indexed_db getObject（） - 如何返回结果 [英] indexed_db getObject() - how to return result
问题描述
我想知道如何定义数据类型，以及如何使用getObject（）返回对象（记录）。目前，我已经能够使用结果（记录）在获取它的函数之外的唯一方法是调用具有结果的另一个函数。这样，不需要指定数据类型。但是，如果我想返回值，我需要定义数据类型，我不能找到它是什么。我试过动态，但似乎没有工作。例如：
fDbSelectOneClient（String sKey，Function fSuccess，String sErmes）{
try {
idb.Transaction oDbTxn = ogDb1.transaction（sgTblClient，'readwrite'）;
idb.ObjectStore oDbTable = oDbTxn.objectStore（sgTblClient）;
idb.Request oDbReqGet = oDbTable。 getObject（sKey）;
oDbReqGet.onSuccess.listen（（val）{
if（oDbReqGet.result == null）{
window.alert（Record $ sKey is found - $ sErmes）;
} else {
/////// return oDbReqGet.result; ///这是我想要做的事
fSuccess（oDbReqGet.result）; // /这是我要做的
}}）;
oDbReqGet.onError.first.then（（e）{window.alert（
读取单个客户端错误。 $ sKey.Error = $ {e}）;}）;
} catch（oError）{
window.alert（尝试读取客户端$ sKey的记录时出错
Error = $ {oError}）;
}
}
fAfterAddOrUpdateClient（oDbRec）{
///这是用作fSuccess的函数之一
正如别人曾经说过（不记得谁），一旦你开始使用一个异步API，一切都需要异步。
典型的Dart模式是使用 Future + Completer pair（虽然你在上面的问题中没有什么错误 - 这更像是一个风格问题）。
概念上， fDbSelectOneClient 函数创建一个完成对象，函数返回 completer.future 。然后，当异步调用完成时，您调用 completer.complete ，传递值。
该函数将调用 fDbSelectOneClient（...）。then（（result）=> print（result））; p>
您的代码可以重构如下：
import'dart：async'; //完成者需要
未来fDbSelectOneClient（String sKey）{
var completer = new Completer（）;
try {
idb.Transaction oDbTxn = ogDb1.transaction（sgTblClient，'readwrite'）;
idb.ObjectStore oDbTable = oDbTxn.objectStore（sgTblClient）;
idb.Request oDbReqGet = oDbTable.getObject（sKey）;
oDbReqGet.onSuccess.listen（（val）=> completer.complete（oDbReqGet.result））;
oDbReqGet.onError.first.then（（err）=> completer.completeError（err））;
}
catch（oError）{
completer.completeError（oError）;
}
return completer.future; // return the future
}
//调用其他地方的代码
foo（）{
var key =Mr Blue;
fDbSelectOneClient（key）
.then（（result）{
//对结果做一些事情（注意，可能为null）
}）
..catchError（（err）{// note method chaining ..
//执行错误
};
}
这个future / completer对只适用于一次镜头（即，如果 onSuccess.listen 被多次调用，那么第二次你会得到一个未来已经完成的错误（我做了一个假设基于函数名 fDbSelectOneClient ，你只是期望选择单个记录。
要多次返回单个未来的值，您可能需要使用未来的新流功能 - 详情： http://news.dartlang.org/2012/11 /introducing-new-streams-api.html
请注意，Futures和Completers支持泛型，因此您可以强烈地键入返回类型，如下所示：
//强类型未来
Future< SomeReturnType> fDbSelectOneClient（String sKey）{
var completer = new Completer< SomeReturnType>（）;
completer.complete（new SomeReturnType（））;
}
foo（）{
//强类型化结果
fDbSelectOneClient（Mr Blue）。 ））;
}
I would like to know how to define the data type and how to return the object (record) using getObject(). Currently, the only way that I have been able to use the result (record) outside of the function that obtains it is to call another function with the result. That way, the data-type does not need to be specified. However if I want to return the value, I need to define the data-type and I can't find what it is. I tried "dynamic" but that didn't appear to work. For example ":
fDbSelectOneClient(String sKey, Function fSuccess, String sErmes) {
try {
idb.Transaction oDbTxn = ogDb1.transaction(sgTblClient, 'readwrite');
idb.ObjectStore oDbTable = oDbTxn.objectStore(sgTblClient);
idb.Request oDbReqGet = oDbTable.getObject(sKey);
oDbReqGet.onSuccess.listen((val){
if (oDbReqGet.result == null) {
window.alert("Record $sKey was not found - $sErmes");
} else {
///////return oDbReqGet.result; /// THIS IS WHAT i WANT TO DO
fSuccess(oDbReqGet.result); /// THIS IS WHAT i'm HAVING TO DO
}});
oDbReqGet.onError.first.then((e){window.alert(
"Error reading single Client. Key = $sKey. Error = ${e}");});
} catch (oError) {
window.alert("Error attempting to read record for Client $sKey.
Error = ${oError}");
}
}
fAfterAddOrUpdateClient(oDbRec) {
/// this is one of the functions used as "fSuccess above
As someone else once said (can't remember who), once you start using an async API, everything needs to be async.
A typical "Dart" pattern to do this would be to use a Future + Completer pair (although there's nothing inherently wrong with what you've done in your question above - it's more a question of style...).
Conceptually, the fDbSelectOneClient function creates a completer object, and the function returns the completer.future. Then, when the async call completes, you call completer.complete, passing the value in.
A user of the function would call fDbSelectOneClient(...).then((result) => print(result)); to make use of the result in an async way
Your code above could be refactored as follows:
import 'dart:async'; // required for Completer
Future fDbSelectOneClient(String sKey) {
var completer = new Completer();
try {
idb.Transaction oDbTxn = ogDb1.transaction(sgTblClient, 'readwrite');
idb.ObjectStore oDbTable = oDbTxn.objectStore(sgTblClient);
idb.Request oDbReqGet = oDbTable.getObject(sKey);
oDbReqGet.onSuccess.listen((val) => completer.complete(oDbReqGet.result));
oDbReqGet.onError.first.then((err) => completer.completeError(err));
}
catch (oError) {
completer.completeError(oError);
}
return completer.future; // return the future
}
// calling code elsewhere
foo() {
var key = "Mr Blue";
fDbSelectOneClient(key)
.then((result) {
// do something with result (note, may be null)
})
..catchError((err) { // note method chaining ..
// do something with error
};
}
This future/completer pair only works for one shot (ie, if the onSuccess.listen is called multiple times, then the second time you will get a "Future already completed" error. (I've made an assumption on the basis of the function name fDbSelectOneClient that you are only expecting to select a single record.
To return a value from a single future multiple times, you'll probably have to use the new streams feature of the Future - see here for more details: http://news.dartlang.org/2012/11/introducing-new-streams-api.html
Note also, that Futures and Completers support generics, so you can strongly type the return type as follows:
// strongly typed future
Future<SomeReturnType> fDbSelectOneClient(String sKey) {
var completer = new Completer<SomeReturnType>();
completer.complete(new SomeReturnType());
}
foo() {
// strongly typed result
fDbSelectOneClient("Mr Blue").then((SomeReturnType result) => print(result));
}
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