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Templates: Flow Control (v 1.x)

• • Conditions
    • Syntax
    • Description
  • Foreach block
    • Syntax
    • Description
      • Type detection
      • Body
    • Custom form rendering
  • Caching Elements
    • Syntax
    • Description
  • Adding Cache Triggers

Conditions

Syntax

Begin or continue conditional statement:

( 'if' | 'elif' ) [ 'not' ] [ 'empty' ] ( VARIABLE | 'rtl' )
( 'if' | 'elif' ) '(' any-c++-expression ')'

Else block:

'else'

Each if/else block should be ended with end statement. For example:

<% if not empty username %>
  <h1>Hello <% username %></h1>
<% else %>
  <h1>Hello Visitor</h1>
<% end %>
<% if ( content.n % 2==0 ) %>
  <% n %> is odd.
<% elif ( content.n % 3 ==0) % >
  <% n %> can be divided by 3 without reminder.
<% end %>

Description

This is ordinary if/else if statement. You can specify following conditions:

• By specifying variable or property you check whether it is true.
• By specifying empty keyword you may check if specific STL container is empty by calling its member empty().
• You can negate the result using not
• You can also check whether current locale referred to Right-To-Left language like Hebrew, Arabic or Persian by specifying rtl keyword.
  This condition actually checks in dictionary if string "LTR" is translated as "RTL".
• You can specify arbitrary C++ conditional expression withing round brackets (). Note: Do not forget to refer to content variables using content. prefix.

Foreach block

Syntax

Major loop:

'foreach' NAME ['as' IDENTIFIER ] [ 'rowid' IDENTIFIER [ 'from' NUMBER ] ] [ 'reverse' ] 'in' VARIABLE 

In case of empty collection:

'empty'

Specify central body:

'item'
'separator'

For example:

<% foreach student in students %>
  <ul>
  <% item %>
     <li><% student.id %>, <% student.name %></li>
  <% end %>
  </ul>
<% empty %>
  <h2>No students</h2>
<% end %>

You can specify delimiter for elements between separator and item block:

<% foreach student in students %>
<% separator %>, <% item %><% student.name %><% end %>
<% end %>

Would generate a list like: "Ron, John, Moshe"

Description

foreach loop creates a for loop that iterates any STL collection. The given name is a reference to the type returned by iterator. For example, if students is std::list<student_t> then student in above example is defined as student_t &student.

• reverse - orders to use reverse iterator (rbegin,rend)
• rowid - defines a special integer index that can be used for enumerating rows and optional from defines initial index value (default 0)

For example

<% foreach line rowid num from 1 in code %>
<% item %><%= num %>:<%=line%><% end %>
<% end %>

Note: reverse and rowid are supported starting from CppCMS 0.99.10

Type detection

Template system uses one of following method for automatic type detection:

1. C++0x auto
2. C++0x decltype
3. GCC's style typeof or __typeof__

At least one of them should be supported by the compiler in order to provide correct template generation without using as keyword.

But some compilers, most noticeable is MSVC8, MSVC9 still not support automatic type detection, thus, specific iterator type should be explicitly specified.

For example:

<% foreach student as students_type::iterator in students %>

Would give you correct solution for MSVC compiler and student would a variable of type students_type::iterator::value_type

Body

empty statement is equivalent to "else" so:

<% foreach a in b %>
<ul>
<% item %><% a %><% end %>
</ul>
<% empty %>
   nothing
<% end %>

Is generated into code like:

if(!content.b.empty()) {
  out()<<"<ul>";
  for(auto a_it=content.b.begin();a_it!=content.b.end();++a_it) {
    auto &a=*a_it;
    out()<<a;
  }
  out()<<"</ul>";
}else{
  out()<<"nothing";
}

You must provide <% item %>...<% end %> block for any foreach statement. If separator is used it should be added between separator and item blocks.

Custom form rendering

CppCMS forms system provides several options for rendering form widgets, but sometimes custom rendering may be needed. In this case, foreach can be used to iterate over cppcms::form as over ordinary collation.

For example, rendering with "new line" separator:

<% foreach w in form %>
  <% separator %><br/><% item %>
    <% if w.has_message() %><% w.message() %>:<% end %>
    <% form input w %>
    <% if not w.valid() %>:<% w.error_message() %><% end %>
  <% end %>
<% end %>

Caching Elements

Syntax

The cached block begins with cache statement

'cache' ( VARIABLE | STRING ) [ 'for' NUMBER ] ['on' 'miss' VARIABLE() ] [ 'no' 'triggers' ] [ 'no' 'recording' ]

and ends with end statement.

For example

<% cache "sidebar" for 10 %> 
  ...
<% end cache %>

Description

This block allows to cache parts of rendered templates. The first parameter ( VARIABLE | STRING ) is mandatory and defines the key that is used to retrieve the frame from the cache. It should be either constant predefined string or some variable stored in the context.

• for NUMBER - defines the number of seconds this block is stored.
• on miss VARIABLE() defines the callback function that should be called if cache miss occurs. It allows for example to populate the context with some more data from the database.

• no triggers - says that the cache key should not be added as trigger to the general cache. By default the key is added to the current triggers set, if no triggers defined it does not happen.

  For example:

    <% cache "foo" %>
        <% cache "bar" %>
        <% end %>
    <% end %>
  

  The bar trigger is added to foo and to all higher level object as dependent trigger. Thus calling rise("bar") would invalidate "foo" frame.

  However if you write:

    <% cache "foo" %>
        <% cache "bar" no triggers %>
        <% end %>
    <% end %>
  

  Then rising "bar" would not invalidate "foo".

• no recording prevents from this object to record all triggers that where added during frame generation.

  For example:

    <% cache "foo" on miss load_data() %>
    ...
    <% end %>
  
    void load_data(some_content &c) 
    {
       c.foo = .. // load some data from S
       cache().add_trigger("my_data");
    }
  

  If load_data() is called it would add a trigger "my_data" to the cache and thus rising my_data would invalidate "foo".

  If no recording is provided

    <% cache "foo" on miss load_data() no recording %>
  

  Then the rising my_data would not invalidate "foo".

Adding Cache Triggers

Strictly speaking this is not flow block but it is strongly connected to the cache block described above.

trigger ( STRING | VARIABLE )

Says to add trigger to the current context effectively calling cache().add_trigger(value)

For example:

<% cache "foo" %>
   ...
   <% trigger "bar" %>
<% end cache %>

Now rising bar as trigger would invalidate foo object.

It is useful for some complex relations where you want to add triggers to cached frames explicitly.

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