iOS - 将 arrayIndex 限制在 0 到 98 之间

Question

在我的项目中，我有一组图像。图像显示在 imageView 上，我可以通过滑动在图像之间移动。但是当我归零并向左滑动时(又名前一张照片，它将 arrayIndex 减少一个)我得到一个错误(SIGABRT)。 这是我负责在图像之间移动的代码:

-(IBAction)nextPhoto:(id)sender{
arrayIndex++;
NSLog(@"rigth! at index %lu", arrayIndex);
if (arrayIndex <= 98){
displayImage.image = [UIImage imageNamed:[imageArray objectAtIndex:arrayIndex]];
} else {
    displayImage.image = [UIImage imageNamed:[imageArray objectAtIndex:0]];
}

}
-(IBAction)previousPhoto:(id)sender{
 arrayIndex--;
 NSLog(@"Left! at index %lu", arrayIndex);
 if (arrayIndex >= 0){
 displayImage.image = [UIImage imageNamed:[imageArray  objectAtIndex:arrayIndex]];
} else {
   displayImage.image = [UIImage imageNamed:[imageArray objectAtIndex:98]];
}

}

Answer 1

您没有向我们展示 arrayIndex 的声明，但我猜它是 unsigned long，因为您用 %lu 打印了它。

考虑当 arrayIndex == 0 时会发生什么:

// arrayIndex == 0
arrayIndex--;
// Now arrayIndex == ULONG_MAX.
// The following condition is always true for unsigned longs.
if (arrayIndex >= 0){
    // This branch is always taken.
    // The following objectAtIndex: fails for ULONG_MAX.
    displayImage.image = [UIImage imageNamed:[imageArray  objectAtIndex:arrayIndex]];

你需要检查 arrayIndex == 0 before 你递减它，像这样:

-(IBAction)nextPhoto:(id)sender {
    ++arrayIndex;
    if (arrayIndex >= imageArray.count) {
        arrayIndex = 0;
    }
    displayImage.image = [UIImage imageNamed:imageArray[arrayIndex]];
}

-(IBAction)previousPhoto:(id)sender {
    if (arrayIndex == 0) {
        arrayIndex = imageArray.count - 1;
    } else {
        --arrayIndex;
    }
    displayImage.image = [UIImage imageNamed:imageArray[arrayIndex]];
}