How can I detect the visibility of a control?_问答_开发者

How to detect if a control is visible to a user? I have a control that displays status of application, and I want it to be updated (it's value to be updated, for example, increase the value of a progress bar, or change a label's Text property) only if a user sees it. No need to update it if a user has minimized the form that contains this control, or if another form overlaps this control. I just don't want to do extra calculations if the control开发者_如何学Python isn't visible any way. Also, how do I detect the events that hide/show this control?

This seems to be one of the topics which is frequently asked and which has many possible solutions, depending on the context.

First, for reference sake, some links to old discussions I stumbled over during my research:

How to check absolute control visibility and how to be notified for changes of absolute visibility
C# winform check if control is physicaly visible

starting position: An IDE for heterogenous systems with plenty of different hardware and many different transport layers where some are really slow. One feature is to display memory values of remote hw in editors. A typical use case is to have >20 editors open, where each displays between 1 and 100 different values.

Because the transport layers are limited in bandwith I was looking for a solution to "prioritise" the data acquisition.

(partial) solution: A visibility tracker which basically manages a specialised adapter for the control itself and every ancestor control. The adapter handles the visibility dependent on the ancestors type, ie. for a Control, it's just instance.Visible, for a TabPage I check which page is selected, ... The manager's housekeeping then is just to keep track of parent-changes for all the ancestors so that it tracks the correct visibilities.

Catching whether or not the form has focus or is minimalized is your best bet, but I am not sure that hits all cases. In general, I would update regardless of whether the user is paying attention, unless it takes tons of cycles to get the information to the form, which I doubt. Setting up conditional bits in an application just adds another place for things to go wrong.

Let's examine this a bit. Minimized forms are one thing. If the form is minimized, you know the user is not even looking at it. But, if another form is in front, then you want to check if the form has focus. the problem here is a person can be using half the screen to watch while typing in word on the other half. Word has focus, so the form does not update, which is not your intention. Same could happen if you create another form in the application. the update no longer has focus.

THere is no magic "is the user looking at me" property in Windows.

This is never necessary. Windows won't generate the Paint event when a window isn't visible. If you implemented painting yourself then be sure to only do this in the Paint event and to call Invalidate() if there's a reason to repaint.

How often does the status change? It is probably better to update the status indicator when the status changes and not worry about visibility. Let's say that status changes on average every 5 minutes. The window/control may not be visible when the status changes but could easily become visible well before the next status update. In this case, the status indicator will be wrong until the next update because you avoided changing the indicator. Unless updating the status indicator is very expensive, it's probably better not to try to project what the user will do.

Edit after reviewing other comments:

I think you might be able to achieve better efficiency improvements by identifying a model for detecting/notifying status changes instead of calculating on demand. You may need to weigh the frequency of status changes against the frequency of visibility changes to your control.

Take a look at Control.Visible, and the Control.VisibleChanged event.