JUCE Threading Model & Component Lifecycle

This skill covers thread safety and component lifecycle patterns critical for avoiding crashes in MAGDA.

Threading Model

Three Thread Contexts

code

MessageThread (UI)     — Component::paint, resized, mouse events, Timer callbacks, callAsync lambdas
Audio Thread (RT)      — processBlock, no allocations, no locks, no MessageManager calls
Background Threads     — juce::Thread, juce::ThreadPool, long-running tasks

Check Which Thread You're On

cpp

// Assert you're on the message thread
jassert(juce::MessageManager::getInstance()->isThisTheMessageThread());

// Check without asserting
if (juce::MessageManager::getInstance()->isThisTheMessageThread())
    component.repaint();

Cross-Thread Communication

cpp

// Audio thread → UI thread (fire-and-forget)
juce::MessageManager::callAsync([this] {
    // Runs on message thread. WARNING: `this` may be dead — see SafePointer below.
    label.setText("Done", juce::dontSendNotification);
});

// Audio thread → UI thread (coalesced, no allocation)
struct MyComp : juce::Component, juce::AsyncUpdater {
    void audioCallback() {
        latestValue.store(newVal);
        triggerAsyncUpdate();  // Safe from any thread, coalesces multiple calls
    }
    void handleAsyncUpdate() override {
        // Runs on message thread, once per coalesced batch
        slider.setValue(latestValue.load());
    }
    std::atomic<float> latestValue { 0.0f };
};

// Periodic polling from UI thread
struct MyComp : juce::Component, juce::Timer {
    MyComp() { startTimerHz(30); }
    ~MyComp() override { stopTimer(); }  // MUST stop in destructor
    void timerCallback() override {
        // Runs on message thread
        repaint();
    }
};

// Delayed one-shot on message thread
juce::MessageManager::callAsync([] {
    juce::Timer::callAfterDelay(500, [] {
        // Runs on message thread after 500ms
    });
});

Audio Thread Rules

cpp

void processBlock(juce::AudioBuffer<float>& buffer, juce::MidiBuffer& midi) override
{
    // NEVER do any of these on the audio thread:
    // - new / delete / malloc
    // - juce::String construction or concatenation
    // - MessageManager::callAsync (allocates — use AsyncUpdater instead)
    // - Lock acquisition (mutex, CriticalSection)
    // - File I/O
    // - DBG() in release builds

    // SAFE on audio thread:
    // - std::atomic reads/writes
    // - Lock-free queues (juce::AbstractFifo)
    // - Raw pointer/buffer arithmetic
    // - triggerAsyncUpdate() (lock-free)
}

Component Lifecycle & Destruction Safety

Destruction Ordering

cpp

struct ParentComp : juce::Component {
    ~ParentComp() override
    {
        stopTimer();            // 1. Stop all timers
        removeListener(this);   // 2. Remove all listener registrations
        // 3. Children are destroyed automatically after this (member order, reverse)
    }

    ChildComp child;  // Destroyed BEFORE ParentComp destructor body runs?
                      // NO — members are destroyed AFTER destructor body.
                      // So stopTimer() in destructor body is safe.
};

Rule: Stop timers and remove listeners in the destructor body, before members are destroyed.

SafePointer for Async Safety

cpp

// DANGEROUS — `this` may be deleted before lambda runs
juce::MessageManager::callAsync([this] {
    setText("done");  // CRASH if component was deleted
});

// SAFE — SafePointer becomes nullptr if component is deleted
juce::MessageManager::callAsync([safeThis = juce::Component::SafePointer<MyComp>(this)] {
    if (auto* self = safeThis.getComponent())
        self->setText("done");
});

// SAFE — shorter form
auto safeThis = juce::Component::SafePointer<MyComp>(this);
juce::MessageManager::callAsync([safeThis]() mutable {
    if (safeThis != nullptr)
        safeThis->setText("done");
});

Timer Safety

cpp

struct MyComp : juce::Component, juce::Timer {
    MyComp() { startTimerHz(30); }

    ~MyComp() override {
        stopTimer();  // MANDATORY — timer callback must not fire after destruction
    }

    void timerCallback() override {
        // Safe to access members here — guaranteed on message thread,
        // and stopTimer() in destructor prevents post-destruction calls
        repaint();
    }
};

LookAndFeel Cleanup

cpp

// DANGEROUS — components reference L&F that's already destroyed
struct MyApp {
    CustomLookAndFeel lnf;       // Destroyed SECOND (after mainWindow)
    std::unique_ptr<MainWindow> mainWindow;  // Destroyed FIRST — but its children
                                              // may still reference lnf during teardown
};

// SAFE — clear default L&F before destroying components
struct MyApp {
    ~MyApp() {
        juce::LookAndFeel::setDefaultLookAndFeel(nullptr);  // Clear global L&F
        mainWindow.reset();  // Now safe to destroy components
    }
    CustomLookAndFeel lnf;
    std::unique_ptr<MainWindow> mainWindow;
};

// SAFE — per-component L&F cleanup
struct MyComp : juce::Component {
    ~MyComp() override {
        setLookAndFeel(nullptr);  // Detach before L&F may be destroyed
    }
};

Listener Removal

cpp

struct MyComp : juce::Component, juce::Value::Listener {
    MyComp(juce::Value& v) : value(v) {
        value.addListener(this);
    }

    ~MyComp() override {
        value.removeListener(this);  // MUST remove before destruction
    }

    void valueChanged(juce::Value&) override {
        repaint();
    }

    juce::Value value;
};

JUCE Leak Detector

cpp

class MyComponent : public juce::Component {
public:
    MyComponent();
    ~MyComponent() override;

private:
    // Place at the END of the class — detects leaked instances on shutdown
    JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR(MyComponent)
};

Common Crash Patterns & Fixes

1. Timer fires after component deleted

cpp

// BUG: no stopTimer() in destructor
struct Bad : juce::Component, juce::Timer {
    ~Bad() override { /* timer still running! */ }
};

// FIX:
struct Good : juce::Component, juce::Timer {
    ~Good() override { stopTimer(); }
};

2. callAsync lambda captures dead `this`

cpp

// BUG:
juce::MessageManager::callAsync([this] { repaint(); });

// FIX:
juce::MessageManager::callAsync(
    [safe = juce::Component::SafePointer<MyComp>(this)] {
        if (safe != nullptr) safe->repaint();
    });

3. LookAndFeel destroyed before components

cpp

// BUG: member destruction order destroys L&F while components still reference it
struct App {
    CustomLookAndFeel lnf;
    std::unique_ptr<MainWindow> window;  // window's children still use lnf during destruction
};

// FIX: explicit teardown order
struct App {
    ~App() {
        juce::LookAndFeel::setDefaultLookAndFeel(nullptr);
        window.reset();
    }
    CustomLookAndFeel lnf;
    std::unique_ptr<MainWindow> window;
};

4. Listener not removed before object destroyed

cpp

// BUG: broadcaster calls dead listener
struct Bad : juce::Button::Listener {
    Bad(juce::Button& b) : button(b) { button.addListener(this); }
    ~Bad() { /* forgot removeListener! */ }
    juce::Button& button;
};

// FIX:
struct Good : juce::Button::Listener {
    Good(juce::Button& b) : button(b) { button.addListener(this); }
    ~Good() { button.removeListener(this); }
    juce::Button& button;
};

5. Component deleted during its own callback

cpp

// BUG: deleting yourself inside a button callback
void buttonClicked(juce::Button*) override {
    owner.removeChildComponent(this);
    delete this;  // CRASH — stack still unwinding through JUCE event dispatch
}

// FIX: defer deletion
void buttonClicked(juce::Button*) override {
    juce::MessageManager::callAsync([this, &owner = this->owner] {
        owner.removeAndDeleteChild(this);
    });
}

6. Accessing MessageManager from audio thread

cpp

// BUG: called from processBlock
void processBlock(...) {
    if (clipping)
        component.repaint();  // WRONG — repaint() posts to message thread internally,
                               // but is not safe from audio thread
}

// FIX: use atomic flag + timer or AsyncUpdater
void processBlock(...) {
    if (clipping)
        clipFlag.store(true);  // Lock-free
}

void timerCallback() override {
    if (clipFlag.exchange(false))
        clipIndicator.repaint();
}

Prefer RAII Over Manual Resource Management

Always use RAII classes instead of manual acquire/release patterns. This prevents resource leaks when exceptions occur or early returns are taken.

Smart Pointers Over Raw new/delete

cpp

// BAD — leak if exception or early return between new and delete
auto* comp = new MyComponent();
addAndMakeVisible(comp);
// ... if something throws or returns, comp is leaked

// GOOD — ownership is automatic
auto comp = std::make_unique<MyComponent>();
addAndMakeVisible(comp.get());
ownedComponents.push_back(std::move(comp));

Scoped Locks

cpp

// BAD — manual lock/unlock, easy to forget unlock on early return
mutex.lock();
doWork();
if (error) return;  // BUG: mutex not unlocked!
mutex.unlock();

// GOOD — RAII lock, always released on scope exit
{
    const juce::ScopedLock sl(mutex);
    doWork();
    if (error) return;  // mutex automatically unlocked
}

Tracktion Engine RAII Helpers

cpp

// Prevent expensive playback graph rebuilds during batch operations
{
    te::TransportControl::ReallocationInhibitor inhibitor(transport);
    // ... make many changes to tracks/plugins ...
}  // Single graph rebuild happens here

// Restore playback state after temporary stop
{
    te::TransportControl::ScopedPlaybackRestarter restarter(transport);
    transport.stop(false, false);
    // ... do work that requires transport stopped ...
}  // Playback automatically resumes

// Group undo operations
{
    te::Edit::UndoTransactionInhibitor inhibitor(*edit);
    // ... multiple operations won't be grouped into one undo step ...
}

RAII for Listener Registration

cpp

// Pattern: register in constructor, unregister in destructor
struct ScopedListener {
    ScopedListener(juce::Value& v, juce::Value::Listener* l)
        : value(v), listener(l) { value.addListener(listener); }
    ~ScopedListener() { value.removeListener(listener); }
    juce::Value& value;
    juce::Value::Listener* listener;
};

RAII for Timer Management

cpp

// If you inherit from Timer, always stop in destructor:
~MyComp() override { stopTimer(); }

// Or use a helper that auto-stops:
struct ScopedTimer : juce::Timer {
    std::function<void()> callback;
    ~ScopedTimer() override { stopTimer(); }
    void timerCallback() override { if (callback) callback(); }
};

Thread-Safe Data Patterns

cpp

// Atomic for simple values
std::atomic<float> gain { 1.0f };
std::atomic<bool> shouldStop { false };

// Lock-free FIFO for messages (audio → UI)
juce::AbstractFifo fifo { 512 };
std::array<float, 512> buffer;

// SpinLock for very short critical sections (avoid on audio thread if possible)
juce::SpinLock lock;
{
    const juce::SpinLock::ScopedLockType sl(lock);
    // Very brief work only
}