Tuesday, September 23, 2008

No SharedObjects Allowed

Client side storage by the Flash player (SharedObjects) has several advantages over traditional client side storage, a.k.a. HTTP cookies. From a security standpoint, it is better because the data is never sent over the wire. However the main advantage to most people is that it is bigger, and when it comes to managing data on the client, size definitely matters.

By default you get 100 KB instead of the 4 KB you get with cookies. If your application tries to store 101KB, it won't fail. Instead the user will be prompted to increase the allocated space by a factor of 10, i.e. from 100 KB to 1 MB. Of course you probably don't want the user to ever see this screen. One of the other advantages of SharedObjects is that people don't delete them. People blow away their cookies all too often, but most people would have no idea how to do the same with SharedObjects. The only you would find out would be if you saw the Flash player settings screen, i.e. the interface that appears when a Flash application tries to go over the 100 KB default limit.

So stick to under 100 KB and all is good, right? Not so fast. The settings interface requires that your Flash app is at last 136x213. If it is smaller than that, then what happens? First let's explain what happens when it is big enough to show the settings interface. When you flush data to local storage, a string is returned with a status. Here is typical code for this.

var testSo:SharedObject = SharedObject.getLocal("test", "/", false);
testSo.data.testValue = "test";
var soStatus:String = testSo.flush();
if (soStatus != null){
switch (soStatus){
case SharedObjectFlushStatus.PENDING:
testSo.addEventListener(NetStatusEvent.NET_STATUS, someHandler);
case SharedObjectFlushStatus.FLUSHED:

There are two possible return values, either "pending" or "flushed." There is no fail. So if you were flushing 101 KB, then you would get a pending return value. Now all you can do is what for an event, or more precisely a NetStatusEvent. This will tell you if the user allowed you to increase the size or not. If not then the NetStatusEvent will come back with a failure code.

If there is not enough space to display the settings interface, then you would think that you would just get an automatic failure, but you don't. Instead you get a "pending" from the return of flush. It's not really pending, since the user can't actually choose to allow it to succeed. It can only fail. But the player pretends this is not the case and that the user denied you request. So you need to still listen for the NetStatusEvent. If you don't catch that event, then it will cause the Flash player to throw an error to the user, and of course you do not want that. Here is a picture of that.

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