The Xcode Debugger

To see the contents of a CGRect named b, we could paste the following statement into our app.

	NSLog(@"b == (%g, %g), %g × %g)",

A debugger gives us an easier way to see, and even to change, the value of a variable.

Apple’s documentation

  1. Debugging in an Xcode window. This is where we will set the breakpoints.
  2. Debugging in the gdb window. This is where we will call methods.

Create and remove a breakpoint

Open the HelloAppDelegate.m file of the Hello, World! project in Xcode. To the left of the file are two margins, a wide one on the left and a narrow, somewhat darker one on the right. Click in the wide one, to the left of the line that says application:didFinishLaunchingWithOptions:. You should see a blue (or blue outlined) pentagon like a horizontally stretched home plate. You have just set a breakpoint at the start of the application:didFinishLaunchingWithOptions: method.

To delete the breakpoint, control-click on it and select Delete Breakpoint.

Stop at a breakpoint

Put the breakpoint back in. Press the Run button. When you stop at the breakpoint, the Xcode Console will say (lldb) or (gdb).

To see the Xcode Console,
View → Debug Area → Activate Console

The green arrow (a very truncated home plate) points to the next line or statement to be executed. It should be pointing to the application:didFinishLaunchingWithOptions: line. This line has not yet been executed.

Let’s walk five statements forward through the program. To the upper left of the Console, there is an icon showing an arrow arcing clockwise over a black rectangle. This is the “Step over” button. Press this button five times. You have just executed the statements that assign values to screen, applicartionFrame, bounds, and view. The green arrow should now be pointing to the statement that assigns a value to sef.window.

Display the fields of a structure and the instance variables of an object

To examine the contents of the CGRect bounds, point at the bounds in the CGRect bounds statement. You should get a red dashed rectangle around the bounds, and a long yellow strip saying

▶ CGRect bounds {…} origin=(x=0,y=0) size=(width=320,height=480)
Point at the equilateral triangle to make it point down and travel down the ▶s. You will see the fields and the fields of the fields, listed one per line. Or click on the pair of tiny triangles, pointing up and down, and select Open in Window. Here are the resulting windows (with the bounds changed to b):

Also examine the variable view, which is much more complicated: bounds was merely a structure, while view is (a pointer to) an object. The view contains the four instance variables _tapInfo, _gestureInfo, _gestureRecognizers, and _viewFlags. In addition to being a View, the view is also a UIView, UIResponder, and NSObject.

Print an object in the gdb debugger

Open the Xcode Console window:
View → Debug Area → Activate Console
The prompt in the window shows the name of the debugger. It should be (gdb). If the prompt is (lldb), change it to (gdb) as follows.
Product → Edit Scheme…
Select Run on left, Info on upper right.
Debugger: GDB
Then run the project again by pressing the Run button. The debugger prompt in the Console should now be (gdb).

view is a pointer to a UIView object.

(gdb) print view
$1 = (View *) 0x6a59840

The $1 = means “this is your first result”. To see the first result again, say

(gdb) print $1

Print the UIView object itself.

(gdb) print *view

Call a method of an object and print the return value

Let’s call the getter method for the bounds property of class UIView. The name of this method is bounds.

(gdb) print (CGRect)[view bounds]
$2 = {origin = {x = 0, y = 0}, size = {width = 320, height = 460}}
(gdb) print ((CGRect)[view bounds]).origin
$3 = {x = 0, y = 0}
(gdb) print ((CGRect)[view bounds]).origin.x
$4 = 0
(gdb) po [view backgroundColor]
UIDeviceRGBColorSpace 1 1 0 1

Create a macro

Better yet, create a gdb macro with one argument. We will name the macro rc, and the argument of the macro is named $arg0. The > is the continuation prompt that gdb displays. Do not type the >.

(gdb) define bo
>print (CGRect)[$arg0 bounds]
(gdb) show user bo
(gdb) show user
(gdb) document bo
>Print the bounds property of the UIView specified as the argument.
(gdb) help bo
Print the bounds property of the UIView specified as the argument.

Execute the macro and pass one argument (the object view) to it.

(gdb) bo view
$5 = {origin = {x = 0, y = 0}, size = {width = 320, height = 460}}

Call a method without attempting to print its return value

Let’s call the setter method for the backgroundColor property of class UIView. The name of this method is . The method has no return value, i.e., it returns void. Say call instead of print.

(gdb) call (void)[view setBackgroundColor: (UIColor *)[UIColor greenColor]]

Then press the “Continue program execution” button to the left of the “Step over” button. The view in the simulator will be green.