Objetos e coleções compartilhados

Shared objects and shared collections are specific objects and collections whose contents are shared between processes. In contrast to interprocess variables, shared objects and shared collections have the advantage of being compatible with preemptive 4D processes: they can be passed by reference as parameters to commands such as New process or CALL WORKER.

Shared objects and shared collections are stored in standard Object and Collection type variables, but must be instantiated using specific commands:

• to create a shared object, use the New shared object command or call the new() function of a shared class,
• to create a shared collection, use the New shared collection command.

nota

Os objectos e coleções partilhados podem ser definidos como propriedades de objetos ou coleções padrão (não partilhados).

In order to modify a shared object/collection, the Use...End use structure must be called. Reading a shared object/collection value does not require Use...End use.

A unique, global catalog returned by the Storage command is always available throughout the application and its components, and can be used to store all shared objects and collections.

Utilização de objetos ou coleções compartidos

Once instantiated with the New shared object or New shared collection commands, shared object/collection properties and elements can be modified or read from any process of the application, under certain conditions.

Modificação

As modificações podem ser aplicadas a objetos partilhados e coleções partilhadas:

• adicionar ou remover propriedades de objectos,
• adicionar ou editar valores (desde que sejam suportados em objetos partilhados), incluindo outros objetos partilhados ou coleções (que criam um grupo partilhado, ver abaixo).

All modification instructions in a shared object or collection require to be protected inside a Use...End use block, otherwise an error is generated.

 $s_obj:=New shared object("prop1";"alpha")
 Use($s_obj)
    $s_obj.prop1:="omega"
 End Use

For conveniency, all collection functions that modify the shared object or collection insert an internal Use...End use block so you do not have to code it yourself. Por exemplo:

$col:=New shared collection()
$col.push("alpha") //.push() desencadeia internamente a utilização Use/End, pelo que não é necessário fazê-lo você mesmo

If you need to execute several modifications on the same collection, you can protect all modifications with a single Use...End use so that modifications are performed atomically.

$col:=Storage.mySharedCollection
Use($col)
    $col[0]:="omega" //modifying an element requires to be performed inside Use/End use
    $col.push("alpha") //.push() internally triggers Use/End use, but we want to do both modifications atomically
End Use

Um objeto/coleção partilhado só pode ser modificado por um processo de cada vez. Use locks the shared object/collection from other threads, while End use unlocks the shared object/collection (if the locking counter is at 0, see below). . Trying to modify a shared object/collection without at least one Use...End use generates an error. When a process calls Use...End use on a shared object/collection that is already in use by another process, it is simply put on hold until the End use unlocks it (no error is generated). Consequently, instructions within Use...End use structures should execute quickly and unlock the elements as soon as possible. Assim, recomenda-se vivamente que se evite modificar um objeto partilhado ou uma coleção diretamente a partir da interface, por exemplo, através de uma caixa de diálogo.

Assigning shared objects/collections to properties or elements of other shared objects/collections is allowed and creates shared groups. Um grupo partilhado é criado automaticamente quando um objeto/coleção partilhado é definido como valor de propriedade ou elemento de outro objeto/coleção partilhado. Os grupos partilhados permitem o aninhamento de objectos e colecções partilhados, mas impõem regras adicionais:

• Calling Use on a shared object/collection belonging to a group locks properties/elements of all shared objects/collections of the group and increments its locking counter. Calling End use decrements the locking counter of the group and when the counter is at 0, all the linked shared objects/collections are unlocked.
• Um objeto/coleção partilhado só pode pertencer a um grupo partilhado. É devolvido um erro se tentar definir um objeto/coleção partilhado já agrupado para um grupo diferente.
• Os objetos/coleções partilhados agrupados não podem ser desagrupados. Uma vez incluído num grupo partilhado, um objeto/coleção partilhado está permanentemente ligado a esse grupo durante toda a sessão. Mesmo que todas as referências de um objeto/coleção sejam removidas do objeto/coleção pai, permanecerão linkadas.

Consulte o exemplo 2 para ver uma ilustração das regras de grupos partilhados.

Note: Shared groups are managed through an internal property named locking identifier. Para obter informações detalhadas sobre este valor, consulte a Referência da Linguagem 4D.

Leitura

Reading properties or elements of a shared object/collection is allowed without having to call the Use...End use structure, even if the shared object/collection is in use by another process.

However, it is necessary to read a shared object/collection within Use...End use when several values are linked together and must be read at once, for consistency reasons.

Duplicação

Calling OB Copy with a shared object (or with an object containing shared object(s) as properties) is possible, but will return a standard (not shared) object including its contained objects (if any).

Armazenamento

Storage is a unique shared object, automatically available on each application and machine. This shared object is returned by the Storage command. É possível utilizar este objeto para fazer referência a todos os objetos/coleções partilhados definidos durante a sessão que se pretende que estejam disponíveis a partir de quaisquer processos preemptivos ou padrão.

Note that, unlike standard shared objects, the storage object does not create a shared group when shared objects/collections are added as its properties. This exception allows the Storage object to be used without locking all connected shared objects or collections.

For more information, refer to the Storage command description.

Use... End use

The formal syntax of the Use...End use structure is:

 Use(Shared_object_or_Shared_collection)
    statement(s)
 End use

The Use...End use structure defines a sequence of statements that will execute tasks on the Shared_object_or_Shared_collection parameter under the protection of an internal semaphore. Shared_object_or_Shared_collection can be any valid shared object or shared collection.

Shared objects and shared collections are designed to allow communication between processes, in particular, preemptive 4D processes. Podem ser passados por referência como parâmetros de um processo para outro. Surrounding modifications on shared objects or shared collections by the Use...End use keywords is mandatory to prevent concurrent access between processes.

• Once the Use line is successfully executed, all Shared_object_or_Shared_collection properties/elements are locked for all other process in write access until the corresponding End use line is executed.
• The statement(s) sequence can execute any modification on the Shared_object_or_Shared_collection properties/elements without risk of concurrent access.
• If another shared object or collection is added as a property of the Shared_object_or_Shared_collection parameter, they become connected within the same shared group.
• If another process tries to access one of the Shared_object_or_Shared_collection properties or connected properties while a Use...End use sequence is being executed, it is automatically put on hold and waits until the current sequence is terminated.
• The End use line unlocks the Shared_object_or_Shared_collection properties and all objects of the same group.
• Several Use...End use structures can be nested in the 4D code. In the case of a group, each Use increments the locking counter of the group and each End use decrements it; all properties/elements will be released only when the last End use call sets the locking counter to 0.

nota

The following functions automatically trigger an internal Use/End use, making an explicit call to the structure unnecessary when the function is executed:

• collection functions that modify shared collections
• shared functions (defined in shared classes).

Exemplo 1

Se quiser lançar vários processos que executem uma tarefa de inventário em diferentes produtos e atualizem o mesmo objeto partilhado. O processo principal instancia um objeto partilhado vazio e, em seguida, lança os outros processos, passando o objeto partilhado e os produtos a contar como parâmetros:

 ARRAY TEXT($_items;0)
 ... //preencher o array com itens para contar
 $nbItems:=Tamanho do array($_items)
 var $inventory : Object
 $inventory:=Novo objeto partilhado
 Use($inventory)
    $inventory.nbItems:=$nbItems
 End use

  //Criar processos
 For($i;1;$nbItems)
    $ps:=New process("HowMany";0; "HowMany_"+$_items{$i};$_items{$i};$inventory)
  //$inventory objeto enviado por referência
 End for

No método "HowMany", o inventário é efetuado e o objeto partilhado $inventory é atualizado o mais rapidamente possível:

    //HowMany
 #DECLARE ($what : Text ; $inventory : Object)

 $count:=CountMethod($what) //method to count products
 Use($inventory) //use shared object
    $inventory[$what]:=$count  //save the results for this item
 End use

Exemplo 2

Os exemplos seguintes destacam regras específicas para o tratamento de grupos partilhados:

 $ob1:=New shared object
 $ob2:=New shared object
 Use($ob1)
    $ob1.a:=$ob2  //group 1 is created
 End use

 $ob3:=New shared object
 $ob4:=New shared object
 Use($ob3)
    $ob3.a:=$ob4  //group 2 is created
 End use

 Use($ob1) //use an object from group 1
    $ob1.b:=$ob4  //ERROR
  //$ob4 already belongs to another group
  //assignment is not allowed
 End use

 Use($ob3)
    $ob3.a:=Null //remove any reference to $ob4 from group 2
 End use

 Use($ob1) //use an object from group 1
    $ob1.b:=$ob4  //ERROR
  //$ob4 still belongs to group 2
  //assignment is not allowed
 End use