iOS AutoLayout使用心得
AutoLayout确实非常方便，比起刀耕火种的手写frame要强大很多。体会一下下面的经典例子，就知道了。
旋转后自动布局
上面的例子可以用下面的代码实现：
[super viewDidLoad];
self.view.backgroundColor=[UIColor whiteColor];
UIView *a = [[UIView alloc] init];
a.backgroundColor = [UIColor redColor];
a.layer.cornerRadius=4;
[a setTranslatesAutoresizingMaskIntoConstraints:NO];
[self.view addSubview:a];
UIView *b = [[UIView alloc] init];
b.backgroundColor = [UIColor greenColor];
b.layer.cornerRadius=4;
[b setTranslatesAutoresizingMaskIntoConstraints:NO];
[self.view addSubview:b];
UIView *c = [[UIView alloc] init];
c.backgroundColor = [UIColor blueColor];
c.layer.cornerRadius=4;
[c setTranslatesAutoresizingMaskIntoConstraints:NO];
[self.view addSubview:c];
NSDictionary *views=NSDictionaryOfVariableBindings(a,b,c);
NSDictionary *metrics=@{@"padding":@10};
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-padding-[a(==b)]-padding-[b]-padding-|" options:NSLayoutFormatAlignAllTop metrics:metrics views:views]];
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-padding-[a(==b)]-padding-[c(==b)]-padding-|" options:0 metrics:metrics views:views]];
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-padding-[c]-padding-|" options:0 metrics:metrics views:views]];
自动计算兄弟元素位置
上面的例子如果用frame是比较麻烦的。因为除了更新自己的height以外，还要更新其他兄弟节点的origin.y。如果用autolayout就很方便：
self.view.backgroundColor=[UIColor whiteColor];
//注意self.view并没有self.view.translatesAutoresizingMaskIntoConstraints=NO;
id lastView=nil;
for (int i=1; i<=10; i++) {
UIButton *v=[UIButton new];
[v setTitle:[NSString stringWithFormat:@"[+] button-%d",i] forState:UIControlStateNormal];
[v setTitle:[NSString stringWithFormat:@"[-] button-%d",i] forState:UIControlStateSelected];
v.backgroundColor=[UIColor colorWithWhite:i*25.0/255 alpha:1];
[self.view addSubview:v];
v.translatesAutoresizingMaskIntoConstraints=NO;
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:v attribute:NSLayoutAttributeTrailing relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTrailing multiplier:1 constant:-10]];
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:v attribute:NSLayoutAttributeLeading relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeading multiplier:1 constant:10]];
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:v attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:(lastView?:self.topLayoutGuide) attribute:NSLayoutAttributeBottom multiplier:1 constant:0]];
NSLayoutConstraint *height=[NSLayoutConstraint constraintWithItem:v attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:0 constant:43];
height.identifier=@"myheight";//通过设置identifier，来找到这个NSLayoutConstraint
[v addConstraint:height];
lastView=v;
//"UIControl+Blocks.h" 或者addTarget
[v addEventHandler:^(id sender) {
UIButton *btn=sender;
NSLog(@"%d clicked\n%@",i,btn.constraints);
for (NSLayoutConstraint *c in btn.constraints) {
if ([c.identifier isEqualToString:@"myheight"]) {
c.constant= (c.constant==100?43:100);
btn.selected=c.constant==100;
}
}
[UIView animateWithDuration:.25 animations:^{
[self.view layoutIfNeeded];
}];
} forControlEvents:UIControlEventTouchUpInside];
}Autolayout和UIScrollView
根据官方文档，UIScrollView使用AutoLayout有两种方法。考虑下面的例子：
#方法1,纯autolayout，注意边界。
self.view.backgroundColor=[UIColor whiteColor];
UIScrollView *sv=[UIScrollView new];
sv.translatesAutoresizingMaskIntoConstraints=NO;
[self.view addSubview:sv];
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[sv]|" options:0 metrics: 0 views:NSDictionaryOfVariableBindings(sv)]];
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[sv]|" options:0 metrics: 0 views:NSDictionaryOfVariableBindings(sv)]];
UIView *lastView=nil;
int length=100;
for (int i=0; i<length; i++) {
UIButton *btn=[UIButton new];
btn.backgroundColor=[UIColor colorWithWhite:i*0.01 alpha:1];
[btn setTitle:[NSString stringWithFormat:@"button-%d",i] forState:UIControlStateNormal];
btn.translatesAutoresizingMaskIntoConstraints=NO;
[sv addSubview:btn];
[sv addConstraint:[NSLayoutConstraint constraintWithItem:btn attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:0 constant:i*4+100]];
[sv addConstraint:[NSLayoutConstraint constraintWithItem:btn attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:sv attribute:NSLayoutAttributeCenterX multiplier:1 constant:0]];
if (i==0) {
[sv addConstraint:[NSLayoutConstraint constraintWithItem:btn attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:sv attribute:NSLayoutAttributeTop multiplier:1 constant:10]];
}else{
[sv addConstraint:[NSLayoutConstraint constraintWithItem:btn attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:lastView attribute:NSLayoutAttributeBottom multiplier:1 constant:1]];
}
if (i==length-1) {//关键点。非常重要!设置contentsize的height,最大的一个设置width，否则会出错。
[sv addConstraint:[NSLayoutConstraint constraintWithItem:btn attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:sv attribute:NSLayoutAttributeBottom multiplier:1 constant:-10]];
[sv addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[btn]|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(btn)]];
}
lastView=btn;#方法2，使用中间view,利用systemLayoutSizeFittingSize设置contentsize
self.automaticallyAdjustsScrollViewInsets=NO;
self.view.backgroundColor=[UIColor whiteColor];
UIScrollView *sv=[[UIScrollView alloc]initWithFrame:CGRectMake(0, MARGIN_TOP, SCREEN_WIDTH, CONTENT_HEIGHT)];
[self.view addSubview:sv];
UIView *view=[UIView new];
view.translatesAutoresizingMaskIntoConstraints=NO;//都添加到view上面。
[sv addSubview:view];
UIView *lastView=nil;
int length=100;
for (int i=0; i<length; i++) {
UIButton *btn=[UIButton new];
btn.backgroundColor=[UIColor colorWithWhite:i*0.01 alpha:1];
[btn setTitle:[NSString stringWithFormat:@"button-%d",i] forState:UIControlStateNormal];
btn.translatesAutoresizingMaskIntoConstraints=NO;
[view addSubview:btn];
[view addConstraint:[NSLayoutConstraint constraintWithItem:btn attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:0 constant:i*4+100]];
[view addConstraint:[NSLayoutConstraint constraintWithItem:btn attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:view attribute:NSLayoutAttributeCenterX multiplier:1 constant:0]];
if (i==0) {
[view addConstraint:[NSLayoutConstraint constraintWithItem:btn attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:view attribute:NSLayoutAttributeTop multiplier:1 constant:10]];
}else{
[view addConstraint:[NSLayoutConstraint constraintWithItem:btn attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:lastView attribute:NSLayoutAttributeBottom multiplier:1 constant:1]];
}
if (i==length-1) {//关键点。非常重要!设置contentsize的height,最大的一个设置width，否则会出错。
[view addConstraint:[NSLayoutConstraint constraintWithItem:btn attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:view attribute:NSLayoutAttributeBottom multiplier:1 constant:-10]];
[view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[btn]|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(btn)]];
}
lastView=btn;
}
//关键点。非常重要.设置contentsize以滚动。
CGSize size=[view systemLayoutSizeFittingSize:UILayoutFittingCompressedSize];
view.frame=CGRectMake(0, 0, size.width, size.height);
sv.contentSize=size;