In a dialogue box, when do you use an Apply button?
Is it correct to assume that the Apply button will only save the changes without closing the dialogue? Does it represent an advantage? Why is Preview button not as commonly used (in window based applications)?
I know this might seem like a lot of questions, but I what I want to understand is the usage of the Apply button, its advantages and disadvantages and common scenarios. The Preview button discussion would be a bonus.
The main reason for the Apply feature is to yield closure in the user. What I mean by that is that you don't want to complicate a task for a user, making the user having to think too much about the mere interactive pattern of an application but rather the main task at hand. In this case it's really to ensure the user that changes the user makes are being saved without the user having to worry about it.
The Apply button is most commonly used in dialogues that feature more than one tab/perspective. The apply button is there to ensure the user that changes being made in one tab will not be lost when the user switches to another tab. This can of course be easily technically avoided, there's usually no problem to store changes that are not currently being displayed to be saved at a later stage, but the user is not aware of this, and that's the problem.
In some ways it can almost be regarded as a placebo control. There is no real use for it, in most cases, since changes can be cached outside the displayed content to be saved when the user finally clicks OK. However, if the user doesn't know this, and worries about whether or not their changes will be lost if they switch tab, they may start clicking OK for every change they make. And that would be very inefficient. I have to point out that I am aware of instances where changes has to be made and saved before continuing, so I know that there can be some actual technical functionality to the Apply button, however those are the rare cases.
Regarding your Preview idea. I see two direct flaws with it. First and foremost Apply and Preview has two different semantic meanings to them. Preview conveys that changes are not actually being made, but that you're rather temporarily tempering with settings to see what WOULD happen. This is not however how Apply works, Apply will save the changes in the same way as if a user clicked OK. Therefore I don't see it transferring correctly.
Secondly the Apply feature is usually used where a "Preview" wouldn't make any sense. Eg:
However, where it does make sense, like in graphical tools, this terminology is also utilized. Eg:
Here it's utilized because it makes sense.
The advantage of the apply button is that it allows the user to see the effect of an operation requested through a dialog, without closing that dialog.
For example in a text editor, the user could appreciate the effect of changing the font to a larger size and specify another size over and over again without having to reopen the dialog.
Notice that in modern computers the "cost" of displaying the effect of such a change is normally negligible, so in MS Word the effect of a style change can be appreciated by simply hovering a "style" and there is no need for the apply thing.
What the apply button is for:
Apply buttons enable saving/applying/submitting current changes without leaving the dialog, thus enabling the user to save some changes and continue making more changes (including changing somethings back).
If the user can see the immediate effect of the changes (on applying them), the apply button enables "previewing"* the changes.
* The reason that previewing is quoted is that it isn't exactly a preview - since the changes are performed and are not necessarily undoable (hopefully they are).
If the changes are not applied immediately, then the apply button can prevent the user from worrying about lossing data, especially if there are a lot of inputs to fill in.
Immediate apply of changes vs. on demand:
There can be different scenarios in which the save or apply buttons vs. immediately applying all changes are more suitable. See my answer here for a calculator example.
Applying changes vs. preview:
The apply button is also useful when the apply has side effects, for example (a similar example to AndroidHustle's) - filters in an image processing application:
Previews of a filter are preformed on previous result original image (on image as it was prior to opening filter dialog).
Changing filter values twice and then applying does not provide the same result as changing values then applying then changing again and then applying, since each apply is relative to previous applied result.
The user may choose to apply multiple filters one after another, including the same filter with different parameters, to reach desired results.
