深入研究了dart 的非同步事件執行流程。   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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將 main() 中的註釋逐行去掉，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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通過仔細分析發現，非同步執行雖然是個好東西，但當非同步呼叫鏈過長時，如果不仔細檢查程式碼，程式碼的執行順序將很難預測正確。