什么时候可以告诉我已在indexedDB中打开连接?

问题描述

在下面的getCursor_函数中，请说明如何确定IndexedDb是否已打开，以及是否不能重新运行它. getCursor_正常运行.但是，由于所有这些调用都是异步的，因此在数据库完成打开之前执行时，该函数将失败.

In the getCursor_ function below, please explain how I could determine if IndexedDb has opened and if not re-run function once it has? The getCursor_ runs correctly. However, since all of these calls are asynchronous, the function fails when executing before the database has completed opening.

此代码在单独的过程中执行:

var ixDb;
var ixDbRequest;

ixDbRequest = window.indexedDB.open(dbName, dbVersion);

ixDbRequest.onsuccess = function (e) {
            ixDb = ixDbRequest.result || e.result;
};

除非ixDbRequest尚未完成执行，否则下面的getCursor_函数可以正常工作.我想出了如何测试的方法，但是不确定在打开的数据库请求仍在运行的实例中如何等待.

The getCursor_ function below works fine unless ixDbRequest has not completed execution. I figured out how to test for that, but I am not sure how to wait in the instance where the open database request is still running.

function getCursor_(objectStoreName) {
    if (!ixDb) {
        if (ixDbRequest) {
            // "Database is still opening.  Need code to wait or re-run
            // once completed here.
            // I tried the following with no luck:
            ixDbRequest.addEventListener ("success",
                getCursor_(objectStoreName), false);
            return;
        }
    }

    var transaction = ixDb.transaction(objectStoreName,
                                       IDBTransaction.READ_ONLY);
    var objectStore = transaction.objectStore(objectStoreName);
    try{
        var request = objectStore.openCursor();
        return request;
    }
    catch(e){
        console.log('IndexedDB Error' + '(' + e.code + '): ' + e.message);
    }
}

更新如下:

@Dangerz的回答无疑帮助我走上了正确的道路.但是，由于函数调用是异步的，因此一旦成功"事件最终触发后，我还不得不添加一个回调，以便实际上能够使用游标，并且我能够获得所请求的IndexedDb游标.最终的工作功能如下(略作重构以消除"if(！ixDb)"上方的负面逻辑.如果有人认为有改进的余地，我完全愿意提出建议！

The answer from @Dangerz definitely helped put me on the right track. However, since the function call is asynchronous, I also ended up having to add a callback in order to actually be able to use the cursor once the "success" event finally fired, and I was able to get the requested IndexedDb cursor. The final working function is below (re-factored slightly to remove the negative logic above "if(!ixDb)" . I am totally open to suggestions, if anyone sees room for improvement!

//****************************************************************************
//Function getCursor - Returns a cursor for the requested ObjectStore
//  objectStoreName  - Name of the Object Store / Table "MyOsName"
//         callback  - Name of the function to call and pass the cursor
//                     request back to upon completion.
//                     Ex. getCursor_("ObjectStore_Name", MyCallBackFunction);
//                     Function MyCallBackFunction(CursorRequestObj) {
//                     CursorRequestObj.onsuccess =
//                               function() {//do stuff here};
//                     }
//****************************************************************************
function getCursor_(objectStoreName, callback) {
  //The the openCursor call is asynchronous, so we must check to ensure a
  // database connection has been established and then provide the cursor
  // via callback.
  if (ixDb) {
      var transaction =
        ixDb.transaction(objectStoreName, IDBTransaction.READ_ONLY);
      var objectStore = transaction.objectStore(objectStoreName);

      try{
        var request = objectStore.openCursor();
        callback(request);
        console.log('ixDbEz: Getting cursor request for '
                    + objectStoreName + ".");
      }
      catch(e){
        console.log('ixDbEz Error' + ' getCursor:('
                    + e.code + '): ' + e.message);
      }
  }
  else {
    if (ixDbRequest) {
      ixDbRequest.addEventListener ("success"
                    , function() { getCursor_(objectStoreName, callback); }
                    , false);
    }
  }
}

推荐答案

将addEventListener行更改为:

Change your addEventListener line to:

ixDbRequest.addEventListener ("success", function() { getCursor_(objectStoreName) }, false);

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