前言
Layout 的过程就是将我们的View根据ViewGroup（例如FramLayout）规则、VIew 的 LayoutParams相关参数以及View Measure之后的大小来确定View的位置的过程
LayoutParams
LayoutParams翻译过来就是布局参数，子View通过父容器对应的LayoutParams来告诉父容器应该如何放置自己。所以基本上每个ViewGroup都拥有一个对应的LayoutParams来支持他们的布局规则，有时候我们也需要自定义LayoutParams来定义自己的布局规则
LayoutParams与View如何建立联系
- 在XML中定义View，这个系统会默认给View添加一个ViewGroup的LayoutParams
- 在java代码中直接生成View对应的实例对象
当我们自定义ViewGroup的时候需要注意下面几点
/**
* 通过代码添加子View，可重载
*/
public void addView(View child) {
addView(child, -1);
}
/**
* 通过代码添加子View，指定位置，可重载
*/
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) {
// 生产默认LayoutParams 需要用户重载下面的方法
params = generateDefaultLayoutParams();
if (params == null) {
throw new IllegalArgumentException("generateDefaultLayoutParams() cannot return null");
}
}
addView(child, index, params);
}
/**
* 通过代码添加子View，指定高宽，可重载
*/
public void addView(View child, int width, int height) {
final LayoutParams params = generateDefaultLayoutParams();
params.width = width;
params.height = height;
addView(child, -1, params);
}
/**
* 注意：通过addView 添加的View，如果不传LayoutParams 则必须通过下面的方法生产默认的LayoutParams
* 重载该方法
*/
protected LayoutParams generateDefaultLayoutParams() {
return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
}
/**
* 添加一个View 指定params
*/
@Override
public void addView(View child, LayoutParams params) {
addView(child, -1, params);
}
/**
* 添加一个View 指定index和params
*/
public void addView(View child, int index, LayoutParams params) {
if (DBG) {
System.out.println(this + " addView");
}
if (child == null) {
throw new IllegalArgumentException("Cannot add a null child view to a ViewGroup");
}
// addViewInner() will call child.requestLayout() when setting the new LayoutParams
// therefore, we call requestLayout() on ourselves before, so that the child's request
// will be blocked at our level
requestLayout();
invalidate(true);
addViewInner(child, index, params, false);
}
private void addViewInner(View child, int index, LayoutParams params,
boolean preventRequestLayout) {
....
if (mTransition != null) {
mTransition.addChild(this, child);
}
//检查传入的参数是否合法
if (!checkLayoutParams(params)) {
//不合法将会进行转化操作
params = generateLayoutParams(params);
}
//是否要阻止重新执行布局流程
if (preventRequestLayout) {
child.mLayoutParams = params;//这个不会引起子View的重新布局（onMeasure -> onLayout -> onDraw）
} else {
child.setLayoutParams(params);//这个会引起子View的重新布局（onMeasure -> onLayout -> onDraw）
}
if (index < 0) {
index = mChildrenCount;
}
addInArray(child, index);
// tell our children
if (preventRequestLayout) {
child.assignParent(this);
} else {
child.mParent = this;
}
.......
}
/**
* 注意：检查LayoutParams 是否合规，重载该方法来自定义检查
*/
protected boolean checkLayoutParams(ViewGroup.LayoutParams p) {
return p != null;
}
/**
* 注意：checkLayoutParams 失败之后会调用该方法，重载该方法 添加完整参数
*/
protected LayoutParams generateLayoutParams(ViewGroup.LayoutParams p) {
return p;
}
/**
* 注意：通过xml添加的子View，是通过这个方法来生产LayoutParams，记得重载该方法
*/
public LayoutParams generateLayoutParams(AttributeSet attrs) {
return new LayoutParams(getContext(), attrs);
}
复制代码自定义LayoutParams
1.创建自定义属性
<declare-styleable name="MyGroupView">
<attr name="layout_simple_attr" format="integer"></attr>
<attr name="android:layout_gravity" format="dimension"></attr>
</declare-styleable>
复制代码2.继承MarginLayoutParams
public class LayoutParams extends ViewGroup.MarginLayoutParams {
public int simpleAttr;
public int gravity;
public LayoutParams(Context c, AttributeSet attrs) {
super(c, attrs);
//解析布局
TypedArray typedArray = c.obtainStyledAttributes(attrs, R.styleable.MyGroupView);
simpleAttr = typedArray.getInteger(R.styleable.MyGroupView_layout_simple_attr, 0);
gravity = typedArray.getInteger(R.styleable.MyGroupView_android_layout_gravity, -1);
typedArray.recycle();//释放资源
}
public LayoutParams(int width, int height) {
super(width, height);
}
public LayoutParams(ViewGroup.MarginLayoutParams source) {
super(source);
}
public LayoutParams(ViewGroup.LayoutParams source) {
super(source);
}
}
复制代码3.重写ViewGroup中几个与LayoutParams相关的方法
// 已FrameLayout 为例
// 检查LayoutParams 是否合法
@Override
protected boolean checkLayoutParams(ViewGroup.LayoutParams p) {
return p instanceof LayoutParams;
}
//生成默认LayoutParams
@Override
protected LayoutParams generateDefaultLayoutParams() {
return new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);
}
//对传入的lp 进行转化
@Override
protected ViewGroup.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);
}
// 通过XML加载的View 通过该方法构造LayoutParams
@Override
public LayoutParams generateLayoutParams(AttributeSet attrs) {
return new FrameLayout.LayoutParams(getContext(), attrs);
}
复制代码Layout
layout流程图
public void layout(int l, int t, int r, int b) {
// 判断需不需要先执行onMesuare，这个通常在measure方法里设置，告诉layout 我还未调用onMeasure 方法，你需要执行一遍
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
// 通常如果你只是移动下视图位置我们只需要调用layout方法。属性动画的位移动画实现方式就是直接调用layout减少measure的计算量，setFrame/setOpticalFrame可能会触发invalidate流程
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);
.....
}
....
}
复制代码