这一期我们来讲一讲LayoutParams这个玩意儿。Android入门的第一行代码就牵扯到这个东西，然而，你真的理解够了吗？

第一层理解

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"xmlns:app="http://schemas.android.com/apk/res-auto"xmlns:tools="http://schemas.android.com/tools"android:layout_width="match_parent"android:layout_height="match_parent"tools:context=".MainActivity">
</RelativeLayout>

layout_width和layout_height这个是不是最开始学的时候，就要搞清楚的基础知识，match_parent代表填充屏幕，wrap_content代表包裹内容。这些其实是系统控件定义的属性，通过TypedArray进行解析。

第二层理解

val lp = RelativeLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT)
lp.addRule(RelativeLayout.ALIGN_PARENT_RIGHT)
lp.addRule(RelativeLayout.CENTER_VERTICAL)
lp.addRule(RelativeLayout.BELOW, viewId)
lp.setMargins(10, 20, 10, 20)

使用代码动态布局的时候设置LayoutParams。

第三层理解

好了，知识是在不断打破旧的认识中进步的，第一层实际还没到LayoutParams，还只是AttributeSet。系统何时将布局中的AttributeSet解析成LayoutParams的呢？

@Override
public LayoutParams generateLayoutParams(AttributeSet attrs) {return new RelativeLayout.LayoutParams(getContext(), attrs);
}protected LayoutParams generateDefaultLayoutParams() {return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
}

ViewGroup有个关键的方法，generateLayoutParams()。

public LayoutParams(Context c, AttributeSet attrs) {super(c, attrs);TypedArray a = c.obtainStyledAttributes(attrs,com.android.internal.R.styleable.RelativeLayout_Layout);final int targetSdkVersion = c.getApplicationInfo().targetSdkVersion;mIsRtlCompatibilityMode = (targetSdkVersion < JELLY_BEAN_MR1 ||!c.getApplicationInfo().hasRtlSupport());final int[] rules = mRules;//noinspection MismatchedReadAndWriteOfArrayfinal int[] initialRules = mInitialRules;final int N = a.getIndexCount();for (int i = 0; i < N; i++) {int attr = a.getIndex(i);switch (attr) {case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignWithParentIfMissing:alignWithParent = a.getBoolean(attr, false);break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_toLeftOf:rules[LEFT_OF] = a.getResourceId(attr, 0);break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_toRightOf:rules[RIGHT_OF] = a.getResourceId(attr, 0);break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_above:rules[ABOVE] = a.getResourceId(attr, 0);break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_below:rules[BELOW] = a.getResourceId(attr, 0);break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignBaseline:rules[ALIGN_BASELINE] = a.getResourceId(attr, 0);break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignLeft:rules[ALIGN_LEFT] = a.getResourceId(attr, 0);break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignTop:rules[ALIGN_TOP] = a.getResourceId(attr, 0);break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignRight:rules[ALIGN_RIGHT] = a.getResourceId(attr, 0);break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignBottom:rules[ALIGN_BOTTOM] = a.getResourceId(attr, 0);break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignParentLeft:rules[ALIGN_PARENT_LEFT] = a.getBoolean(attr, false) ? TRUE : 0;break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignParentTop:rules[ALIGN_PARENT_TOP] = a.getBoolean(attr, false) ? TRUE : 0;break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignParentRight:rules[ALIGN_PARENT_RIGHT] = a.getBoolean(attr, false) ? TRUE : 0;break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignParentBottom:rules[ALIGN_PARENT_BOTTOM] = a.getBoolean(attr, false) ? TRUE : 0;break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_centerInParent:rules[CENTER_IN_PARENT] = a.getBoolean(attr, false) ? TRUE : 0;break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_centerHorizontal:rules[CENTER_HORIZONTAL] = a.getBoolean(attr, false) ? TRUE : 0;break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_centerVertical:rules[CENTER_VERTICAL] = a.getBoolean(attr, false) ? TRUE : 0;break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_toStartOf:rules[START_OF] = a.getResourceId(attr, 0);break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_toEndOf:rules[END_OF] = a.getResourceId(attr, 0);break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignStart:rules[ALIGN_START] = a.getResourceId(attr, 0);break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignEnd:rules[ALIGN_END] = a.getResourceId(attr, 0);break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignParentStart:rules[ALIGN_PARENT_START] = a.getBoolean(attr, false) ? TRUE : 0;break;case com.android.internal.R.styleable.RelativeLayout_Layout_layout_alignParentEnd:rules[ALIGN_PARENT_END] = a.getBoolean(attr, false) ? TRUE : 0;break;}}mRulesChanged = true;System.arraycopy(rules, LEFT_OF, initialRules, LEFT_OF, VERB_COUNT);a.recycle();
}

这个代码熟悉吧，这就是我们之前讲过的自定义属性啊！没错，xml布局中的属性会先被解析成LayoutParams。那么我问你个问题，你觉得generateLayoutParams()和generateDefaultLayoutParams()的这个LayoutParams是给自己用的呢？还是给它的子控件用的呢？它是给子控件用的。自己的那个直接在构造方法中就从AttributeSet解析出来了。这样你就理解了，为什么RelativeLayout的那些个

android:layout_centerVertical="true"
android:layout_alignParentEnd="true"

怎么全部定义在子控件里面了。然后ViewGroup的addView()方法中就可以带上这个LayoutParams了。

/*** 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);
}

你不重写generateLayoutParams()方法，怎么在添加子控件的时候，让子控件用你的LayoutParams呢？

public static class LayoutParams extends ViewGroup.MarginLayoutParams {
}

以上是LinearLayout.LayoutParams的摘要，我们自定义ViewGroup的时候，是不是也可以继承个ViewGroup的LayoutParams玩一玩呢？然后重写generateLayoutParams()和generateDefaultLayoutParams()方法。

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

这里return你的ViewGroup的LayoutParams，然后在你的ViewGroup的LayoutParams的构造方法中就可以解析自定义属性attrs了。如果忘记了解析方式，我给你个提示，使用context的obtainStyledAttributes()方法。

大部分停留在第二层理解，你如果学会了第三层，那么你自定义View又可以玩出新的高度了。