Archive:Development/Tutorials/D-Bus/Introduction (zh CN)
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该系统很好的反映了桌面应用的内部工作流程：方法调用（信息）产生将会得到一些运行后可能的返回值。D-Bus消息的一个主要附加功能就是，他们异步的：一个信息发出后，响应可能不知道什么时候才会到达（内建超时机制防止'永远等待'的死锁） 。当然，便利的Qt4 D-Bus编程接口调用看上去似乎是同步的。
应用程序自身就可以发送消息。These are "short-circuited" and kept local to the application, so it is not necessary for code in an application to worry about whether or not it might actually be calling a remote or local application. This is often useful in highly componentized apps and prevents possible deadlock situations.
Since multiple applications can be on the same bus, and one application may provide multiple objects to which messages can be sent, it is necessary to have a means to effectively and unambiguously address any given object on any given bus, similar to the way a street address uniquely identifies any given residence or office. There are 3 pieces of information which, when taken together, create a unique address for any given object on a bus: interface, service and object name.
An interface is a set of callable methods and signals that are advertised on the bus. An interface provides a "contract" between the applications passing messages that defines the name, parameters (if any) and return values (if any) of the interface. These methods may not map directly in a one-to-one fashion to methods or API in the application that is advertising the interface, though they often do. This allows multiple applications to provide similar or the same interfaces, regardless of internal implementation, while allowing applications to use these interfaces without worrying about the internal design of the applications.
Interfaces can be described for documentation and code re-use purposes using XML. Not only can users and programmers reference the XML description of the interface, but developers can use classes that are auto-generated from the XML — making using D-Bus much easier and less error-prone (e.g. the compiler can check the syntax of messages at compile time).
service here corresponds to "well-known" Bus names in the D-Bus specification terminology. A term "Bus name" is a bit confusing. Regardless of how it sounds like, Bus names are the names of connections(!) on the bus, not names of buses(!). So here the term service used, as QT Documentation calls it.
These are kept unique by using a "reverse domain name" approach, as can be seen in many other systems that need to namespace for multiple components. Most services provided by applications from the KDE project itself use the org.kde prefix to their service name. So one may find "org.kde.screensaver" advertised on the session bus.
You should use the domain name for your organization or application for your service names. For example if your domain is awesomeapps.org and the name of your application is wickedwidget you would probably use org.awesomeapps.wickedwidget as the service name on the bus.
If an application has more than one connection to a bus, or if multiple instances of the same application may be active at once, it will need to use a unique service name for each connection. Often this is done by appending the process ID to the service name.
Of course, an application is likely to advertise access to more than one object on the bus. This many-to-one relationship between objects and services is accommodated by providing a path component to the address. Each path associated with a service represents a different, unique object. An example might be /MainInterface or /Documents/Doc1. The actual path structure is completely arbitrary and is up to the application providing the service as to what the paths should be. These paths simply provide a way to identify and logically group objects for applications that send messages to the application.
Some libraries export object paths with their "reverse domain" prepended to it, so as to properly namespace their objects. This is quite common for libraries and plugins that join an arbitrary service and must therefore avoid all clashing with objects exported by the application and other components. However, this practice is not in use in KDE applications and libraries.
Objects provide access to interfaces. In fact, a given object can provide access to multiple interfaces at the same time.
A D-Bus message contains an address made up of all the above components so that it can be routed to the correct application, object and method call. Such an address might look like this:
org.kde.krunner /ScreenSaver org.kde.screensaver.setBlankOnly
In this case org.kde.krunner is the service, /ScreenSaver is the path to the object, org.kde.screensaver is the interface the object exports and setBlankOnly is a method in the interface. If the /ScreenSaver object only provides the org.kde.screensaver interface (or the setBlankOnly method is unique amongst the services it implements) then this would work equally well as an address:
org.kde.krunner /ScreenSaver setBlankOnly
In this way each possible destination is uniquely and reliably addressable.
Now that we have a way to address any given end point on the bus, we can examine the possibilities when it comes to actually sending or receiving messages.
Methods are messages that are sent to cause code to be executed in the receiving application. If the method is not available because, for instance, the address was wrong or the requested application is not running, an error will be returned to the calling application. If the method is successfully called, an optional return value will be returned to the calling application. Even if there is no return value provided, a success message will be returned. This round trip does have overhead, and it is important to keep this in mind for performance critical code.
Such method calls are always initiated by the calling application and the resulting messages have exactly one source and one destination address.
Signals are like method calls except that they happen in the "reverse" direction and are not tied to a single destination. A signal is emitted by the application which is exporting the interface and is available to any application on the same bus. This allows an application to spontaneously advertise changes in state or other events, to any applications which may be interested in tracking those changes.
If this sounds a lot like the signal and slots mechanism in Qt, that's because it is. For all intents and purposes it is a non-local version of the same functionality.
There are several useful tools for exploring the D-Bus busses as well as developing applications that use D-Bus. We will now look briefly at end-user tools as the articles that follow cover the development tools in greater detail and context.
qdbus is a command line tool which can be used to list the services, objects and interfaces on a given bus as well as send messages to a given address on the bus. It can be used to explore both the system and the default session bus. If the --system switch is passed, qdbus will connect to the system bus, otherwise it uses the session bus.
qdbus uses the rest of the supplied arguments on the command as an address and, if any, parameters to pass to a given object. If a full address is not supplied, then it lists all the objects available from that point on the bus. For instance, if no addresses are provided a list of available services is listed. If a service name is provided, object paths will be provided. If a path is also provided all methods in all interfaces will be listed. In this way one can quite easily explore and interact with objects on the bus, making qdbus very useful for testing, scripting and even idle exploration.
qdbusviewer is a Qt application that provides a graphical interface to essentially the same set of features that qdbus provides on the command line, thus providing a more user friendly mechanism to interact with the bus. qdbusviewer ships with Qt4 itself and is easy for anyone who is familiar with the basic D-Bus concepts, such as object addresses, to use.