How to reverse engineer _printHierarchy
I wanted to traverse the view controllers hierarchy the same way as the private method that is available on UIKit -UIViewController._printHierarchy.
You can run it from lldb:
exp -l objc -o -- [UIApplication.sharedApplication.keyWindow.rootViewController _printHierarchy]
or using a class method
exp -l objc -o -- [UIViewController _printHierarchy]
and it should return something like:
<UITabBarController 0x12c01d200>, state: appeared, view: <UILayoutContainerView 0x129e07b40>
| <UINavigationController 0x12a808200>, state: appeared, view: <UILayoutContainerView 0x129b10880>
| | <App.ViewController 0x129b04620>, state: disappeared, view: (view not loaded)
| | <App.ViewController 0x129b049c0>, state: disappeared, view: <UIView 0x129b27620> not in the window
| | <App.ViewController 0x129f06550>, state: disappeared, view: <UIView 0x129f0b7c0> not in the window
| | <App.ViewController 0x129b24540>, state: appeared, view: <UIView 0x129b2b750>
| <UINavigationController 0x12b80a000>, state: disappeared, view: <UILayoutContainerView 0x12d0052c0> not in the window
| | <App.ViewController 0x129c07220>, state: disappeared, view: (view not loaded)
| | <App.ViewController 0x129c075c0>, state: disappeared, view: <UIView 0x129e0c730> not in the window
| | <App.ViewController 0x12d204900>, state: disappeared, view: <UIView 0x129e11550> not in the window
| | <App.ViewController 0x12d204ca0>, state: disappeared, view: <UIView 0x129b16200> not in the window
+ <App.ViewController 0x12d2065e0>, state: appeared, view: <UIView 0x129b26a90>, presented with: <_UIPageSheetPresentationController 0x12d205aa0>
| + <App.ViewController 0x129b4b130>, state: appeared, view: <UIView 0x129b4b4d0>, presented with: <_UIPageSheetPresentationController 0x129b4b7f0>
| | + <App.ViewController 0x129e2ada0>, state: appeared, view: <UIView 0x129e25950>, presented with: <_UIPageSheetPresentationController 0x129e27ec0>
I have found a few existing solutions that for example use the UIResponder chain to traverse the hierarchy like this one. But the problem with UIResponder.next is that it won't traverse all of the controllers if there is a branch of more than one UIViewController stack like you might have with e.g. UITabViewController and modally presented view controllers. UIResponder.next traverses bottom-up the view hierarchy but only from the specific leaf.
I thought, that why not reverse engineer the original _printHierarchy method, to check all of the quirks of its internal implementation and understand how Apple engineers traverse the view controllers hierarchy top-down. It turned out to be quite straightforward after using Hopper Disassembler app. The _printHierachy core method is actually implemented in _appendDescription. On UIKit 15.0 with disassembled UIKitCore using x86_64 which is a bit more readable than ARM64.
function _appendDescription {
stack[-200] = rcx;
r14 = rdx;
r12 = [rdi retain];
rbx = [rsi retain];
if ([rbx length] != 0x0) {
[rbx appendString:@"\n"];
}
stack[-208] = rbx;
rax = [r12 _parentViewController];
rax = [rax retain];
stack[-188] = r14 & 0xff;
stack[-216] = r12;
if (rax != 0x0) {
[rax release];
r15 = stack[-208];
if (stack[-200] != 0x0) {
r12 = sign_extend_64(stack[-200]);
rbx = 0x1;
do {
[r15 appendString:@" | "];
rbx = rbx + 0x1;
} while (rbx <= r12);
}
r13 = stack[-216];
rax = -[UIViewController _descriptionForPrintingHierarchyIncludingInsets:](r13, stack[-188]);
rax = [rax retain];
rbx = rax;
[r15 appendString:rax];
}
else {
if ([r12 _isRootViewController] != 0x0) {
r15 = stack[-208];
if (stack[-200] != 0x0) {
r12 = sign_extend_64(stack[-200]);
rbx = 0x1;
do {
[r15 appendString:@" | "];
rbx = rbx + 0x1;
} while (rbx <= r12);
}
r13 = stack[-216];
rax = -[UIViewController _descriptionForPrintingHierarchyIncludingInsets:](r13, stack[-188]);
rax = [rax retain];
rbx = rax;
[r15 appendString:rax];
}
else {
r12 = stack[-208];
if (stack[-200] >= 0x2) {
rbx = sign_extend_64(stack[-200]) - 0x1;
do {
[r12 appendString:@" | "];
rbx = rbx - 0x1;
} while (rbx != 0x0);
}
[r12 appendString:@" + "];
r15 = [-[UIViewController _descriptionForPrintingHierarchyIncludingInsets:](stack[-216], stack[-188]) retain];
rax = [stack[-216] presentingViewController];
rax = [rax retain];
stack[-248] = rax;
rax = [rax _presentationController];
rax = [rax retain];
stack[-240] = rax;
rax = [rax _descriptionForPrintingViewControllerHierarchy];
rax = [rax retain];
stack[-224] = rax;
rax = [NSMutableString stringWithFormat:@"%@, presented with: %@", r15, rax];
rax = [rax retain];
r13 = stack[-216];
[r12 appendString:rax];
rbx = r15;
[rax release];
[stack[-224] release];
[stack[-240] release];
[stack[-248] release];
}
}
[rbx release];
*(int128_t *)(&stack[-312] + 0x30) = intrinsic_movaps(*(int128_t *)(&stack[-312] + 0x30), 0x0);
*(int128_t *)(&stack[-312] + 0x20) = intrinsic_movaps(*(int128_t *)(&stack[-312] + 0x20), 0x0);
*(int128_t *)(&stack[-312] + 0x10) = intrinsic_movaps(*(int128_t *)(&stack[-312] + 0x10), 0x0);
*(int128_t *)&stack[-312] = intrinsic_movaps(*(int128_t *)&stack[-312], 0x0);
rax = [r13 childViewControllers];
rax = [rax retain];
rbx = rax;
rcx = &stack[-184];
rdx = &stack[-312];
rax = [rax countByEnumeratingWithState:rdx objects:rcx count:0x10];
if (rax != 0x0) {
r13 = rax;
r12 = **(&stack[-312] + 0x10);
r15 = stack[-200] + 0x1;
do {
r14 = 0x0;
do {
if (*stack[-296] != r12) {
objc_enumerationMutation(rbx);
}
_appendDescription(*(stack[-304] + r14 * 0x8), stack[-208], stack[-188], r15, 0x10);
r14 = r14 + 0x1;
} while (r13 != r14);
rdx = &stack[-312];
rcx = &stack[-184];
rax = [rbx countByEnumeratingWithState:rdx objects:rcx count:0x10];
r13 = rax;
} while (rax != 0x0);
}
[rbx release];
r14 = stack[-216];
rsi = @selector(childModalViewController);
rax = (*_objc_msgSend)(r14, rsi);
rax = [rax retain];
[rax release];
rbx = stack[-208];
if (rax != 0x0) {
rax = [r14 childModalViewController];
rax = [rax retain];
rcx = stack[-200] + 0x1;
rsi = rbx;
rdx = stack[-188];
_appendDescription(rax, rsi, rdx, rcx);
[rax release];
}
stack[-56] = **___stack_chk_guard;
[rbx release];
[r14 release];
rax = *___stack_chk_guard;
rax = *rax;
if (rax != stack[-56]) {
rax = __stack_chk_fail();
}
return rax;
}
There are few if statements that split the code, but they are mostly used to properly append to the print indentation, like " | " and " + ". Overall if you forget about the lines generated by the assembly code (e.g. registry assignments) and ARC (e.g. [rax retain], [r14 release]) the code is quite short and simple. Inside if statements blocks you can find the while/repeat loops that add the indentation required depending on the view hierarchy tree.
I transferred the code into Swift and this is what I got:
extension UIViewController {
public func printHierarchy() {
var prints: [String] = []
self.appendDescription(output: &prints, deep: 0)
let printString = prints.reduce("") { partialResult, elem in
return partialResult.appending(elem)
}
print(printString)
}
private func appendDescription(output: inout [String], deep: Int) {
if !output.isEmpty {
output.append("\n")
}
if self.parent != nil {
var i = 0
while i < deep {
output.append(" | ")
i += 1
}
output.append(self.description)
} else {
if self.isRootViewController {
var i = 1
while i < deep {
output.append(" | ")
output.append(self.description)
i += 1
}
output.append(self.description)
} else {
if deep >= 2 {
var i = deep - 1
repeat {
output.append(" | ")
i -= 1
} while i != 0
}
output.append(" + ")
output.append(self.description)
output.append(", presented with: \(self.presentationController?.description ?? "N/A")")
}
}
if !self.children.isEmpty {
var i = 0
while i < self.children.count {
self.children[i].appendDescription(output: &output, deep: deep + 1)
i += 1
}
}
if let childModalVC = self.childModalViewController {
childModalVC.appendDescription(output: &output, deep: deep + 1)
}
}
}
extension UIViewController {
var isRootViewController: Bool {
UIApplication.shared.windows.compactMap(\.rootViewController).contains(self)
}
var childModalViewController: UIViewController? {
if self.presentedViewController?.presentingViewController == self {
return self.presentedViewController
} else {
return nil
}
}
}
I intentionally left the while/repeat loops to make the comparison with disassembled method easier. I had to reimplement the isRootViewController and childModalViewController that is used in the original implementation and it is not avaiable in the public methods of UIKit API. appendDescription goes recursively trought child and child modal view controllers. What I learned from this, is that Apple engenieers smartly do not check for the specific UIViewController subclasses to verify if there are sub view controllers like you can do with UINavigationController.viewControllers. They just check if there are child/modal view controllers, and recursively visit all of them. The ouput of my reimplemented Swift printHierarchy looks the same beside slight differences of the UIViewControllers descriptions, as again there is custom implementation used internally like _descriptionForPrintingHierarchyIncludingInsets.
Reimplemented
<UITabBarController: 0x12c01d200>
| <UINavigationController: 0x12a808200>
| | <App.ViewController: 0x129b04620>
| | <App.ViewController: 0x129b049c0>
| | <App.ViewController: 0x129f06550>
| | <App.ViewController: 0x129b24540>
| <UINavigationController: 0x12b80a000>
| | <App.ViewController: 0x129c07220>
| | <App.ViewController: 0x129c075c0>
| | <App.ViewController: 0x12d204900>
| | <App.ViewController: 0x12d204ca0>
+ <App.ViewController: 0x12d2065e0>, presented with: <_UIPageSheetPresentationController: 0x12d205aa0>
| + <App.ViewController: 0x129b4b130>, presented with: <_UIPageSheetPresentationController: 0x129b4b7f0>
| | + <App.ViewController: 0x129e2ada0>, presented with: <_UIPageSheetPresentationController: 0x129e27ec0>
Original
<UITabBarController 0x12c01d200>, state: appeared, view: <UILayoutContainerView 0x129e07b40>
| <UINavigationController 0x12a808200>, state: appeared, view: <UILayoutContainerView 0x129b10880>
| | <App.ViewController 0x129b04620>, state: disappeared, view: (view not loaded)
| | <App.ViewController 0x129b049c0>, state: disappeared, view: <UIView 0x129b27620> not in the window
| | <App.ViewController 0x129f06550>, state: disappeared, view: <UIView 0x129f0b7c0> not in the window
| | <App.ViewController 0x129b24540>, state: appeared, view: <UIView 0x129b2b750>
| <UINavigationController 0x12b80a000>, state: disappeared, view: <UILayoutContainerView 0x12d0052c0> not in the window
| | <App.ViewController 0x129c07220>, state: disappeared, view: (view not loaded)
| | <App.ViewController 0x129c075c0>, state: disappeared, view: <UIView 0x129e0c730> not in the window
| | <App.ViewController 0x12d204900>, state: disappeared, view: <UIView 0x129e11550> not in the window
| | <App.ViewController 0x12d204ca0>, state: disappeared, view: <UIView 0x129b16200> not in the window
+ <App.ViewController 0x12d2065e0>, state: appeared, view: <UIView 0x129b26a90>, presented with: <_UIPageSheetPresentationController 0x12d205aa0>
| + <App.ViewController 0x129b4b130>, state: appeared, view: <UIView 0x129b4b4d0>, presented with: <_UIPageSheetPresentationController 0x129b4b7f0>
| | + <App.ViewController 0x129e2ada0>, state: appeared, view: <UIView 0x129e25950>, presented with: <_UIPageSheetPresentationController 0x129e27ec0>