dart 异步事件执行流程分析（一）

深入研究了dart 的异步事件执行流程。

 

main() {

/// testNoAwait() and testWithCallerAwait() exe order is same.

// testNoAwait();

// testWithCallerAwait();

 

/// testWithCalleeAwait() and testBothAwait() exe order is same;

// testWithCalleeAwait();

// testBothAwait();

 

/// testAllAwait() exe order is same as the code text order;

// testAllAwait();

}

 

testNoAwait() {

print('testNoAwait start…');

runNoAwait(delayedNoAwait, 10);

runNoAwait(delayedNoAwait, 5);

runNoAwait(delayedNoAwait, 1);

print('testNoAwait completed…\n');

}

 

testWithCallerAwait() {

print('testWithCallerAwait start…');

runWithAwait(delayedNoAwait, 10);

runWithAwait(delayedNoAwait, 5);

runWithAwait(delayedNoAwait, 1);

print('testWithCallerAwait completed…\n');

}

 

testWithCalleeAwait() {

print('testWithCalleeAwait start…');

runNoAwait(delayedWithAwait, 10);

runNoAwait(delayedWithAwait, 5);

runNoAwait(delayedWithAwait, 1);

print('testWithCalleeAwait completed…\n');

}

 

testBothAwait() {

print('testBothAwait start…');

runWithAwait(delayedWithAwait, 10);

runWithAwait(delayedWithAwait, 5);

runWithAwait(delayedWithAwait, 1);

print('testBothAwait completed…\n');

}

 

testAllAwait() async {

print('testAllAwait start…');

await runWithAwait(delayedWithAwait, 10);

await runWithAwait(delayedWithAwait, 5);

await runWithAwait(delayedWithAwait, 1);

print('testAllAwait completed…\n');

}

 

void runNoAwait(void delayed(int s), int seconds) {

delayed(seconds);

}

 

void runWithAwait(void delayed(int s), int seconds) async {

await delayed(seconds);

}

 

void delayedNoAwait(int s) {

print('delayedNoAwait $s start —');

Future.delayed(

Duration(seconds: s), () => print('delayedNoAwait $s task start…'))

.then((_) => print('delayedNoAwait $s task completed.'));

print('delayedNoAwait $s completed —\n');

}

 

void delayedWithAwait(int s) async {

print('delayedWithAwait $s start —');

await Future.delayed(Duration(seconds: s),

() => print('delayedWithAwait $s task start…'))

.then((_) => print('delayedWithAwait $s task completed.'));

print('delayedWithAwait $s completed —\n');

}

 

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将 main() 中的注释逐行去掉，output 如下：

testNoAwait start…
delayedNoAwait 10 start —
delayedNoAwait 10 completed —

delayedNoAwait 5 start —
delayedNoAwait 5 completed —

delayedNoAwait 1 start —
delayedNoAwait 1 completed —

testNoAwait completed…

delayedNoAwait 1 task start…
delayedNoAwait 1 task completed.
delayedNoAwait 5 task start…
delayedNoAwait 5 task completed.
delayedNoAwait 10 task start…
delayedNoAwait 10 task completed.
Exited

testWithCallerAwait start…
delayedNoAwait 10 start —
delayedNoAwait 10 completed —

delayedNoAwait 5 start —
delayedNoAwait 5 completed —

delayedNoAwait 1 start —
delayedNoAwait 1 completed —

testWithCallerAwait completed…

delayedNoAwait 1 task start…
delayedNoAwait 1 task completed.
delayedNoAwait 5 task start…
delayedNoAwait 5 task completed.
delayedNoAwait 10 task start…
delayedNoAwait 10 task completed.
Exited

testWithCalleeAwait start…
delayedWithAwait 10 start —
delayedWithAwait 5 start —
delayedWithAwait 1 start —
testWithCalleeAwait completed…

delayedWithAwait 1 task start…
delayedWithAwait 1 task completed.
delayedWithAwait 1 completed —

delayedWithAwait 5 task start…
delayedWithAwait 5 task completed.
delayedWithAwait 5 completed —

delayedWithAwait 10 task start…
delayedWithAwait 10 task completed.
delayedWithAwait 10 completed —

Exited

testBothAwait start…
delayedWithAwait 10 start —
delayedWithAwait 5 start —
delayedWithAwait 1 start —
testBothAwait completed…

delayedWithAwait 1 task start…
delayedWithAwait 1 task completed.
delayedWithAwait 1 completed —

delayedWithAwait 5 task start…
delayedWithAwait 5 task completed.
delayedWithAwait 5 completed —

delayedWithAwait 10 task start…
delayedWithAwait 10 task completed.
delayedWithAwait 10 completed —

Exited

testAllAwait start…
delayedWithAwait 10 start —
delayedWithAwait 10 task start…
delayedWithAwait 10 task completed.
delayedWithAwait 10 completed —

delayedWithAwait 5 start —
delayedWithAwait 5 task start…
delayedWithAwait 5 task completed.
delayedWithAwait 5 completed —

delayedWithAwait 1 start —
delayedWithAwait 1 task start…
delayedWithAwait 1 task completed.
delayedWithAwait 1 completed —

testAllAwait completed…

Exited

 

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通过仔细分析发现，异步执行虽然是个好东西，但当异步调用链过长时，如果不仔细检查代码，代码的执行顺序将很难预测正确。