Support for auto layout

[miensol]

Obviously we would like an easy way to layout our views. The question is how we will declare layout configuration? Let's discuss them here.

For simplicity let's assume we declare them inline and we can reference other views with #id

An example (which I don't like):

<UIView>
    <UIButton id="myButton" layout_width="match_parent" layout_height="match_parent" layout_center="horizontal | vertical | true">
    </UIButton>
</UIView>

[NOtherDev]

General format for the constraint is:

/* accessors
 firstItem.firstAttribute {==,<=,>=} secondItem.secondAttribute * multiplier + constant
 */
@property (readonly, assign) id firstItem;
@property (readonly) NSLayoutAttribute firstAttribute;
@property (readonly) NSLayoutRelation relation;
@property (readonly, assign) id secondItem;
@property (readonly) NSLayoutAttribute secondAttribute;
@property (readonly) CGFloat multiplier;
@property CGFloat constant;
@property UILayoutPriority priority;

Ideally we should support all of it, so the structure might look like:

<UIView>
    <UIButton id="myButton">
      <Constraints>
        <Constraint on="NSLayoutAttributeLeft" relation="NSLayoutRelationEqual" 
            with="#anotherControl.NSLayoutAttributeLeft" multiplier="0.8" 
            constant="100" priority="9" />
      </Constraints>
    </UIButton>
</UIView>

But this is awfully verbose. Some defaults may be introduced:

• relation can be NSLayoutRelationEqual if not specified
• in with, another control attribute may be assumed to be the same as on value if omitted
• multiplier = 1 when another control defined, 0 otherwise
• priority = default
• constant = 0

This gives something like this in most cases:

 <Constraint on="NSLayoutAttributeLeft" with="#anotherControl" />

And this looks OK for me.

More ideas:

• skip NSLayoutAttribute part - it can be just <Constraint on="left" with="#anotherControl" />.
• define aliases for commonly used constraint sets, like fill parent edges: <UIButton constraints="fillParentEdges"> - this would mimic helper methods known from AutoLayout etc. and allow skipping the whole <Constraints> element.

[miensol]

Here is what so far is working:

<UIView>
    <Constraint on="width,height" with=":superview"/>

    <UILabel text="First label which become longer" id="firstLabel">
        <Constraint on="topMargin" with=":superview" constant="40" id="topMarginConstraint" />
        <Constraint on="width" with=":superview" constant="-20"/>
        <Constraint on="centerx" with=":superview"/>
    </UILabel>

    <UIButton>
        <Constraint on="left" with="#firstLabel"/>
        <Constraint on="width" with=":previous" multiplier="0.5" constant="-10"/>
        <Constraint on="top" with="#firstLabel.bottom" constant="30"/>
    </UIButton>

    <UIButton>
        <Constraint on="width,top" with=":previous" id="buttonWidthAndTopConstraint" />
        <Constraint on="left" with=":previous.right" constant="20"/>
    </UIButton>
</UIView>

I'm still not sure about the shortcut form <UIButton constraints="fillParentEdges">. It's obviously more readable than <Constraint on="width,height,top,left" with=":superview"/>. How would we define a padding? Maybe this would work <UIButton constraints="fillParentEdges(10,20,10,20)"> ?

[miensol]

Most of commonly used costructs is supported now.