Android 探究-----继承ViewGroup后我们可以做什么？

假设我们用CustomView继承ViewGroup，

在xml文件中，假设CustomView里面的23个childView，那么在生成CustomView的对象时自动回调23次父类ViewGroup的generateLayoutParams(AttributeSet attributeSet)和checkLayoutParams(ViewGroup.LayoutParams p)；

上面的规律告诉我们，有多少个childView就调用多少次这两个方法：generateLayoutParams(AttributeSet attributeSet)和checkLayoutParams(ViewGroup.LayoutParams p)，通过进一步研究我们发现，generateLayoutParams(AttributeSet attributeSet)方法中的attributeSet就是childView的属性集，所以如果我们要对childView加入自定义属性时，在CustomView中重写generateLayoutParams(AttributeSet
attributeSet) 方法；

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

上面的LayoutParams类就是CustomView的内部静态类，示例代码：

public static class LayoutParams extends ViewGroup.LayoutParams {
private static int NO_SPACING = -1;
private int x;
private int y;
private int horizontalSpacing = NO_SPACING;
private int verticalSpacing = NO_SPACING;
private boolean newLine = false;

public LayoutParams(Context context, AttributeSet attributeSet) {
super(context, attributeSet);
this.readStyleParameters(context, attributeSet);
}

public LayoutParams(int width, int height) {
super(width, height);
}

public LayoutParams(ViewGroup.LayoutParams layoutParams) {
super(layoutParams);
}

public boolean horizontalSpacingSpecified() {
return horizontalSpacing != NO_SPACING;
}

public boolean verticalSpacingSpecified() {
return verticalSpacing != NO_SPACING;
}

public void setPosition(int x, int y) {
this.x = x;
this.y = y;
}

private void readStyleParameters(Context context, AttributeSet attributeSet) {
TypedArray a = context.obtainStyledAttributes(attributeSet, R.styleable.FlowLayout_LayoutParams);
try {
horizontalSpacing = a.getDimensionPixelSize(R.styleable.FlowLayout_LayoutParams_layout_horizontalSpacing, NO_SPACING);
verticalSpacing = a.getDimensionPixelSize(R.styleable.FlowLayout_LayoutParams_layout_verticalSpacing, NO_SPACING);
newLine = a.getBoolean(R.styleable.FlowLayout_LayoutParams_layout_newLine, false);
} finally {
a.recycle();
}
}
}

其中用到的声明自定义属性集的文件attrs.xml代码为：

<resources>
<declare-styleable name="FlowLayout">
<attr name="horizontalSpacing" format="dimension"/>
<attr name="verticalSpacing" format="dimension"/>
<attr name="orientation" format="enum">
<enum name="horizontal" value="0"/>
<enum name="vertical" value="1"/>
</attr>
<attr name="debugDraw" format="boolean" />
</declare-styleable>
<declare-styleable name="FlowLayout_LayoutParams">
<attr name="layout_newLine" format="boolean"/>
<attr name="layout_horizontalSpacing" format="dimension"/>
<attr name="layout_verticalSpacing" format="dimension"/>
</declare-styleable>
</resources>

当给childView加入属性后，如果childView调用getLayoutParams()，一定要记得LayoutParams lp = (LayoutParams) child.getLayoutParams();

而CustomView当通过代码去addView时，如果被add的View不调用setLayoutParams，则回调父类ViewGroup的generateLayoutParams()和checkLayoutParams(ViewGroup.LayoutParams p)；如果被add的View调用setLayoutParams，则只回调父类ViewGroup的checkLayoutParams(ViewGroup.LayoutParams
p)；

参考：https://github.com/ApmeM/android-flowlayout