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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 873
842
CASE STUDY 1: UNIX, LINUX, AND ANDROID
CHAP. 10
receives a URI of the data to share, but does not know where it came from—in the
figure here it comes from the camera, but any other application could use this to let
the user email its data, from audio files to word-processing documents.
The email
application only needs to read that URI as a byte stream to add it as an attachment.
However, with permissions it would also have to specify up-front the permissions
for all of the data of all of the applications it may be asked to send an email from.
We have two problems to solve. First, we do not want to give applications ac-
cess to wide swaths of data that they do not really need. Second, they need to be
given access to any data sources, even ones they do not have a priori knowledge
about.
There is an important observation to make: the act of emailing a picture is ac-
tually a user interaction where the user has expressed a clear intent to use a specific
picture with a specific application.
As long as the operating system is involved in
the interaction, it can use this to identify a specific hole to open in the sandboxes
between the two applications, allowing that data through.
Android supports this kind of implicit secure data access through intents and
content providers. Figure 10-65 illustrates how this situation works for our picture
emailing example. The camera application at the bottom-left has created an intent
asking to share one of its images,
content://pics/1
.
In addition to starting the email
compose application as we had seen before, this also adds an entry to a list of
‘‘granted URIs,’’ noting that the new ComposeActivity now has access to this URI.
Now when ComposeActivity looks to open and read the data from the URI it has
been given, the camera application’s PicturesProvider that owns the data behind the
URI can ask the activity manager if the calling email application has access to the
data, which it does, so the picture is returned.
This fine-grained URI access control can also operate the other way.
There is
another intent action,
android.intent.action.GET
CONTENT
, which an application
can use to ask the user to pick some data and return to it.
This would be used in
our email application, for example, to operate the other way around: the user while
in the email application can ask to add an attachment, which will launch an activity
in the camera application for them to select one.
Figure 10-66 illustrates this new flow. It is almost identical to Fig. 10-65, the
only difference being in the way the activities of the two applications are com-
posed, with the email application starting the appropriate picture-selection activity
in the camera application.
Once an image is selected, its URI is returned back to
the email application, and at this point our URI grant is recorded by the activity
manager.
This approach is extremely powerful, since it allows the system to maintain
tight control over per-application data, granting specific access to data where need-
ed, without the user needing to be aware that this is happening.
Many other user
interactions can also benefit from it.
An obvious one is drag and drop to create a
similar URI grant, but Android also takes advantage of other information such as
current window focus to determine the kinds of interactions applications can have.
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