Overview

MvvmCross is a well-known cross-platform framework that facilitates the development of mobile applications by implementing the MVVM (Model-View-ViewModel) pattern. Despite its capabilities, developers often encounter certain challenges, a common one being UI freezing during the initialization phase when an async method is invoked. Understanding the root causes and best practices for handling async operations in MvvmCross can significantly enhance the responsiveness and fluidity of your applications.

Understanding UI Freezing

UI freezing occurs when the application becomes unresponsive, usually because the UI thread is blocked, preventing the app from reacting to user inputs. This is particularly problematic during initialization due to several attempts to perform time-consuming tasks synchronously or inept handling of asynchronous operations.

Asynchronous Initialization in MvvmCross

In MvvmCross, the ViewModels serve as the binding context for Views. Initialization often involves fetching data required to set up the ViewModel. When such tasks are CPU-bound or involve I/O operations, they should be performed asynchronously to ensure the UI remains responsive.

Common Patterns Leading to Freeze

1. Blocking Calls in Async Methods:
   • Initiating asynchronous operations with Async suffix methods, but blocking them using .Result or .Wait(). This blocks the UI as these calls are synchronous and run on the main thread.
2. Complex Initialization Logic:
   • Methods marked as async but performing complex initialization directly on the UI thread instead of offloading work to a background thread.
3. Improper Use of ConfigureAwait:
   • Defaulting to ConfigureAwait(true) or missing it can continue post-await execution on the UI thread, which can freeze the UI if operations take substantial time to complete.

Example of a Problematic Method

• Async Operations: Ensure operations are properly awaited. Always await any asynchronous work fully.
• Avoid Synchronous Blocks: Avoid blocking the result of tasks using .Result or .Wait().
• Task Parallelization: Use Task.Run to execute CPU-bound work off the UI thread. However, ensure that UI-bound operations are appropriately awaited and context is captured only when necessary.
• Configure Context: Use ConfigureAwait(false) wherever applicable, especially when the continuation after await does not need to run on the UI thread.
• Profiling and Diagnostics: Utilize debugging tools to identify bottlenecks in code that may cause freezes.
• ViewModel Lifecycles: Consider the lifecycle of ViewModels in MvvmCross and defer initialization tasks to periods where the UI is stable.
• Retries and Fault Handling: Implement strategies to handle retries and faults gracefully, ensuring that the app remains responsive under exceptional circumstances.