A 4D component is a set of 4D methods and forms representing one or more functionalities that can be installed in different applications. For example, you can develop a 4D e-mail component that manages every aspect of sending, receiving and storing e-mails in 4D applications.
Creating and installing 4D components is carried out directly from 4D. Basically, components are managed like plug-ins according to the following principles:
- A component consists of a regular structure file (compiled or not) having the standard architecture or in the form of a package (see .4dbase Extension).
- To install a component in an application project, you simply need to copy it into the "Components" folder of the project, at the same level as the Project folder.
- A component can call on most of the 4D elements: project methods, project forms, menu bars, choice lists, pictures from the library, and so on. It cannot call database methods and triggers.
- You cannot use standard tables or data files in 4D components. However, a component can create and/or use tables, fields and data files using mechanisms of external databases. These are separate 4D databases that you work with using SQL commands.
The component management mechanisms in 4D require the implementation of the following terms and concepts:
- Matrix Project: 4D project used for developing the component. The matrix project is a standard project with no specific attributes. A matrix project forms a single component. The matrix project is intended to be copied, compiled or not, into the Components folder of the project that will be using the component (host application project).
- Host Project: Application project in which a component is installed and used.
- Component: Matrix project, compiled or not, copied into the Components folder of the host application and whose contents are used in the host applications.
It should be noted that a project can be both a “matrix” and a “host,” in other words, a matrix project can itself use one or more components. However, a component cannot use “sub-components” itself.
Protection of components: compilation
By default, all the project methods of a matrix project installed as a component are potentially visible from the host project. In particular:
- The shared project methods are found on the Methods Page of the Explorer and can be called in the methods of the host project. Their contents can be selected and copied in the preview area of the Explorer. They can also be viewed in the debugger. However, it is not possible to open them in the Method editor nor to modify them.
- The other project methods of the matrix project do not appear in the Explorer but they too can be viewed in the debugger of the host project.
To protect the project methods of a component effectively, simply compile the matrix project and provide it in the form of a .4dz file. When a compiled matrix project is installed as a component:
- The shared project methods are shown on the Methods Page of the Explorer and can be called in the methods of the host project. However, their contents will not appear in the preview area nor in the debugger.
- The other project methods of the matrix project will never appear.
Sharing of project methods
All the project methods of a matrix project are by definition included in the component (the project is the component), which means that they can be called and executed by the component.
On the other hand, by default these project methods will not be visible, nor can they be called in the host projects. In the matrix project, you must explicitly designate the methods that you want to share with the host project. These project methods can be called in the code of the host project (but they cannot be modified in the Method editor of the host database). These methods form entry points in the component.
Note: Conversely, for security reasons, by default a component cannot execute project methods belonging to the host project. In certain cases, you may need to allow a component to access the project methods of your host project. To do this, you must explicitly designate the project methods of the host project that you want to make accessible to the components.
The local, process and interprocess variables are not shared between components and host projects. The only way to access component variables from the host project and vice versa is using pointers.
Example using an array:
//In the host project: ARRAY INTEGER(MyArray;10) AMethod(->MyArray) //In the component, the AMethod project method contains: APPEND TO ARRAY($1->;2)
Examples using variables:
C_TEXT(myvariable) component_method1(->myvariable) C_POINTER($p) $p:=component_method2(...)
When you use pointers to allow components and the host project to communicate, you need to take the following specificities into account:
Get pointercommand will not return a pointer to a variable of the host project if it is called from a component and vice versa.
The component architecture allows the coexistence, within the same interpreted project, of both interpreted and compiled components (conversely, only compiled components can be used in a compiled project). In order to use pointers in this case, you must respect the following principle: the interpreter can unpoint a pointer built in compiled mode; however, in compiled mode, you cannot unpoint a pointer built in interpreted mode. Let’s illustrate this principle with the following example: given two components, C (compiled) and I (interpreted), installed in the same host project.
If component C defines the
myCvarvariable, component I can access the value of this variable by using the pointer
If component I defines the
myIvarvariable, component C cannot access this variable by using the pointer
->myIvar. This syntax causes an execution error.
The comparison of pointers using the
RESOLVE POINTERcommand is not recommended with components since the principle of partitioning variables allows the coexistence of variables having the same name but with radically different contents in a component and the host project (or another component). The type of the variable can even be different in both contexts. If the
myptr2pointers each point to a variable, the following comparison will produce an incorrect result:
RESOLVE POINTER(myptr1;vVarName1;vtablenum1;vfieldnum1) RESOLVE POINTER(myptr2;vVarName2;vtablenum2;vfieldnum2) If(vVarName1=vVarName2) //This test returns True even though the variables are different
In this case, it is necessary to use the comparison of pointers:
If(myptr1=myptr2) //This test returns False
Access to tables of the host project
Although components cannot use tables, pointers can permit host projects and components to communicate with each other. For example, here is a method that could be called from a component:
// calling a component method methCreateRec(->[PEOPLE];->[PEOPLE]Name;"Julie Andrews")
Within the component, the code of the
C_POINTER($1) //Pointer on a table in host project C_POINTER($2) //Pointer on a field in host project C_TEXT($3) // Value to insert $tablepointer:=$1 $fieldpointer:=$2 CREATE RECORD($tablepointer->) $fieldpointer->:=$3 SAVE RECORD($tablepointer->)
Scope of language commands
Except for Unusable commands, a component can use any command of the 4D language.
When commands are called from a component, they are executed in the context of the component, except for the
EXECUTE METHOD command that uses the context of the method specified by the command. Also note that the read commands of the “Users and Groups” theme can be used from a component but will read the users and groups of the host project (a component does not have its own users and groups).
SET DATABASE PARAMETER and
Get database parameter commands are an exception: their scope is global to the application. When these commands are called from a component, they are applied to the host application project.
Furthermore, specific measures have been specified for the
Structure file and
Get 4D folder commands when they are used in the framework of components.
COMPONENT LIST command can be used to obtain the list of components that are loaded by the host project.
The following commands are not compatible for use within a component because they modify the structure file — which is open in read-only. Their execution in a component will generate the error -10511, “The CommandName command cannot be called from a component”:
ON EVENT CALL
Method called on event
SET PICTURE TO LIBRARY
REMOVE PICTURE FROM LIBRARY
ARRAY TO LIST
CREATE USER FORM
DELETE USER FORM
Set group properties
Set user properties
BLOB TO USERS
SET PLUGIN ACCESS
Current form tablecommand returns
Nilwhen it is called in the context of a project form. Consequently, it cannot be used in a component.
- SQL data definition language commands (
DROP TABLE, etc.) cannot be used on the component project. However, they are supported with external databases (see
CREATE DATABASESQL command).
An error-handling method installed by the
ON ERR CALL command only applies to the running application. In the case of an error generated by a component, the
ON ERR CALL error-handling method of the host project is not called, and vice versa.
Use of forms
- Only “project forms” (forms that are not associated with any specific table) can be used in a component. Any project forms present in the matrix project can be used by the component.
- A component can call table forms of the host project. Note that in this case it is necessary to use pointers rather than table names between brackets  to specify the forms in the code of the component.
Note: If a component uses the
ADD RECORD command, the current Input form of the host project will be displayed, in the context of the host project. Consequently, if the form includes variables, the component will not have access to it.
- You can publish component forms as subforms in the host projects. This means that you can, more particularly, develop components offering graphic objects. For example, Widgets provided by 4D are based on the use of subforms in components.
Use of tables and fields
A component cannot use the tables and fields defined in the 4D structure of the matrix project. However, you can create and use external databases, and then use their tables and fields according to your needs. You can create and manage external databases using SQL. An external database is a 4D project that is independent from the main 4D project, but that you can work with from the main 4D project. Using an external database means temporarily designating this database as the current database, in other words, as the target database for the SQL queries executed by 4D. You create external databases using the SQL
CREATE DATABASE command.
The following code is included in a component and performs three basic actions with an external database:
- creates the external database if it does not already exist,
- adds data to the external database,
- reads data from the external database.
Creating the external database:
<>MyDatabase:=Get 4D folder+"\MyDB" // (Windows) stores the data in an authorized directory Begin SQL CREATE DATABASE IF NOT EXISTS DATAFILE :[<>MyDatabase]; USE DATABASE DATAFILE :[<>MyDatabase]; CREATE TABLE IF NOT EXISTS KEEPIT ( ID INT32 PRIMARY KEY, kind VARCHAR, name VARCHAR, code TEXT, sort_order INT32 ); CREATE UNIQUE INDEX id_index ON KEEPIT (ID); USE DATABASE SQL_INTERNAL; End SQL
Writing in the external database:
$Ptr_1:=$2 // retrieves data from the host project through pointers $Ptr_2:=$3 $Ptr_3:=$4 $Ptr_4:=$5 $Ptr_5:=$6 Begin SQL USE DATABASE DATAFILE :[<>MyDatabase]; INSERT INTO KEEPIT (ID, kind, name, code, sort_order) VALUES (:[$Ptr_1], :[$Ptr_2], :[$Ptr_3], :[$Ptr_4], :[$Ptr_5]); USE DATABASE SQL_INTERNAL; End SQL
Reading from an external database:
$Ptr_1:=$2 // accesses data of the host project through pointers $Ptr_2:=$3 $Ptr_3:=$4 $Ptr_4:=$5 $Ptr_5:=$6 Begin SQL USE DATABASE DATAFILE :[<>MyDatabase]; SELECT ALL ID, kind, name, code, sort_order FROM KEEPIT INTO :$Ptr_1, :$Ptr_2, :$Ptr_3, :$Ptr_4, :$Ptr_5; USE DATABASE SQL_INTERNAL; End SQL
Use of resources
Components can use resources. In conformity with the resource management principle, if the component is of the .4dbase architecture (recommended architecture), the Resources folder must be placed inside this folder.
Automatic mechanisms are operational: the XLIFF files found in the Resources folder of a component will be loaded by this component.
In a host project containing one or more components, each component as well as the host projects has its own “resources string.” Resources are partitioned between the different projects: it is not possible to access the resources of component A from component B or the host project.
On-line help for components
A specific mechanism has been implemented in order to allow developers to add on-line help to their components. The principle is the same as that provided for 4D projects:
- The component help must be provided as a file suffixed .htm, .html or (Windows only) .chm,
- The help file must be put next to the structure file of the component and have the same name as the structure file,
- This file is then automatically loaded into the Help menu of the application with the title “Help for...” followed by the name of the help file.