等待for循环的承诺 [英] Wait for promises from a for loop
问题描述
以下代码并没有真正做到我想要的.
The following code does not really do what I want.
function doIt () {
return new Promise (function (resolve, reject) {
var promises = [];
db.transaction(function(tx1){
tx1.executeSql(function(tx2, rs) {
for (var i = i; i < N; i++) {
promises.push(db.transaction(function(tx3){
...
}));
}
});
});
Promise.all(promises).then(resolve);
});
}
现在它不起作用了,因为Promise.all()被执行了,在所有promise都在数组中之前,至少我认为这是正确的.
Now it does not work, because Promise.all() gets executed, before all promises are in the array, at least I think that's correct.
是否有一种优雅的方法可以保证在兑现结束之前完成所有这些承诺?
Is there a elegant way to guarantee that all these promises are finished, before doIt ends?
推荐答案
您可以移动Promise.all()
所在的位置,以便在for
循环完成填充数组之后就可以将其正确:
You can just move where the Promise.all()
is located so that it's right AFTER the for
loop has finished populating the array:
function doIt () {
return new Promise (function (resolve, reject) {
var promises = [];
db.transaction(function(tx1){
tx1.executeSql(function(tx2, rs) {
for (var i = i; i < N; i++) {
promises.push(db.transaction(function(tx3){
...
}));
}
Promise.all(promises).then(resolve);
});
});
});
}
仅供参考,混淆承诺和回调可能会造成混淆,并使一致的错误处理特别困难. tx1.executeSql()
已经返回承诺了吗?如果是这样,您可以仅使用数据库功能已经创建的promise来做一些更清洁的事情:
FYI, mixing promises and callbacks can be confusing and makes consistent error handling particularly difficult. Does tx1.executeSql()
already return a promise? If so, you can do something cleaner using only promises that are already created by your database functions like this:
function doIt() {
return db.transaction.then(function(tx1) {
return tx1.executeSql().then(function(tx2, rs) {
var promises = [];
for (var i = i; i < N; i++) {
promises.push(db.transaction().then(function(tx3) {
...
}));
}
return Promise.all(promises).then(resolve);
});
});
}
这将返回来自.then()
处理程序的诺言,以将诺言自动链接在一起.
This returns promises from .then()
handlers to auto-chain promises together.
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