Monday, May 26, 2014

Step By Step How to create partial views with Child Action Methods

        By Carmel Schvartzman

In this tutorial we'll learn how to create a partial view using Child Action Methods in MVC 4. Child Methods allow us to reuse code and markup, fully applying the DRY Software Paradigm ("Don't Repeat Yourself") , and avoiding the WET ("Write Everything Twice") approaches. The Child Actions cannot be invoked via regular user requests, instead they are invoked directly from the View. A Child Action method is to an Action method as a Partial View is to a View. 

In MVC there are 4 similar methods to render a Partial View, and it's important that we decide which one to use according to their properties:
1) The most widely used method is RenderPartial(), which is fast because it writes directly to the HTTP response stream, and is easy to use as it doesn't require to code a Child Action. But the data for the partial view must already be loaded in the view Model. The problem with this approach is, we can be tempted to manipulate the data INSIDE the PRESENTATION LAYER, to send it to the partial view. When we pass data from a view to another view we are specifying how to instantiate entity objects, and that shoud be a Bussiness Layer task.

2) Partial() is essentially the same thing but, instead RenderPartial(), it returns a string with HTML encoding, and you can use it to instantiate a variable if you like.
3) RenderAction() requires 
from the developer to code a Child Action, and therefore you could separate presentation layer from the Repository or Bussiness Layer. Is fast because it writes directly to the HTTP response stream.

4) Action() method is the same but it returns a string with HTML encoding. Using Action() you leave to the Controller (Child Action) the task of deciding which partial view to render. That way we leave to the Controller the responsability to make traffic decisions and to invoke Bussiness Logic. Also, this way we can make refactoring more neatly.
Child Actions are useful in building widgets and embedding them in the View. 

In this tutorial we'll create a Partial View and render it using an Action() call, being the presentation layer as this:



We'll display all Posts in a <TABLE> saved in a Partial View, and all Bloggers in another table from another Partial View. Each Partial View will be populated by a different Controller's Child Action.

First of all, let's create a new View inside the Shared folder at Views. This will be the "PostsList" Partial View:


 Now   for the "PostsList" Partial View, select "Blog" class for the scaffolding Model, and "List" for the template:



Open the _PostsList Partial View and take a look at the markup. It will be something like this (depending on your Entity Model):

Add the classes style definitions, CSS style that we'll add in a little bit, and look at the loop built by the template:

Notice we added the style classes to the <table> tag. Also, we've applied some formatting to properly display dates and pictures.

Now in order to display some style , add a new .css stylesheet file to the "Contents" folder:



Name the .css as GridStyle:




In the stylesheet we include all the style for the WebGrid, footer, header, hyperlinks, even the style for displaying adecuately the pictures:



The code (to copy-paste) is the following:
        .webgrid-table
        {
            font:italic 11px Verdana;
            width: 100%;
            display:grid;
            border-collapse: separate;
            border: solid 1px #98BF21;
            background-color: #f0c9a0;
            padding: 5px 5px 5px 5px;
        } 
        .webgrid-header
        {
            background-color: #c67f1c !important;
            color: #FFFFFF !important;
            font: 900 14px Verdana !important;
            padding:5px 5px 5px 5px;            
            text-align: center;
            
        }
        .details-div
        {
            background-color: #c67f1c !important;
            color: #FFFFFF !important;
            font: 900 14px Verdana !important;
            padding:5px 5px 5px 5px;
            text-align: center;
            
        }
        .ActionsTH
        {
            background-color: #c67f1c !important;
            color: #FFFFFF !important;
            font: 900 14px Verdana !important;
            padding:5px 5px 5px 5px;
            text-align: center;
            width:180px;
        }
        .webgrid-footer, .webgrid-footer a
        {
            background-color: #c67f1c;
            color: #FFF;
            font: 900 14px Verdana;
            padding:3px 3px 3px 3px;
        }
        .webgrid-alternating-row
        {
            background-color: #e5d773;
            padding:5px 5px 5px 5px;
        }
        .title-column
        {
            font:900 13px Verdana;
            text-align:center;
        }
        .webgrid-img
        {
            width: 150px;
            height: 150px;
        }




Now open the "Index" View and invoke the Child Action using the Action() method:



Notice that we added a <link> tag , to include the .css in the Index View:



Finally, let's code the Child Method, creating first the Context in the Controller:



Now append the Child Action:
Notice that this Child Action deals directly with the data from a Repository, and also can decide whether to render some specific Partial View or another one.


Save and run the application:



That's all!! 
In this tutorial we've learn how to create a partial view using Child Action Methods in MVC 4, thus allowing us to reuse code and markup in future refactoring.  

Happy programming.....


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