我有一个saveAsync
方法,可以从应用程序的其他几个部分调用该方法。这些代码路径中的某些不是源自主UI线程,而是源自其他异步操作,例如从套接字/文件读取数据。
在我的saveAsync
函数中,我调用WinRT
的StorageFile
流编写函数。一次只允许一个文件写入者。因此,我必须将对saveAsync
函数的多个连续调用排队,并按顺序执行它们。这是我现在正在使用的代码:
SaveableMixin = {
savesPending: 0,
saveAsync: function() {
self = this
logger.debug("Save requested (_cachedFile=" + self._cachedFile + ")");
if (self.saving) {
self.savesPending += 1;
logger.debug("saveAsync: already saving, " + self.savesPending + " pending now");
return self.saving;
}
return self.saving = getFileNameAsync(self.serialNumber).then(function(file) {
logger.debug("saveAsync: saving...");
return self.node.ownerDocument.saveToFileAsync(file);
}).then(null, (function(error) {
return logger.error("saveAsync: failed but " + self.savesPending + " saves pending - good luck!", error);
}).then((function() {
logger.debug("saveAsync: saved and " + self.savesPending + " pending");
self.saving = null;
if (self.savesPending) {
self.savesPending = 0;
return self.saveAsync();
}
});
}
}
CoffeeScript原始代码:
savesPending: 0
saveAsync: () ->
logger.debug("Save requested (_cachedFile=#{@_cachedFile})")
if @saving
@savesPending += 1
logger.debug("saveAsync: already saving, #{@savesPending} pending now")
return @saving
@saving = getCachedFile(@serialNumber)
.then (file) =>
logger.debug("saveAsync: saving...")
@node.ownerDocument.saveToFileAsync(file)
.then null, (error) =>
logger.error("saveAsync: failed with inspection #{@serialNumber} but #{@savesPending} saves pending - good luck!", error)
.then =>
logger.debug("saveAsync: saved and #{@savesPending} pending")
@saving = null
if @savesPending
@savesPending = 0
return @saveAsync()
如您所见,我记得最后一个
saving
承诺会知道某个操作正在运行。但是可能发生的是,我仍然会以某种方式导致Access Denied
异常,某些代码路径设法都将saving
视为空,因此现在考虑进行保存操作。当即将为第一个调用方分配变量,第二次异步操作(例如,读取套接字读取)完成并在第一个调用方获取分配的saveAsync
变量的同时调用saving
时,似乎发生了这种情况。在C ++中,我将使用锁来防止这种情况。在JavaScript世界中可以做什么?
更新资料
_saving: WinJS.Promise.as(),
_saves: 0,
saveAsync: function() {
var currentSave;
currentSave = this._saves;
this._saves += 1;
return this._saving = this._saving.then((function(_this) {
return function() {
logger.debug("saveAsync: get file for save " + currentSave);
return getCachedFile(_this.serialNumber);
};
})(this)).then((function(_this) {
return function(file) {
logger.debug("saveAsync: got file for save " + currentSave + " ...");
return _this.node.ownerDocument.saveToFileAsync(file);
};
})(this)).then(null, function(error) {
logger.debug("saveAsync: error for " + currentSave);
});
这将在调试器中产生以下输出:
2014-12-07 23:46:52,286 - [inspection - DEBUG] - saveAsync: get file for save 0
2014-12-07 23:46:52,289 - [inspection - DEBUG] - saveAsync: get file for save 0
2014-12-07 23:46:52,349 - [inspection - DEBUG] - saveAsync: got file for save 0 ...
2014-12-07 23:46:52,444 - [inspection - DEBUG] - saveAsync: got file for save 0 ...
2014-12-07 23:46:52,446 - [inspection - DEBUG] - saveAsync: get file for save 1
2014-12-07 23:46:52,447 - [inspection - DEBUG] - saveAsync: got file for save 1 ...
当然,这是对相关文件的双重访问,并且因AV而崩溃。
最佳答案
在JavaScript世界中,您将使用promise来做到这一点,promise会抽象一系列异步操作,您可以将其用作队列。
SaveableMixin = {
queue: WinJS.promise.as(), // empty start of queue
saveAsync: function() {
this.queue = this.queue.then(function(){ // update the queue
return getFileNameAsync(self.serialNumber).then(function(file) {
logger.debug("saveAsync: saving...");
return self.node.ownerDocument.saveToFileAsync(file);
})..then(null, (function(error) {
return logger.error("saveAsync: failed good luck!", error);
}).then((function() {
logger.debug("saveAsync: saved and " + self.savesPending + " pending");
});
}
}