屏幕兼容的问题
为了让我们的应用在不容尺寸的屏幕下都能 “正常”的表示，我们尽量不要把数据写死。
大多数可视元素都是一个矩形区域，当然这个矩形区域有坐标的，我们有了这个区域坐标就能确定可视元素的现实位置了。
但是iphone5 和以前的屏幕不一样了，在以前的设备中，我们可以添加一个 xx.@2x.png 来适应retina屏幕，但是iphoen5咋办呢？
ios 引入了 Auto Layout 的东东，这个要和UIViewAutoresizing 区分下。
看下面代码
- (void)viewDidLoad
{ [super viewDidLoad]; UIView *aView = [[UIView alloc] init]; aView.backgroundColor = [UIColor redColor];//为了不和autosizing冲突，我们设置No
[aView setTranslatesAutoresizingMaskIntoConstraints:NO];
[self.view addSubview:aView]; UIView *bView = [[UIView alloc] init]; bView.backgroundColor = [UIColor blueColor]; [bView setTranslatesAutoresizingMaskIntoConstraints:NO]; [self.view addSubview:bView]; NSDictionary *views = NSDictionaryOfVariableBindings(aView, bView);//NSDictionaryOfVariableBindings 宏 其实 NSDictionaryOfVariableBindings(v1, v2, v3) 等效于 [NSDictionary dictionaryWithObjectsAndKeys:v1, @"v1", v2, @"v2", v3, @"v3", nil];
[self.view addConstraints: [NSLayoutConstraint constraintsWithVisualFormat:@"H:|-(>=50)-[aView(100)]" options:0 metrics:nil views:views]]; [self.view addConstraints: [NSLayoutConstraint constraintsWithVisualFormat:@"V:|-(>=100)-[aView(50)]" options:0 metrics:nil views:views]]; [self.view addConstraints: [NSLayoutConstraint constraintsWithVisualFormat:@"H:[bView(==aView)]" options:0 metrics:nil views:views]]; [self.view addConstraints: [NSLayoutConstraint constraintsWithVisualFormat:@"V:[bView(==aView)]" options:0 metrics:nil views:views]]; [self.view addConstraint: [NSLayoutConstraint constraintWithItem:bView attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:aView attribute:NSLayoutAttributeRight multiplier:1 constant:10]]; //添加一个限制 等效于 bView.frame.origin.x = (aView.frame.origin.x +aView.frame.size.width) * 1 + 10,好像是这样的！个人觉得！它是一种依赖关系，bView依赖aView，这样就算aView变了，bView也会跟着变换。
[self.view addConstraint: [NSLayoutConstraint constraintWithItem:bView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:aView attribute:NSLayoutAttributeTop multiplier:1 constant:0]]; [aView release]; [bView release];}
constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:
Create a constraint of the form "view1.attr1 <relation> view2.attr2 * multiplier + constant".
属性
typedef NS_ENUM(NSInteger, NSLayoutAttribute) {
NSLayoutAttributeLeft = 1, NSLayoutAttributeRight, NSLayoutAttributeTop, NSLayoutAttributeBottom, NSLayoutAttributeLeading, NSLayoutAttributeTrailing, NSLayoutAttributeWidth, NSLayoutAttributeHeight, NSLayoutAttributeCenterX, NSLayoutAttributeCenterY, NSLayoutAttributeBaseline, NSLayoutAttributeNotAnAttribute = 0};关系
typedef NS_ENUM(NSInteger, NSLayoutRelation) {
NSLayoutRelationLessThanOrEqual = -1, NSLayoutRelationEqual = 0, NSLayoutRelationGreaterThanOrEqual = 1,};最后的结果就是 “view1.attr1 < >= 或者 == 或者 <= > view2.attr2 * multiplier + constant”