headerloop = @fragments.enum_for(:each_with_index).collect { |f, i| [b::Uint32be, f.id, b::Uint32be, offset((f.content.size) ,i)]}.flatten  

#Then calculating the offset outside that block
...

File.open( "binary.bin", "wb" ) do |io|
  obj = BinData::String.new(:initial_value => "Binary file start-point").write(io)      
  obj = BinData::Uint32be.new(:initial_value => 0).write(io)
...