Develop Reference

Can an Android app interact with another app?

I want to be able to tap the statusbar and the contents in the displayed app to be scrolled up to the top.
Is it technically possible that an app intercept my tap and send the appropriate command to the active app? I have noticed for example that AntTek quick settings shows a drop-down window when swiping down from statusbar. While using the app I did also notice that even by just tapping the statusbar (before beginning to move the finger down), the app seems to already interact with the touch as it dims the screen brightness in preparation to display it's "window" (sorry I use the MS Windows term), so clearly a statusbar tap CAN be sensed by an app.
Starting from this, I wonder if such an app could then send a message to the active program telling it to scroll up.
Is that possible? And if yes, the message must be customized to a particular app (let's say the browser as the most important) or is it standardized so the apps speak the same language between themselves?
I am not a programmer so answers with codesamples might be less helpful than a plain english explanation. Finding out that is possible would lead rather to pursuing a programmer to implement the idea rather than starting to develop it myself.
Thanks :-)

There is an XPosed-module which seems to do exactly what you want.
To use XPosed-modules, you'll need to root your phone and install the XPosed-framework.
The XPosed-module is called "Statusbar Scroll to Top" and its repository can be found here:
http://repo.xposed.info/module/com.mohammadag.statusbarscrolltotop
This will work for almost all app-lists, but for example won't work for browser-content.
If you want to scroll to the top of the page in a browser, then you'll probably do best to get a browser which can do that on its own. (For example Habit Browser has it built-in and respective plugins are available for Firefox.)

Yes it is possible. HiroMacro and Frep can do this, but it requires root. https://play.google.com/store/apps/details?id=com.prohiro.macro&hl=en
(how do they simulate mouse and keyboard interactions on other applications? i have no clue :/ anyone?)

Is it technically possible that an app intercept my tap and send the appropriate command to the active app?
No. One app cannot send fake input to another app, for security reasons.

An android app comprises of several activities. Each activity display a GUI that allows the user to perform a specific task. To take the user from one activity to another, app must use an Intent to define our app’s intent to do something.
An intent can be explicit in order to start a specific component (a specific Activity instance) or implicit in order to start any component that can handle the intended action.
Interacting one app to other app in android
google docs link

Only displaying pre-installed applications on android device

So I am using the Home sample to build an application that creates a second home screen for the user. The idea is to be able to have only one user account yet restrict certain access to chosen applications. I have managed to ensure that all of the applications are invisible in the XML yet I am struggling with how to change this to make certain apps visible.
Is it possible to write a whitelist of accepted apps for instance the preinstalled apps or child friendly apps for children who game using the android device and then put in a Java method to access this white list? This is the only way I can think to make it work.
If anyone knows the correct way can you please help.
Thanks.

Ok so I discovered how to do this.
In the home sample they provide a for loop in the Home.java file that covers all apps and displays them. It take a simple if statement to restrict the apps that can be viewed -
// for loop is here
if (info.activityInfo.applicationInfo.packageName.contains("com.android"))
//then the rest of the home sample is here.
Still very basic but provides me with a good enough UI so that kids cannot see apps I don't want them to.

Is it possible to block Android market programatically?

I want to enable/disable the Android market programmatically for my kids app (with a checkbox). Is this possible? How?

Generally, no. You cannot affect the behaviour of other apps from your app. There are tricks that allow you to monitor what has been stated and you can bring your own app in the foreground to sort of 'block' it, but this is neither supported, nor guaranteed to work on all devices and Android versions.

To disable the play store, do as you would with any other app if you were going to delete it, however in replace of the delete buttin wil be a disable button.
If you do not want to disable it all together, you can open the app, click menu, and then settings, and then half way down on the list you are abe to set a passcode for purchases.
EDIT
I know hat you said programmatically, however this could still be a solution as to what you may be looking for instead as a workaround.

How can I set a place a custom button on incomming call screen?

I went through some posts, but most of them discuss placing a image or a string with information on it.
I however need to place button that manipulates some call features.
Is there a method to do that universally works throughout all android distributions?
(I thought to pop up custom screen with my button above usual screen. Is that good idea, or is there more straightforward way to achieve this?)
Thanks a lot

On stock devices there isn't going to be a legit way to put your own button "on" any portion of the Dialer application Activities (including incoming call screen). This kind of functionality would require the Dialer to explicitly provide an API for it. The stock system prevents applications in the background from placing their own clickable regions "on top of" whatever is currently in the foreground . (It's a good thing too, the bad reasons to do this far outweigh the legit ones.)
If you are wanting to do this you're going to have to look into building your own version of the OS that allows for it. OR potentially if you unlock your device enough to be able to install your own application that handles all of the functionality of the Dialer. Then you could provide an API for a third party application to do it(or just "bake in" your button to the Dialer) But I imagine that being able to get it all hooked up correctly to actually make your calls would be mighty difficult.

modify a an existing Android ROM to control user actions

I am creating an Android application for a customer which will be pre-installed and distributed together with the handsets. Now the customer asked me to lock down the ROM to prevent the future users from using anything else apart from this one app. I.e. no browsing, no email, nothing which could create any costs etc.
Now after some googling it seems to be relatively straightforward to remove applications from an Android image. But even if I can remove the web browser, email client, the Android Market App etc, how can I make sure that the user will not reinstall those apps via the USB connector? As Android is just Linux I am sure there are ways of denying all users the right to install new applications (or actually denying them pretty much anything apart from using this one app).
If somebody could point me in the right direction it would be much appreciated.
EDIT:
To clarify things a bit, the customer does not want to use the android devices as phones, even though they are phones. With 100€ touch screen phones becoming available, it becomes viable to use those phones for just one specific purpose. In my customers case, the device will be used as something like a POS system: the employee uses the phone to process a certain kind of customer request, and for this the app needs to have a data connection which will accrue costs of course. Now obviously contracts can be set up that will oblige employees to pay any irregular costs themselves, but why make it that complicated?
This is not about taking freedom away from users but rather about using android phones as general purpose touch screen devices with a data connection that employees can use in a business environment without shooting themselves in the foot with unexpected high data connection cost.

There's not really a whole lot you need to do to make a single-purpose device. If you play your cards right, it should be something you can do without having to tinker with the ROM.
The quick-and-easy route would be to deploy your application as a replacement for the stock launcher, just like any of the other home replacements that are available. Setting that as the default would cover most of the opportunities for casual tampering, since it would leave no other way to launch or install anything else. The only other things I can think of off the top of my head that you'd need to do are snag the search button so it doesn't bring up the default Android search box atop your app and the green key so the phone app won't come up.
You would, of course, need a way to get to the original launcher to maintain the device and install new versions of your app. I'd accomplish that using a "Maintenance" menu item somewhere that asks for some form of authentication (e.g., a password), changes the home app back to the original and launches it. When you're done doing what you need to do, set your app as the default launcher and you're back in business.
Edit to address MAINERROR (now Octavian Damiean)'s comment:
Any activity in any application can register itself as a home application by adding an intent filter on the android.intent.category.HOME category. It's literally four lines in the manifest, and you don't have to write any code to support it. Take a look at lines 77-82 in the stock launcher's manifest for an example of how this is done. (Ignore the filters on DEFAULT and MONKEY; they're not necessary.) Once the activity is selected as the default handler for the category, it becomes the first thing launched at boot and what comes up when the Home key is pressed. HTC Sense, aHome, Panda Home, etc. all use this mechanism.
Launching the stock home (or any other application) explicitly is about five lines of code.
Side note: There's a application on the Market called Home Switcher that lets you launch any of the activities filtering on the HOME category or set one as the default.
Unless the handset manufacturer adds a lot of shovelware, the stuff that runs in the background should be inconsequential and won't get in the way.

There was a similar question already somewhere. You can indeed limit the functionality of your device by the amount you want or have to. In order to achieve this you will definitely have to build your own modified ROM.
You will have to touch the ROM because you will have to get rid of several applications running in the background. One you won't need them anyway and two as you don't need them they would only consume resources.
You might want to take a look at http://source.android.com there you will find more information about the sources which will hopefully direct you where you need.

Blrfl's answer is great, but it still has a problem: if the user long presses the HOME button, the recent applications popup will appear an the user will be able to launch another app.