ViewGroup的LayoutParams理解[通俗易懂]

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LayoutParams是ViewGroup的一个内部类,声明方式如下

public static class LayoutParams { 
   
        public static final int MATCH_PARENT = -1;
        public static final int WRAP_CONTENT = -2;
        public int width;
        public int height;
        
        public LayoutParams(Context c, AttributeSet attrs) { 
   
            TypedArray a = c.obtainStyledAttributes(attrs, R.styleable.ViewGroup_Layout);
            setBaseAttributes(a,R.styleable.ViewGroup_Layout_layout_width, R.styleable.ViewGroup_Layout_layout_height);
            a.recycle();
        }

        public LayoutParams(int width, int height) { 
   
            this.width = width;
            this.height = height;
        }
        
        public LayoutParams(LayoutParams source) { 
   
            this.width = source.width;
            this.height = source.height;
        }

 }

可以分析到,系统默认为ViewGroup自定义了宽高属性width和height,将其获取方式封装在LayoutParams中,系统考虑所有的View肯定都有宽高,所以就直接统一定义了,有一个疑问为啥不定义在View中,因为子View定义的宽高都是layout_width,layout_height,都是相对于父容器的
接下来分析,系统是如何将这个统一的ViewGroup.LayoutParams宽高属性给到View的呢？

阅读源码布局中View的加载流程,主要分析LayoutInflater#inflate这一步

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) { 
   
       
            final View temp = createViewFromTag(root, name, inflaterContext, attrs);

            ViewGroup.LayoutParams params = null;

            if (root != null) { 
   
                params = root.generateLayoutParams(attrs);
                if (!attachToRoot) { 
   
                    temp.setLayoutParams(params);
                 }
            }                    
}

分析关键代码,是root(ViewGroup)调用了generateLayoutParams(AttributeSet attrs);解析attrs中的layout_width和layout_height属性,将其值封装到了LayoutParams中，之后调用子View的setLayoutParams方法设置到子View中,之后子View就可以获取到width和height了

所以默认情况下父容器的generateLayoutParams只是解析layout_width和layout_height。如果子view需要支持margin属性,不重写父容器的generateLayoutParams的话,之后是子View是获取不到margin属性的

再分析下ViewGroup#generateDefaultLayoutParams,在上面我们分析了ViewGroup#generateLayoutParams它是作用在xml布局解析阶段为子View设置LayoutParams,那么ViewGroup#generateDefaultLayoutParams则是在代码中ViewGroup#addView中调用,我们看下源码

public void addView(View child, int index) { 
   
        if (child == null) { 
   
            throw new IllegalArgumentException("Cannot add a null child view to a ViewGroup");
        }
        LayoutParams params = child.getLayoutParams();
        if (params == null) { 
   
            params = generateDefaultLayoutParams();
            if (params == null) { 
   
                throw new IllegalArgumentException("generateDefaultLayoutParams() cannot return null");
            }
        }
        addView(child, index, params);
}

分析源码得知,generateDefaultLayoutParams的调用确实在动态addView中,同样的它也是默认创建的ViewGroup.LayoutParams,只支持width和height的

再分析下ViewGroup#generateLayoutParams(LayoutParams p),分析ViewGroup#addViewInner源码

private void addViewInner(View child, int index, LayoutParams params,boolean preventRequestLayout) { 
   
        if (!checkLayoutParams(params)) { 
   
            params = generateLayoutParams(params);
        }
}

分析可以知道,在addViewInner中会对params进行校验,如果不合法则调用generateLayoutParams(params)帮我们生成一个合法的LayoutParams.我们看下校验规则

protected boolean checkLayoutParams(ViewGroup.LayoutParams p) { 
   
        return  p != null;
}

ViewGroup默认的校验规则仅仅是不为空,相关宽松,LinearLayout等一般都会重写,有兴趣可以看源码

再分析下ViewGroup默认的generateLayoutParams(ViewGroup.LayoutParams p)实现

protected LayoutParams generateLayoutParams(ViewGroup.LayoutParams p) { 
   
        return p;
}

仅仅是返回了这个不合法的params,这并不是我们期望的,所以这个用法一般是我们重写checkLayoutParams实现合法的校验规则,然后在重写generateLayoutParams创建合法的LayoutParams,看下LinearLayout的实现

@Override
protected boolean checkLayoutParams(ViewGroup.LayoutParams p) { 
   
        return p instanceof LinearLayout.LayoutParams;
}

@Override
protected LayoutParams generateLayoutParams(ViewGroup.LayoutParams lp) { 
   
        if (sPreserveMarginParamsInLayoutParamConversion) { 
   
            if (lp instanceof LayoutParams) { 
   
                return new LayoutParams((LayoutParams) lp);
            } else if (lp instanceof MarginLayoutParams) { 
   
                return new LayoutParams((MarginLayoutParams) lp);
            }
        }
        return new LayoutParams(lp);
}

前面分析了ViewGroup默认的LayoutParams只是支持layout_width和layout_height属性的，因为它内部只有获取这两个属性的逻辑,但是我们实际很多子View都是需要支持margin属性的,怎么办呢?

ViewGroup还为我们定义了一个MarginLayoutParams,是不是相当的便利

public static class MarginLayoutParams extends ViewGroup.LayoutParams { 
   
        public int leftMargin;
        public int topMargin;
        public int rightMargin;
        public int bottomMargin;
        public MarginLayoutParams(Context c, AttributeSet attrs) { 
   
            super();
            ...//省略若干获取margin属性操作代码
        }

}