Swift 4中AXValueGetValue返回nil但调试器显示有值及指针取值问题

Hey there, let's work through your AXValueGetValue headaches in Swift 4—super common pitfall with Accessibility APIs, so let's break it down.

First Issue: AXValueGetValue "returns nil" but debugger shows the value exists #

Wait, quick clarification: In Swift 4, AXValueGetValue actually returns a Bool (true for success, false for failure), not an optional. My guess is you're seeing the target variable you're trying to populate stay at its default value (like CGRect.zero) while the debugger shows the AXValue itself holds valid data.

The #1 cause here is type mismatch. You're passing an AXValueType (like .cgRect) that doesn't match what's actually stored in the AXValue. For example, if the AXValue contains a CGPoint but you try to fetch it as a CGRect, the function will return false and leave your target variable untouched—even though the debugger can see the raw data inside the AXValue.

Fix this first by verifying the actual type of the AXValue:

let valueType = AXValueGetType(yourAXValue)
print("Actual AXValue type code: \(valueType.rawValue)")
print("Expected CGRect type code: \(AXValueType.cgRect.rawValue)")

If these don't match, you're using the wrong AXValueType for fetching.

Second Issue: Extracting values from a pointer that looks like a tuple #

If the debugger shows the pointer points to a tuple instead of a standard CGRect, that means the AXValue is storing a tuple type (not the system CGRect struct). This usually happens if the AXValue was created with a custom tuple instead of using the built-in .cgRect type.

Here's how to handle it:

1. Define a matching tuple type: First, figure out the exact tuple structure from the debugger (e.g., (CGFloat, CGFloat, CGFloat, CGFloat) for x/y/width/height).
2. Fetch directly into the tuple: Use that tuple type as your target variable when calling AXValueGetValue:

// Match the tuple structure you see in the debugger
typealias RectTuple = (CGFloat, CGFloat, CGFloat, CGFloat)

var targetTuple: RectTuple = (0, 0, 0, 0)
let success = AXValueGetValue(yourAXValue, yourCustomAXValueType, &targetTuple)

if success {
    // Convert the tuple to a CGRect if needed
    let rect = CGRect(x: targetTuple.0, y: targetTuple.1, width: targetTuple.2, height: targetTuple.3)
    print("Extracted rect: \(rect)")
}

Note: You'll need to use the correct AXValueType that matches the custom tuple type stored in the AXValue (this might be a custom code if it's not a system-defined type).

If you're working with raw pointers directly (not recommended unless necessary), bind the pointer to your tuple type first:

let tupleSize = MemoryLayout<RectTuple>.stride
let ptr = UnsafeMutableRawPointer.allocate(byteCount: tupleSize, alignment: MemoryLayout<RectTuple>.alignment)
defer { ptr.deallocate() } // Always clean up allocated memory

let success = AXValueGetValue(yourAXValue, yourCustomAXValueType, ptr)
if success {
    let fetchedTuple = ptr.bindMemory(to: RectTuple.self, capacity: 1).pointee
    let rect = CGRect(x: fetchedTuple.0, y: fetchedTuple.1, width: fetchedTuple.2, height: fetchedTuple.3)
    print(rect)
}

Quick Pro Tip #

Stick to using system-defined AXValueTypes (like .cgRect, .cgPoint) whenever possible—they're designed to work seamlessly with AXValueGetValue and avoid these tuple/pointer mismatches.

内容的提问来源于stack exchange，提问作者Doug