NSLayoutConstraint-代碼實現自動佈局的函數用法說明

[NSLayoutConstraint constraintWithItem:(id)item                              attribute:(NSLayoutAttribute)attribute                              relatedBy:(NSLayoutRelation)relation                                 toItem:(id)otherItem                              attribute:(NSLayoutAttribute)otherAttribute                             multiplier:(CGFloat)multiplier                               constant:(CGFloat)constant]

參數說明:

第一個參數:指定約束左邊的視圖view1

第二個參數:指定view1的屬性attr1，具體屬性見文末。

第三個參數:指定左右兩邊的視圖的關係relation，具體關係見文末。

第四個參數:指定約束右邊的視圖view2

第五個參數:指定view2的屬性attr2，具體屬性見文末。

第六個參數:指定一個與view2屬性相乘的乘數multiplier

第七個參數:指定一個與view2屬性相加的浮點數constant

這個函數的對照公式爲:

view1.attr1 <relation> view2.attr2 * multiplier + constant

注意:

1.若是你想設置的約束裏不須要第二個view，要將第四個參數設爲nil，第五個參數設爲NSLayoutAttributeNotAnAttribute

舉例:

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[NSLayoutConstraint constraintWithItem:view1                              attribute:NSLayoutAttributeLeft                              relatedBy:NSLayoutRelationEqual                                 toItem:view2                              attribute:NSLayoutAttributeRight                             multiplier:1                               constant:10]

翻譯過來就是:view1的左側，在，view2的右側，再多10個點，的地方。

附視圖的屬性和關係的值:

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typedef NS_ENUM(NSInteger, NSLayoutRelation) {     NSLayoutRelationLessThanOrEqual = -1,          //小於等於     NSLayoutRelationEqual = 0,                     //等於     NSLayoutRelationGreaterThanOrEqual = 1,        //大於等於 }; typedef NS_ENUM(NSInteger, NSLayoutAttribute) {     NSLayoutAttributeLeft = 1,                     //左側     NSLayoutAttributeRight,                        //右側     NSLayoutAttributeTop,                          //上方     NSLayoutAttributeBottom,                       //下方     NSLayoutAttributeLeading,                      //首部     NSLayoutAttributeTrailing,                     //尾部     NSLayoutAttributeWidth,                        //寬度     NSLayoutAttributeHeight,                       //高度     NSLayoutAttributeCenterX,                      //X軸中心     NSLayoutAttributeCenterY,                      //Y軸中心     NSLayoutAttributeBaseline,                     //文本底標線                                                                                                                                                          NSLayoutAttributeNotAnAttribute = 0            //沒有屬性 };

NSLayoutAttributeLeft/NSLayoutAttributeRight 和NSLayoutAttributeLeading/NSLayoutAttributeTrailing的區別是left/right永遠是指左右，而leading/trailing在某些從右至左習慣的地區會變成，leading是右邊，trailing是左邊。

屏幕兼容的問題

爲了讓咱們的應用在不容尺寸的屏幕下都能 「正常」的表示，咱們儘可能不要把數據寫死。

大多數可視元素都是一個矩形區域，固然這個矩形區域有座標的，咱們有了這個區域座標就能肯定可視元素的現實位置了。

可是iphone5 和之前的屏幕不同了，在之前的設備中，咱們能夠添加一個 xx.@2x.png 來適應retina屏幕，可是iphoen5咋辦呢？

ios 引入了 Auto Layout 的東東，這個要和UIViewAutoresizing 區分下。

看下面代碼

- (void)viewDidLoad
{
    [super viewDidLoad];
        
    UIView *aView = [[UIView alloc] init];
    aView.backgroundColor = [UIColor redColor];

//爲了避免和autosizing衝突，咱們設置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  其實 NSDictionaryOfVariableBindings(v1, v2, v3) 等效於 [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]];
    //添加一個限制  等效於 bView.frame.origin.x  = (aView.frame.origin.x +aView.frame.size.width)  * 1  + 10,好像是這樣的！我的以爲！

  它是一種依賴關係，bView依賴aView，這樣就算aView變了，bView也會跟着變換。
    [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".

屬性

最後的結果就是 「view1.attr1  <       >=    或者 ==   或者    <=       >  view2.attr2 * multiplier + constant」