如何将 IndexedDB 表数据检索到变量？

Question

我在 IndexedDB 表中有一些数据，其中非常简单地包含这些数据：

var Customers = [
   { ssn: "123-45-6666", name: "Andrew", age: 22, email: "andrew@hotmail.com" },
   { ssn: "555-66-7777", name: "Gail", age: 25, email: "gail@email.me" }
];

然后我有这个函数从 IndexedDB 取回数据：

function RetrieveTableRows(Table) {
   var returnData = [];

   var db = window.indexedDB.db;
   var trans = db.transaction([Table], "readwrite");
   var store = trans.objectStore(Table);

   var keyRange = IDBKeyRange.lowerBound(0);
   var cursorRequest = store.openCursor(keyRange);

   cursorRequest.onerror = window.indexedDB.onerror;

   cursorRequest.onsuccess = function(e) {
      var result = e.target.result;
      if(!!result == false) {
         return;
      }
      returnData.push(result.value);

      result.continue();
   };    

   return returnData; 

}

我意识到它不起作用，因为 onsuccess 函数是异步的，但是我无法解决解决方案。

简单地说，我希望能够写：

var myCustomers = RetrieveTableRows('customers');

然后能够使用变量 myCustomers - 这可能吗？

我试过使用JQuery.deferred();方法，但这似乎不起作用，我知道我可能会做这样的事情：

    transaction.oncomplete = function() {
        ReturnTableRows(returnData);
    };                  
}
function ReturnTableRows(data) {
   //do something with the array of data
}

但我不知道如何将它传递回myCustomers变量。

Answer 1

使用延迟对象，你应该能够做这样的事情

function RetrieveTableRows(Table) {
   var returnData = [];
   //setup deferred object
   var defer = $.Deferred();
   var db = window.indexedDB.db;
   var trans = db.transaction([Table], "readwrite");
   var store = trans.objectStore(Table);

   var keyRange = IDBKeyRange.lowerBound(0);
   var cursorRequest = store.openCursor(keyRange);

   cursorRequest.onerror = window.indexedDB.onerror;

   cursorRequest.onsuccess = function(e) {
      var result = e.target.result;
      if(!!result == false) {
         //when done resolve the promise, you could also do more 
         //checking and reject the data so you can handle
         //errors
         defer.resolve(returnData);

         //Make sure we exit the function once we run out of data!
         return
      }
      returnData.push(result.value);

      result.continue();
   };    

   //return the promise
   return defer.promise(); 

}

//########### then to use this ###########

//this is now a promise
var myCustomersPromise = RetrieveTableRows('customers');
var myCustomers;

//action todo when promise is resolved/rejected
$.when(myCustomersPromise ).done(function(data){
   //do something with the data/assign it to you var here
   myCustomers= data;
}).fail(function(data){

});

虽然我之前没有实际使用过 indexedDB，所以可能误解了查询如何知道它已完成（我假设再次result.continue()调用，onSuccess并且在遍历所有数据时结果为 false）但这是我在异步执行任何操作时使用的设置在我的应用程序中