Best way to change jqGrid rowNum from ALL to -1 before pass to a web service

I'm looking to find the best way to allow users to choose to show ALL records in a jqGrid. I know that a -1 value passed for the rows parameter denotes ALL, but I want the word "ALL" not a -1 to appear in the rowList select element, ie. rowList: [15, 50, 100, 'ALL'].

I'm passing the grid request to a web service which accepts an int for "rows", and I'm trying find how and when I should change the user selected value of "ALL" to a -1 before it gets sent to the web service.

Below is my cleaned up grid code. I tried some various code blocks before my $.ajax in the datatype function. But most attempts just seemed like I have to be doing this the most convoluted way I possibly could. For example,

datatype: function(postdata) {
   if ($("#gridTableAssets").jqGrid('getGridParam', 'rowNum') == 'ALL') {
      $("#gridTableAssets").appendPostData({ "rows": -1, "page": 1 });
   }
   $.ajax({...

But doing that seemed to cause the actual "page" GridParam to be nulled out on subsequent grid actions, forcing me handle that in other places. There just seems like this is something that would be frequently done out there and have clean way of doing it.

Cleaned grid code:

$("#gridTableAssets").jqGrid({
    datatype: function(postdata) {
       $.ajax({
           url: "/Service/Repository.asmx/GetAssets",
           data: JSON.stringify(postdata),
           type: 'POST',
           contentType: "application/json; charset=utf-8",
           error: function(XMLHttpRequest, textStatus, errorThrown) {
              alert('error');
           },
           success: function(msg) {
              var assetsGrid = $("#gridTableAssets")[0];
              assetsGrid.addJSONData(JSON.parse(msg));
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           }
       });
    },
    ...
    pager: $('#pagerAssets'),
    rowNum: 15,
    rowList: [15, 50, 100, 'ALL'],
    ...                    
    onPaging: function(index, colindex, sortorder) {
       SessionKeepAlive();
    }
});

And here is the web service

[WebMethod]
public string GetAssetsOfAssetStructure(bool _search, int rows, int page, 
    string sidx, string sord, string filters)

First of all I find your question very good. As I started with jqGrid I was searching for "All" in the rowList and made some experiments, but without any success. Then I forgot about this problem. Now after your question I think about using this feature in all my jqGrids.

Now about the solution. It is very easy, but befor all I suggest to replace datatype as function to the datatype: 'json'. All what you do inside of the function you can do with standard 'json' type. To change contentType to "application/json; charset=utf-8" one can use ajaxGridOptions option (see Setting the content-type of requests performed by jQuery jqGrid for details). One can use postData parameter as a replacement of data parameter of jQuery.ajax, but you will need it only if you want to send some additional parameters to server (like described in How to filter the jqGrid data NOT using the built in search/filter box). At the end we receive something like following:

$("#gridTableAssets").jqGrid({
    datatype: 'json',
    gridview: true,
    url: '/Service/Repository.asmx/GetAssets',
    //postData: postdata, // add some additional parameters
    ajaxGridOptions: { contentType: 'application/json; charset=utf-8' },
    mtype: 'POST',
    // ...
    pager: $('#pagerAssets'),
    rowNum: 15,
    rowList: [15, 50, 100, 'ALL'],
    serializeGridData: function (postData) {
        if (typeof postData.rows === "string") {  // "ALL"
            postData.rows = 10000;  // or -1 if your server 
        }
        if (isNaN(postData.page)) { // fix NaN in page for rows="ALL"
            postData.page = 1;
        }
        return JSON.stringify(postData);
    },
    // ...                    
});

If you plan to use serializeGridData, ajaxGridOptions and some other parameters in all your jqGrids you can define this function inside of $.jgrid.defaults (see one more time Setting the content-type of requests performed by jQuery jqGrid).