true if the Web page is performing a postback or the client-side control is loading content; otherwise, false.

When the client-side control is loading content, it can be performing a postback, such as refreshing a report with new data, or loading its content in the browser after a postback. This property does not cause an exception when the Web page or the client-side control is performing a postback. Use this property to check whether the report or page is being updated and whether you can access the methods and properties.

If you are implementing a custom toolbar, you can use this property to toggle the enabled and disabled states of the toolbar items. For example, you can add a script reference to the following JavaScript code in your ScriptManager control to toggle the enabled and disabled state of an external button.

Note
The ScriptManager control ensures that the referenced script executes before the client-side control is loaded. This allows the event handler to handle all changes to the isLoading property.

Sys.Application.add_load(function () {
    $find("ReportViewer1").add_propertyChanged(viewerPropertyChanged);
});

function viewerPropertyChanged(sender, e) {
    if (e.get_propertyName() === "isLoading") {
        var viewer = $find("ReportViewer1");
        var button = document.getElementById("Button1");
        button.disabled = viewer.get_isLoading();
    }
}

In case the client-side control is performing a long-running postback, you can use the Sys.WebForms.PageRequestManager.abortPostBack method to cancel the postback. For example:

function cancelPostBack() {
    var prm = Sys.WebForms.PageRequestManager.getInstance();
    if (prmn.get_isInAsyncPostBack()) {
        prm.abortPostBack();
    }
}

Reference

Other Resources