Creating Applications with Mozilla-Chapter 8. XPCOM- P1

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Creating Applications with Mozilla-Chapter 8. XPCOM- P1

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Nội dung Text: Creating Applications with Mozilla-Chapter 8. XPCOM- P1

  1. Chapter 8. XPCOM- P1 This chapter provides a high-level introduction to XPCOM component technology. XPCOM can be difficult to master, but after reading this chapter, you should have a good sense of what it is and the important part it plays as Mozilla's core technology. You should be able to find and use existing scriptable components in your own applications and create a simple XPCOM component by using JavaScript or C++. XPCOM permits a reusable code module to be globally accessible to a Mozilla-based application. You do not need to worry about including external source files in your application distribution and you can distribute components by using XPInstall. This type of architecture makes the development of core application services flexible and entirely modular. The section Section 8.2.1 lets you create an interface from start to finish -- writing the implementation for that interface, compiling it into a type library, registering it with Mozilla, and then testing the new component. One advantage of using XPCOM is that you can create multiple implementations for a single interface; following the JavaScript component section, we will take the same nsISimple interface and implement it in C++ as well. The section Section 8.2.5 includes some techniques and programming tasks that are particular to C++ components, such as handling return values and generating header files and useful macros. The section Section 8.2.7 introduces the XPCOM bindings for the Python language (pyXPCOM). First, it provides an overview of XPCOM and how it relates to other technologies used in Mozilla. 8.1. What Is XPCOM?
  2. XPCOM is Mozilla's cross-platform component object model. Although it is similar to Microsoft's COM technology, this chapter points out some important differences. Essentially, when you program in a component-based environment, you do one of three things: you create a new component using existing components, write a component that implements other components, and establish interdependencies and a service network. 8.1.1. What Is a Component? You've already seen components used in this book. In some cases, you may have used the services of Mozilla components without knowing it -- for example, when you created a XUL tree widget in the section Section 3.4.2 in Chapter 3, and used its built-in layout and view capabilities. Some of this functionality is defined in an interface called nsITreeView, which provides specific methods and properties for a XUL tree, persisting its state, row, cell, and column properties, navigation, and other object metadata used in a tree object. Behind the scenes, you'll find an XPCOM-instantiated tree view object where methods and properties associated with the XUL element are accessed via DOM > JavaScript > XPConnect > XPCOM layers. A component is a reusable or modular piece of code that implements a clearly defined interface. In Mozilla, this code can exist as a singleton service or an object instance. A singleton service is an object instance that is created only once and then used by other code (usually called "callers," "clients," or "consumers"). An object instance is an object that is instantiated once or many times. Components are written as classes that typically have member variables and methods. The basic purpose of a component is to implement a clearly defined set of APIs that exist in a public interface. The
  3. interface exists separately so that the implementation is abstracted away, and it can be changed without affecting the interface or breaking binary compatibility. When interfaces are deployed in a production environment, they are frozen, which means they are held in an immutable state -- theoretically for as long as the application exists. While MSCOM provides a component-based programming model on Microsoft platforms, XPCOM provides it on all platforms where Mozilla is available. Example 8-1 shows how simple using XPCOM components can be. In two lines, an XPConnect-wrapped nsIBookmarksService object is instantiated, and one of its methods is called, providing easy access to this XPCOM component from JavaScript. Example 8-1. Using an XPCOM object in script // create a bookmark service object in JS var bmks = Components.classes["@mozilla.org/browser/bookmarks- service;1"]. getService(Components.interfaces.nsIBookmarksServic e); // call one of the object's methods: // flush the bookmarks to disk if they've been touched. bmks.Flush( );
  4. As you can see, the assignment of an XPCOM object to the variable bmks takes only a single line. Once you are comfortable using XPCOM from JavaScript, you can use any of Mozilla's scriptable interfaces in your application. Once an object like bmks is created, as in Example 8-1, it can be used to call any method in the nsIBookmarksService interface, of which Flush( ) is an example. 8.1.2. XPConnect and the Component Object As shown the previous example, the XPCOM object is called and instantiated from script. For an interpreted language like JavaScript to call and instantiate it, a bridge must bind JavaScript types to XPCOM types. These type bindings are part of a technology called XPConnect. In XPConnect, XPCOM interfaces, classIDs, and progIDs are stored as global JavaScript objects and properties that can be manipulated directly through a top-level object called Components. This object accesses any component that is declared "scriptable" in an XPCOM IDL interface. Through the Components object, you can access and use the services that these interfaces provide. The Component object's top-level properties and methods include: QueryInterface A method used to match an interface with a desired implementation. The implementation can be in C, C++, JavaScript, Python, and other languages for which appropriate bindings are created. You can have multiple implementations for an interface. Through QueryInterface, you can ask for and assign the desired interface to its implementation. Each XPCOM object needs to implement
  5. QueryInterface in order to return an instance of that object's class: js> var clz = Components.classes['@mozilla.org/file/local;1']; js> var inst = clz.getService( ); js> inst.QueryInterface(C.interfaces.nsILocalFile); [xpconnect wrapped nsILocalFile @ 0x81b7040] interfaces A read-only object array containing all the interfaces declared scriptable in the IDL file. The object name has the same name as the interface it represents. Components.interfaces.nsILocalFile The source file for this particular interface, for example, is nsILocalFile.idl. This XPIDL compiler compiles this file to produce a cross-platform binary type library, nsILocalFile.xpt, which contains tokenized IDL in an efficiently parsed form. classes A read-only array of all the XPCOM component classes indexed by the ProgID (or human-readable name) of the component class. The classes object has these properties associated with it: toString Returns the string progID. QueryInterface Used to QI this interface. name Returns the string progid name.
  6. number Returns the string components uuid number. valid Boolean verifies if the instance is valid. equals The Boolean used to match identical instances. initialize I don't know what this does. createInstance Will create an instance of the component; you can have many instances. getService Will instantiate the component as a service; you can have only one instance of a service. classesByID The same as classes, except this time the array is indexed by the "canonical" or "well-established" form of their CLSID: Components.classesByID['{dea98e50-1dd1-11b2-9344- 8902b4805a2e}']; The classesByID object has the same properties object associated with it as the class object. The properties are also used in the same way: toString QueryInterface
  7. name number valid equals initialize createInstance getService stack A read-only property that represents a snapshot of the current JavaScript call stack. JavaScript handles each code interpretation one call at a time and then places that code onto a call stack. This property can be used for recondite diagnostic purposes: js> var C=Components; js> C.stack; JS frame :: typein :: :: line 2 js> C.stack; JS frame :: typein :: :: line 3 results An object array of nserror results: Components.results.NS_ERROR_FILE_ACCESS_DENIED; 2152857621 manager
  8. A reflection of the XPCOM global native component manager service. Using the component manager is the only way for a component to actually be created. It uses the components factory to create an instance of the class object. ID A constructor used for a component written in JavaScript This component needs to register itself with the component manager by using its own nsID (an ID that is not already registered and thus does not appear in Components.classes). Exception A JavaScript constructor used to create exception objects. When implementing XPCOM interfaces in JavaScript, these exception objects are the preferred types of exceptions. When an XPCOM exception is thrown in your JS code, it takes the form of an Exception object that has properties associated with this object. Exceptions are usually caught in a "catch" block. Constructor A JavaScript constructor object that constructs new instances of XPCOM components: js> var File=new Components.Constructor( "@mozilla.org/file/local;1", "nsILocalFile", "initWithPath"); The interface nsILocalFile and the method initWithPath are optional. This example creates and initializes the nsILocalFile component.
  9. isSucessCode A function that determines if the results code argument is successful. It takes an argument of nsresult and returns the Boolean values true or false: js> Components.isSuccessCode(Components.results.NS_OK); true js> Components.isSuccessCode(Components.results.NS_ERRO R_FAILURE); false The methods and properties of the Components object listed above provide the only means to instantiate and access XPCOM objects from JavaScript. They are found often in the Mozilla codebase. In the sections that follow, they will be used frequently. 8.1.3. XPCOM Interfaces and the IDL All XPCOM interfaces are defined with the Interface Definition Language (IDL). IDL provides a language-neutral way to describe the public methods and properties of a component. Mozilla actually uses a modified, cross- platform version of IDL called XPIDL to compile interface source files. The separation of interface and implementation is a key distinction of COM programming. If the application programming interface (API) is abstracted from the implementation language and then frozen, consumers of that API will receive a guaranteed, established contract with the interface that ensures
  10. it will not be changed. This is perhaps the main reason why COM was invented: to maintain compatibility on a binary level so the client code can find and use the library it needs without worrying about linking to it. To make this sort of modularity possible at runtime, IDL interfaces are compiled into binary files called type libraries, which are described later in the section Section 8.1.4. 8.1.3.1. Interfaces versus components It is important to understand that most XPCOM components implement at least two interfaces. Like COM, each component needs the QueryInterface, AddRef, and Release functions to be available as an XPCOM object. These methods are derived from a basic interface called nsISupports, which is the XPCOM equivalent to Microsoft COM's IUnknown, shown in Table 8-1. Table 8-1. The IUnknown interface Parameters / Name Type Description return value Returns: int, which is the Increments the new ULONG reference count incremented AddRef AddRef(void) on the COM reference count object. on the object. This value may be useful for
  11. Parameters / Name Type Description return value diagnostics or testing. Parameters: iid, which is an [in] identifier of the requested interface. ppvObject, which is an HRESULT [out] pointer to QueryInterface(/* Retrieves a the interface [in] */ REFIID pointer to the pointer QueryInterface riid, /* requested identified by [iid_is][out] */ interface. iid. If the object void **ppvObject) does not support this interface, ppvObject is set to NULL. Returns: HRESULT, which is the
  12. Parameters / Name Type Description return value standard HRESULT value. Returns: int, which is the new Decrements the decremented ULONG reference count reference count Release Release(void) on the COM on the object. object. This value may be useful for diagnostics or testing. Tables 8-1 and 8-2 illustrate the minor differences between Microsoft's nsIUnknown and Mozilla's nsISupports root interfaces. The usage is covered in detail throughout this chapter. Table 8-2. The nsISupports interface Descripti Name Type Parameters / return value on AddRef NS_IMETHOD_(nsre Increases Returns:
  13. Descripti Name Type Parameters / return value on fcnt) the The resulting reference AddRef(void) reference count. count for this interface. The associate d instance will not be deleted unless the reference count is returned to zero. A Parameters: NS_IMETHOD runtime param aIID [in], which is a QueryInterf QueryInterface(REF mechanis requested interface IID. ace NSIID aIID, void** m for param aInstancePtr [out], aInstancePtr) interface which is a pointer to an discovery interface pointer that receives
  14. Descripti Name Type Parameters / return value on . the result. Returns: NS_OK if the interface is supported by the associated instance; NS_NOINTERFACE if it is not; and NS_ERROR_INVALID_POI NTER if aInstancePtr is NULL. Decreases the reference count for this Returns: NS_IMETHOD_(nsre interface. Release fcnt) Release(void) = The resulting reference Generally 0; count. , if the reference count returns to zero, the
  15. Descripti Name Type Parameters / return value on associate d instance is deleted.
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