本文基于compile 'com.android.support:percent:26+'分析

在PercentFrameLayout和PercentRelativeLayout中都重写了
generateDefaultLayoutParams()和generateLayoutParams()方法。

PercentFrameLayout的重写

public class PercentFrameLayout extends FrameLayout {
    //省略其他代码

    @Override
    protected LayoutParams generateDefaultLayoutParams() {
        return new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);
    }

    @Override
    public LayoutParams generateLayoutParams(AttributeSet attrs) {
        return new LayoutParams(getContext(), attrs);
    }
}

PercentRelativeLayout

public class PercentRelativeLayout extends RelativeLayout {
//省略其他代码
      @Override
    protected LayoutParams generateDefaultLayoutParams() {
        return new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);
    }

    @Override
    public LayoutParams generateLayoutParams(AttributeSet attrs) {
        return new LayoutParams(getContext(), attrs);
    }
}

可以看到它们两者的实现是很相似的，那么generateDefaultLayoutParams()和generateLayoutParams()到底有何用？什么时候回调？

一下分析以PercentFrameLayout为例，PercentRelativeLayout类似。

generateDefaultLayoutParams

generateDefaultLayoutParams()方法属于ViewGroup,那么我们看一看ViewGroup中对于generateDefaultLayoutParams()方法的解析。

  /** * Returns a set of default layout parameters. These parameters are requested * when the View passed to {@link #addView(View)} has no layout parameters * already set. If null is returned, an exception is thrown from addView. * * @return a set of default layout parameters or null */
    protected LayoutParams generateDefaultLayoutParams() {
        return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
    }

generateDefaultLayoutParams()在ViewGroup中有两处调用到，这里展示其中一处:

 /** * Adds a child view. If no layout parameters are already set on the child, the * default parameters for this ViewGroup are set on the child. * * <p><strong>Note:</strong> do not invoke this method from * {@link #draw(android.graphics.Canvas)}, {@link #onDraw(android.graphics.Canvas)}, * {@link #dispatchDraw(android.graphics.Canvas)} or any related method.</p> * * @param child the child view to add * @param index the position at which to add the child * * @see #generateDefaultLayoutParams() */
    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);
    }

通过读上面的注释和代码，可以知道原来平常我们调用addView的方法时，不指定LayoutParams，是通过generateDefaultLayoutParams()获取默认的LayoutParams属性的。