event) => event*event);
}

结果

I/flutter ( 7129): tian.shm =listen2:4
I/flutter ( 7129): tian.shm =listen2:225
I/flutter ( 7129): tian.shm =listen2:1764
I/flutter ( 7129): tian.shm =listen2:2809
I/flutter ( 7129): tian.shm =listen2:4096
I/flutter ( 7129): tian.shm =listen2:5329
I/flutter ( 7129): tian.shm =listen2:5929
I/flutter ( 7129): tian.shm =listen2:9801

我学习flutter的整个过程都记录在里面了
https://www.jianshu.com/c/36554cb4c804

最后附上demo 地址

https://github.com/tsm19911014/tsm_flutter