Canvas Developer Guide

Getting Started
Using Force.com Canvas
Force.com Canvas SDK
Canvas Apps and Visualforce Pages
Canvas Apps in a Page Layout or a Mobile Card
Canvas Apps in the Publisher
Canvas Apps in the Chatter Feed
Canvas in the Salesforce1 App
Customizing Your App Lifecycle
Creating a CanvasLifecycleHandler
Associating Your CanvasLifecycleHandler with Your App
Filtering CanvasRequest Context Data
Controlling Canvas App Behavior
Presenting User Error Messages
Testing Your CanvasLifecycleHandler Implementation
Distributing Your CanvasLifecycleHandler Class
Reference
PDF

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Presenting User Error Messages

You can use Canvas.CanvasRenderException to display error messages to the user.

If you throw a CanvasRenderException from your onRender() implementation, the error message in that exception will be displayed to the user in the Salesforce UI. This exception will be managed only within the onRender() method.

CanvasRenderException supports all the standard Apex exception class methods and constructors. See ”Exception Class and Build-In Exceptions” in the Force.com Apex Code Developer’s Guide for more information on Apex exceptions.

Here’s an example onRender() implementation that throws a Canvas.CanvasRenderException. Salesforce uses this exception and displays “Error: Could not set URL” as the user error message.
If the canvas app is run from the Salesforce Chatter tab, the error message resembles:Screenshot of Salesforce Chatter tab with custom canvas app error