iOS 仿抖音 Object-C

# Masonry [](https://travis-ci.org/SnapKit/Masonry) [](https://coveralls.io/r/SnapKit/Masonry) [](https://github.com/Carthage/Carthage)  **Masonry is still actively maintained, we are committed to fixing bugs and merging good quality PRs from the wider community. However if you're using Swift in your project, we recommend using [SnapKit](https://github.com/SnapKit/SnapKit) as it provides better type safety with a simpler API.** Masonry is a light-weight layout framework which wraps AutoLayout with a nicer syntax. Masonry has its own layout DSL which provides a chainable way of describing your NSLayoutConstraints which results in layout code that is more concise and readable. Masonry supports iOS and Mac OS X. For examples take a look at the **Masonry iOS Examples** project in the Masonry workspace. You will need to run `pod install` after downloading. ## What's wrong with NSLayoutConstraints? Under the hood Auto Layout is a powerful and flexible way of organising and laying out your views. However creating constraints from code is verbose and not very descriptive. Imagine a simple example in which you want to have a view fill its superview but inset by 10 pixels on every side ```obj-c UIView *superview = self.view; UIView *view1 = [[UIView alloc] init]; view1.translatesAutoresizingMaskIntoConstraints = NO; view1.backgroundColor = [UIColor greenColor]; [superview addSubview:view1]; UIEdgeInsets padding = UIEdgeInsetsMake(10, 10, 10, 10); [superview addConstraints:@[ //view1 constraints [NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:superview attribute:NSLayoutAttributeTop multiplier:1.0 constant:padding.top], [NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:superview attribute:NSLayoutAttributeLeft multiplier:1.0 constant:padding.left], [NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:superview attribute:NSLayoutAttributeBottom multiplier:1.0 constant:-padding.bottom], [NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:superview attribute:NSLayoutAttributeRight multiplier:1 constant:-padding.right], ]]; ``` Even with such a simple example the code needed is quite verbose and quickly becomes unreadable when you have more than 2 or 3 views. Another option is to use Visual Format Language (VFL), which is a bit less long winded. However the ASCII type syntax has its own pitfalls and its also a bit harder to animate as `NSLayoutConstraint constraintsWithVisualFormat:` returns an array. ## Prepare to meet your Maker! Heres the same constraints created using MASConstraintMaker ```obj-c UIEdgeInsets padding = UIEdgeInsetsMake(10, 10, 10, 10); [view1 mas_makeConstraints:^(MASConstraintMaker *make) { make.top.equalTo(superview.mas_top).with.offset(padding.top); //with is an optional semantic filler make.left.equalTo(superview.mas_left).with.offset(padding.left); make.bottom.equalTo(superview.mas_bottom).with.offset(-padding.bottom); make.right.equalTo(superview.mas_right).with.offset(-padding.right); }]; ``` Or even shorter ```obj-c [view1 mas_makeConstraints:^(MASConstraintMaker *make) { make.edges.equalTo(superview).with.insets(padding); }]; ``` Also note in the first example we had to add the constraints to the superview `[superview addConstraints:...`. Masonry however will automagically add constraints to the appropriate view. Masonry will also call `view1.translatesAutoresizingMaskIntoConstraints = NO;` for you. ## Not all things are created equal > `.equalTo` equivalent to **NSLayoutRelationEqual** > `.lessThanOrEqualTo` equivalent to **NSLayoutRelationLessThanOrEqual** > `.greaterThanOrEqualTo` equivalent to **NSLayoutRelationGreaterThanOrEqual** These three equality constraints accept one argument which can be any of the following: #### 1. MASViewAttribute ```obj-c make.centerX.lessThanOrEqualTo(view2.mas_left); ``` MASViewAttribute | NSLayoutAttribute ------------------------- | -------------------------- view.mas_left | NSLayoutAttributeLeft view.mas_right | NSLayoutAttributeRight view.mas_top | NSLayoutAttributeTop view.mas_bottom | NSLayoutAttributeBottom view.mas_leading | NSLayoutAttributeLeading view.mas_trailing | NSLayoutAttributeTrailing view.mas_width | NSLayoutAttributeWidth view.mas_height | NSLayoutAttributeHeight view.mas_centerX | NSLayoutAttributeCenterX view.mas_centerY | NSLayoutAttributeCenterY view.mas_baseline | NSLayoutAttributeBaseline #### 2. UIView/NSView if you want view.left to be greater than or equal to label.left : ```obj-c //these two constraints are exactly the same make.left.greaterThanOrEqualTo(label); make.left.greaterThanOrEqualTo(label.mas_left); ``` #### 3. NSNumber Auto Layout allows width and height to be set to constant values. if you want to set view to have a minimum and maximum width you could pass a number to the equality blocks: ```obj-c //width >= 200 && width <= 400 make.width.greaterThanOrEqualTo(@200); make.width.lessThanOrEqualTo(@400) ``` However Auto Layout does not allow alignment attributes such as left, right, centerY etc to be set to constant values. So if you pass a NSNumber for these attributes Masonry will turn these into constraints relative to the view&rsquo;s superview ie: ```obj-c //creates view.left = view.superview.left + 10 make.left.lessThanOrEqualTo(@10) ``` Instead of using NSNumber, you can use primitives and structs to build your constraints, like so: ```obj-c make.top.mas_equalTo(42); make.height.mas_equalTo(20); make.size.mas_equalTo(CGSizeMake(50, 100)); make.edges.mas_equalTo(UIEdgeInsetsMake(10, 0, 10, 0)); make.left.mas_equalTo(view).mas_offset(UIEdgeInsetsMake(10, 0, 10, 0)); ``` By default, macros which support [autoboxing](https://en.wikipedia.org/wiki/Autoboxing#Autoboxing) are prefixed with `mas_`. Unprefixed versions are available by defining `MAS_SHORTHAND_GLOBALS` before importing Masonry. #### 4. NSArray An array of a mixture of any of the previous types ```obj-c make.height.equalTo(@[view1.mas_height, view2.mas_height]); make.height.equalTo(@[view1, view2]); make.left.equalTo(@[view1, @100, view3.right]); ```` ## Learn to prioritize > `.priority` allows you to specify an exact priority > `.priorityHigh` equivalent to **UILayoutPriorityDefaultHigh** > `.priorityMedium` is half way between high and low > `.priorityLow` equivalent to **UILayoutPriorityDefaultLow** Priorities are can be tacked on to the end of a constraint chain like so: ```obj-c make.left.greaterThanO