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How to detect and use mouse wheel scrolling in Scratch

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IM

InputMaster_Alex

Posted on April 3, 2022 • Intermediate

🖱️ Mouse wheel detection mystery

Hey fellow Scratchers! 👋 I’ve been exploring some amazing projects that respond to mouse wheel scrolling - like zooming in and out of maps, scrolling through menus, and controlling camera movement. It looks so professional!

I’m really curious about how this works. I’ve tried looking for mouse wheel blocks in the sensing category, but I can’t find any specific mouse wheel detection blocks. How do these projects detect when I scroll my mouse wheel?

If anyone knows the secret behind this, I’d love to learn! I’m working on a project where mouse wheel control would be perfect for navigating through different levels. Thanks in advance! 🙏

TC

TechCoder_Mario

Replied 26 minutes later • ⭐ Best Answer

Great question @InputMaster_Alex! This is actually a clever workaround that many advanced Scratchers use. Here’s the secret behind mouse wheel detection:

🔍 The Mouse Wheel Mystery Solved

Scratch doesn’t have direct mouse wheel blocks, but here’s what happens behind the scenes:

  • When you scroll the mouse wheel UP, browsers send an “up arrow” key signal
  • When you scroll the mouse wheel DOWN, browsers send a “down arrow” key signal
  • But these signals are different from actual keyboard presses!

🎯 The Clever Detection Method

Here’s how to detect mouse wheel scrolling:

flowchart TD A[🖱️ Mouse Wheel Scroll] --> B{Direction?} B -->|Scroll Up| C[Browser sends UP arrow signal] B -->|Scroll Down| D[Browser sends DOWN arrow signal] C --> E[Scratch detects UP arrow key event] D --> F[Scratch detects DOWN arrow key event] E --> G{Is keyboard UP actually pressed?} F --> H{Is keyboard DOWN actually pressed?} G -->|No| I[✅ Mouse wheel UP detected!] G -->|Yes| J[❌ Ignore - keyboard input] H -->|No| K[✅ Mouse wheel DOWN detected!] H -->|Yes| L[❌ Ignore - keyboard input] I --> M[Execute scroll up action] K --> N[Execute scroll down action] style A fill:#e1f5fe style I fill:#e8f5e8 style K fill:#e8f5e8 style J fill:#ffebee style L fill:#ffebee

💻 Basic Mouse Wheel Detection Code

Here’s the fundamental technique:

    // Mouse wheel UP detection
when [up arrow v] key pressed
if <not <key [up arrow v] pressed?>> then
// This is mouse wheel scroll UP!
change [scroll position v] by [10]
say [Scrolled up!] for (0.5) seconds
end

// Mouse wheel DOWN detection
when [down arrow v] key pressed
if <not <key [down arrow v] pressed?>> then
// This is mouse wheel scroll DOWN!
change [scroll position v] by [-10]
say [Scrolled down!] for (0.5) seconds
end

🚀 Advanced Scrolling Applications

Here are some practical uses:

📋 Scrollable Menu System
    // Variables: menu_position, menu_items, visible_items
when flag clicked
set [menu position v] to [0]
set [visible items v] to [5]

// Scroll up through menu
when [up arrow v] key pressed
if <not <key [up arrow v] pressed?>> then
if <(menu position) > [0]> then
change [menu position v] by [-1]
broadcast [update menu display v]
end
end

// Scroll down through menu
when [down arrow v] key pressed
if <not <key [down arrow v] pressed?>> then
if <(menu position) < ((length of [menu items v]) - (visible items))> then
change [menu position v] by [1]
broadcast [update menu display v]
end
end
🔍 Zoom Control System
    // Variables: zoom_level, min_zoom, max_zoom
when flag clicked
set [zoom level v] to [100]
set [min zoom v] to [50]
set [max zoom v] to [200]

// Zoom in with mouse wheel up
when [up arrow v] key pressed
if <not <key [up arrow v] pressed?>> then
if <(zoom level) < (max zoom)> then
change [zoom level v] by [10]
set size to (zoom level) %
say (join [Zoom: ] (join (zoom level) [%])) for (0.3) seconds
end
end

// Zoom out with mouse wheel down
when [down arrow v] key pressed
if <not <key [down arrow v] pressed?>> then
if <(zoom level) > (min zoom)> then
change [zoom level v] by [-10]
set size to (zoom level) %
say (join [Zoom: ] (join (zoom level) [%])) for (0.3) seconds
end
end
🎮 Camera Movement Control
    // Variables: camera_y, scroll_speed, world_height
when flag clicked
set [camera y v] to [0]
set [scroll speed v] to [20]

// Scroll camera up
when [up arrow v] key pressed
if <not <key [up arrow v] pressed?>> then
if <(camera y) < (world height)> then
change [camera y v] by (scroll speed)
broadcast [move world down v]
end
end

// Scroll camera down
when [down arrow v] key pressed
if <not <key [down arrow v] pressed?>> then
if <(camera y) > [0]> then
change [camera y v] by (0 - (scroll speed))
broadcast [move world up v]
end
end

⚡ Pro Tips for Smooth Scrolling

  • Debouncing: Add small delays to prevent too-rapid scrolling
  • Acceleration: Increase scroll speed for continuous scrolling
  • Boundaries: Always check limits to prevent scrolling beyond content
  • Visual feedback: Show scroll indicators or position markers
    // Smooth scrolling with acceleration
when flag clicked
set [scroll momentum v] to [0]
set [max momentum v] to [5]

forever
// Apply momentum-based movement
if <(scroll momentum) > [0]> then
change y by (scroll momentum)
change [scroll momentum v] by [-0.2]  // Friction
if <(scroll momentum) < [0.1]> then
set [scroll momentum v] to [0]
end
else
if <(scroll momentum) < [0]> then
change y by (scroll momentum)
change [scroll momentum v] by [0.2]  // Friction
if <(scroll momentum) > [-0.1]> then
set [scroll momentum v] to [0]
end
end
end
wait (0.02) seconds
end

// Mouse wheel adds to momentum
when [up arrow v] key pressed
if <not <key [up arrow v] pressed?>> then
change [scroll momentum v] by [2]
if <(scroll momentum) > (max momentum)> then
set [scroll momentum v] to (max momentum)
end
end

This technique opens up so many possibilities for creating professional-feeling interfaces! 🎯

IM

InputMaster_Alex

Replied 1 minute later

@TechCoder_Mario This is absolutely mind-blowing! 🤯 I had no idea that’s how it worked!

I just implemented the basic version and it works perfectly! My level navigation system is now so much more intuitive. Thank you for the detailed explanation and all those examples!

Following you as promised! 🎉

UI

UIExpert_Sarah

Replied 3 years later

This technique is still incredibly useful! 💪 I’ve been using it in my latest projects and it makes such a difference for user experience.

One additional tip: You can combine this with touch gestures for mobile compatibility by detecting swipe movements. Makes your projects work great on tablets too!

Thanks for keeping this knowledge alive in the community! 🙌

VB

Vibelf_Community

Pinned Message • Moderator

🖱️ Master Advanced Input Techniques

Excellent discussion on mouse wheel detection! For developers building sophisticated user interfaces, our expert tutors can help you implement:

  • 🎮 Multi-input control systems
  • 📱 Cross-platform input handling
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