To see weak references in action, let’s add another array to the mix. What if we wanted an array of all assets – even ones that have not been assigned to a particular employee? We could add the assets to an array as they are created. Add a few lines of code to main.m:
#import <Foundation/Foundation.h>
#import "Employee.h"
#import "Asset.h"
int main(int argc, const char * argv[])
{
@autoreleasepool {
// Create an array of Employee objects
NSMutableArray *employees = [[NSMutableArray alloc] init];
for (int i = 0; i < 10; i++) {
// Create an instance of Employee
Employee *person = [[Employee alloc] init];
// Give the instance variables interesting values
[person setWeightInKilos:90 + i];
[person setHeightInMeters:1.8 - i/10.0];
[person setEmployeeID:i];
// Put the employee in the employees array
[employees addObject:person];
}
NSMutableArray *allAssets = [[NSMutableArray alloc] init];
// Create 10 assets
for (int i = 0; i < 10; i++) {
// Create an asset
Asset *asset = [[Asset alloc] init];
// Give it an interesting label
NSString *currentLabel = [NSString stringWithFormat:@"Laptop %d", i];
[asset setLabel:currentLabel];
[asset setResaleValue:i * 17];
// Get a random number between 0 and 9 inclusive
NSUInteger randomIndex = random() % [employees count];
// Find that employee
Employee *randomEmployee = [employees objectAtIndex:randomIndex];
// Assign the asset to the employee
[randomEmployee addAssetsObject:asset];
[allAssets addObject:asset];
}
NSLog(@"Employees: %@", employees);
NSLog(@"Giving up ownership of one employee");
[employees removeObjectAtIndex:5];
NSLog(@"allAssets: %@", allAssets);
NSLog(@"Giving up ownership of arrays");
allAssets = nil;
employees = nil;
}
sleep(100);
return 0;
}
Before you build and run your program, think about what you expect your output to look like. You’ll see the contents of the allAssets array – after Employee #5 has been deallocated. What will the status of Employee #5’s assets be at this point? These assets lose one owner (Employee #5), but they are still owned by allAssets, so they won’t be deallocated.
What about holder for the assets previously owned by Employee #5? When the object that a weak reference points to is deallocated, the pointer variable is zeroed, or set to nil. So Employee #5’s assets will not be deallocated, and their holder variables will be automatically set to nil.
Now build and run the program and check your output:
Employees: ( "<Employee 0: $0 in assets>", ... "<Employee 9: $136 in assets>" ) Giving up ownership of one employee deallocating <Employee 5: $136 in assets> allAssets: ( "<Laptop 0: $0, assigned to <Employee 3: $68 in assets>>", "<Laptop 1: $17, assigned to <Employee 6: $119 in assets>>", "<Laptop 2: $34, assigned to <Employee 7: $34 in assets>>", "<Laptop 3: $51 unassigned>", "<Laptop 4: $68, assigned to <Employee 3: $68 in assets>>", "<Laptop 5: $85 unassigned>", "<Laptop 6: $102, assigned to <Employee 6: $119 in assets>>", "<Laptop 7: $119, assigned to <Employee 2: $119 in assets>>", "<Laptop 8: $136, assigned to <Employee 9: $136 in assets>>", "<Laptop 9: $153, assigned to <Employee 1: $153 in assets>>" ) Giving up ownership of arrays deallocing <Laptop 3: $51 unassigned> ... deallocing <Laptop 8: $136 unassigned>
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