Compiled Error in Xcode for iphone game and other questions

Question

Dear all,
Hi, I am a newbie of xcode and objective-c and I have a few questions regarding to the code sample of a game attached below. It is written in objective C, Xcode for iphone4 simulator. It is part of the code of 'ball bounce against brick" game. Instead of creating the image by IB, the code supposes to create (programmatically) 5 X 4 bricks using 4 different kinds of bricks pictures (bricktype1.png...). I have the bricks defined in .h file properly and method written in .m.
My questions are for the following code:
- (void)initializeBricks
brickTypes[0] = @"bricktype1.png";
brickTypes[1] = @"bricktype2.png";
brickTypes[2] = @"bricktype3.png";
brickTypes[3] = @"bricktype4.png";
int count = 0;`
for (int y = 0; y < BRICKS_HEIGHT; y++)
for (int x = 0; x < BRICKS_WIDTH; x++)
- Line1 UIImage *image = [ImageCache loadImage:brickTypes[count++ % 4]];
- Line2 bricks[x][y] = [[[UIImageView alloc] initWithImage:image] autorelease];
- Line3 CGRect newFrame = bricks[x][y].frame;
- Line4 newFrame.origin = CGPointMake(x * 64, (y * 40) + 50);
- Line5 bricks[x][y].frame = newFrame;
- Line6 [self.view addSubview:bricks[x][y]]
1) When it is compiled, error "ImageCache undeclared" in Line 1. But I have already added the png to the project. What is the problem and how to fix it? (If possible, please suggest code and explain what it does and where to put it.)
2) How does the following in Line 1 work? Does it assign the element (name of .png) of brickType to image?
brickTypes[count ++ % 4]
For instance, returns one of the file name bricktype1.png to the image object? If true, what is the max value of "count", ends at 5? (as X increments 5 times for each Y). But then "count" will exceed the max 'index value' of brickTypes which is 3!
3) In Line2, does the image object which is being allocated has a name and linked with the .png already at this line *before* it is assigned to brick[x][y]?
4) What do Line3 and Line5 do? Why newFrame on left in line3 but appears on right in Line5?
5) What does Line 4 do?
Thanks
North

Answer 1

Hi North -
macbie wrote:
1) When it is compiled, error "ImageCache undeclared" in Line 1. ...
UIImage *image = [ImageCache loadImage:brickTypes[count++ % 4]]; // Line 1
The compiler is telling you it doesn't know what ImageCache refers to. Is ImageCache the name of a custom class? In that case you may have omitted #import "ImageCache.h". Else if ImageCache refers to an instance of some class, where is that declaration made? I can't tell you how to code the missing piece(s) because I can't guess the answers to these questions.
Btw, if the png file images were already the correct size, it looks like you could substitute this for Line 1:
UIImage *image = [UIImage imageNamed:brickTypes[count++ % 4]]; // Line 1
2) How does the following in Line 1 work? Does it assign the element (name of .png) of brickType to image?
brickTypes[count ++ % 4]
Though you don't show the declaration of brickTypes, it appears to be a "C" array of NSString object pointers. Thus brickTypes[0] is the first string, and brickTypes[3] is the last string.
The expression (count++ % 4) does two things. Firstly, the trailing ++ operator means the variable 'count' will be incremented as soon as the current expression is evaluated. Thus 'count' is zero (its initial value) the first time through the inner loop, its value is one the second time, and two the third time. The following two code blocks do exactly the same thing::
int index = 0;
NSString *filename = brickTypes[index++];
int index = 0;
NSString *filename = brickTypes[index];
index = index + 1;
The percent sign is the "modulus operator" so x%4 means "x modulo 4", which evaluates to the remainder after x is divided by 4. If x starts at 0, and is incremented once everytime through a loop, we'll get the following sequence of values for x%4: 0, 1, 2, 3, 0, 1, 2, 3, 0, 1, ...
So repeated evaluation of (brickTypes[count++ % 4]) produces the sequence: @"bricktype1.png", @"bricktype2.png", @"bricktype3.png", @"bricktype4.png", @"bricktype1.png", @"bricktype2.png", @"bricktype3.png", @"bricktype4.png", @"bricktype1.png", @"bricktype2.png", ...
3) In Line2, does the image object which is being allocated has a name and linked with the .png already at this line *before* it is assigned to brick[x][y]?
Line 2 allocs an object of type UIImageView and specifies the data at 'image' as the picture to be displayed by the new object. Since we immediately assign the address of the new UIImageView object to an element of the 'bricks' array, that address isn't stored in any named variable.
The new UIImageView object is not associated with the name of the png file from which its picture originated. In fact the UIImage object which inited the UIImageView object is also not associated with that png filename. In other words, once a UIImage object is initialized from the contents of an image file, it's not possible to obtain the name of that file from the UIImage object. Note when you add a png media object to a UIImageView object in IB, the filename of the png resource will be retained and used to identify the image view object. But AFAIK, unless you explicitly save it somewhere in your code, that filename will not be available at run time.
4) What do Line3 and Line5 do? Why newFrame on left in line3 but appears on right in Line5?
5) What does Line 4 do?
In Line 2 we've set the current element of 'bricks' to the address of a new UIImageView object which will display one of the 4 brick types. By default, the frame of a UIImageView object is set to the size of the image which initialized it. So after Line 2, we know that frame.size for the current array element is correct (assuming the image size of the original png file was what we want to display, or assuming that the purpose of [ImageCache loadImage:...] is to adjust the png size).
Then in Line 3, we set the rectangle named newFrame to the frame of the current array element, i.e. to the frame of the UIImageView object whose address is stored in the current array element. So now we have a rectangle whose size (width, height) is correct for the image to be displayed. But where will this rectangle be placed on the superview? The placement of this rectangle is determined by its origin.
Line 4 computes the origin we want. Now we have a rectangle with both the correct size and the correct origin.
Line 5 sets the frame of the new UIImageView object to the rectangle we constructed in Lines 3 and 4. When that object is then added to the superview in Line 6, it will display an image of the correct size at the correct position.
- Ray