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');

}

 

---

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

---

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。