Android: How to get a custom view to redraw partially?

Android: How to Get a Custom View to Redraw Partially

In Android development, custom views provide a powerful way to create unique and interactive UI elements. However, efficiently redrawing a custom view when only a portion of it needs updating can be tricky. This article will guide you through the process of redrawing parts of a custom view in Android, optimizing performance and ensuring a smooth user experience.

Understanding View Invalidation

Android’s view system relies on the concept of invalidation. When a view needs to be redrawn, it marks itself as “invalid”. The framework then triggers a redraw process, which includes calling the view’s onDraw() method. However, redrawing the entire view every time a small change occurs can be computationally expensive, leading to lag and performance issues.

Techniques for Partial Redrawing

1. Invalidate Specific Areas

Instead of invalidating the entire view, you can target the specific regions that require redrawing. This approach provides a more efficient solution.

Using invalidate(Rect):

// Example: Redrawing a rectangle within a view
invalidate(new Rect(10, 10, 50, 50)); 

This code snippet invalidates the area defined by the specified rectangle, resulting in only that portion being redrawn.

2. Using View.postInvalidate()

In some scenarios, you may need to invalidate a view from a different thread, such as a background thread. The View.postInvalidate() method is designed for this purpose. It posts a runnable to the view’s message queue, which will then trigger a redraw.

// Example: Redrawing a view from a background thread
new Thread(new Runnable() {
    @Override
    public void run() {
        // Perform some operation on the background thread
        view.postInvalidate();
    }
}).start();

3. Optimizing with View.onDraw()

You can also optimize redrawing by carefully controlling the drawing process within your view’s onDraw() method. This includes:

• Using Canvas.clipRect() to clip the drawing area and prevent unnecessary rendering outside the required region.
• Drawing only the elements that need to be updated.
• Avoiding redundant calculations within the onDraw() method.

Consider caching complex or expensive calculations in the background thread for improved performance.

Example: Custom Progress Bar

Let’s illustrate the principles with a custom progress bar example.

public class MyProgressBar extends View {
    // ... view properties ...

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        // Draw background of the progress bar
        canvas.drawRect(0, 0, getWidth(), getHeight(), mBackgroundPaint);

        // Calculate the progress filled area
        float progressWidth = getWidth() * (mProgress / 100f);

        // Draw the progress filled area
        canvas.drawRect(0, 0, progressWidth, getHeight(), mProgressPaint);
    }

    // Update the progress value and invalidate the view
    public void setProgress(int progress) {
        mProgress = progress;
        invalidate();
    }
}

Choosing the Right Technique

Conclusion

Effectively managing partial redraws in Android custom views is crucial for maintaining responsiveness and a smooth user experience. By employing the techniques discussed, you can optimize view updates, reduce performance bottlenecks, and create more engaging and efficient Android applications.