本文介绍View的onLayout方法。onLayout通常在View给其孩子设置尺寸和位置时被调用。

onLayout简介

点击查看：onLayout示例的完整源码

在view给其孩子设置尺寸和位置时被调用。在我们自定义ViewGroup时，通常需要覆盖该方法。示例如下：

public class MyGroupView extends ViewGroup {

    public MyGroupView(Context context) {
        super(context);
    }

    public MyGroupView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public MyGroupView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    ...

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        // 记录总高度
        int mTotalHeight = 0;
        // 遍历所有子视图
        int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            View childView = getChildAt(i);

            // 获取在onMeasure中计算的视图尺寸
            int measureHeight = childView.getMeasuredHeight();
            int measuredWidth = childView.getMeasuredWidth();

            childView.layout(l, mTotalHeight, measuredWidth, mTotalHeight
                    + measureHeight);

            mTotalHeight += measureHeight;
        }
    }
}

自定义了ViewGroup之后，就可以使用该ViewGroup了。示意如下：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#00f0f0"
    tools:context=".MainActivity" >

    <com.skw.viewtest.MyGroupView
        android:id="@+id/myViewGroup"
        android:layout_width="480dp"
        android:layout_height="300dp"
        android:background="#0f0f0f" >

        <TextView
            android:layout_width="fill_parent"
            android:layout_height="60dp"
            android:background="#000000"
            android:gravity="center"
            android:text="First TextView" />

        <TextView
            android:layout_width="200dp"
            android:layout_height="100dp"
            android:background="#ffffff"
            android:gravity="center"
            android:text="Second TextView" />
    </com.skw.viewtest.MyGroupView>

</RelativeLayout>