2.点开源码可以看到，主要有两个布局类PercentFrameLayout和PercentRelativeLayout，一个工具类PercentLayoutHelper。
3.点开布局类比如PercentRelativeLayout的源码，可以看到实现的很简单。
public class PercentRelativeLayout extends RelativeLayout {
private final PercentLayoutHelper mHelper = new PercentLayoutHelper(this);
/**省略若干行构造方法之类的代码**/
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//重点在这
mHelper.adjustChildren(widthMeasureSpec, heightMeasureSpec);
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
if (mHelper.handleMeasuredStateTooSmall()) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
}
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
super.onLayout(changed, left, top, right, bottom);
mHelper.restoreOriginalParams();
}
public static class LayoutParams extends RelativeLayout.LayoutParams
implements PercentLayoutHelper.PercentLayoutParams {
private PercentLayoutHelper.PercentLayoutInfo mPercentLayoutInfo;
public LayoutParams(Context c, AttributeSet attrs) {
super(c, attrs);
mPercentLayoutInfo = PercentLayoutHelper.getPercentLayoutInfo(c, attrs);
}
/**省略若干行构造方法之类的代码**/
@Override
public PercentLayoutHelper.PercentLayoutInfo getPercentLayoutInfo() {
if (mPercentLayoutInfo == null) {
mPercentLayoutInfo = new PercentLayoutHelper.PercentLayoutInfo();
}
return mPercentLayoutInfo;
}
@Override
protected void setBaseAttributes(TypedArray a, int widthAttr, int heightAttr) {
PercentLayoutHelper.fetchWidthAndHeight(this, a, widthAttr, heightAttr);
}
}
}
就是在onMeasure和onLayout里面调用了PercentLayoutHelper 的一些方法，另外在里面定义了自己的LayoutParams ，而这个LayoutParams 也相当简单。
这里关键的一行代码是在onMeasure方法里面，mHelper.adjustChildren(widthMeasureSpec, heightMeasureSpec);通过PercentLayoutHelper 的adjustChildren 遍历子view来设置 子view的宽高，宽高在PercentLayoutHelper 的内部类PercentLayoutInfo通过在布局文件中设置的值计算好了。
public void adjustChildren(int widthMeasureSpec, int heightMeasureSpec) {
// Calculate available space, accounting for host's paddings
int widthHint = View.MeasureSpec.getSize(widthMeasureSpec) - mHost.getPaddingLeft()
- mHost.getPaddingRight();
int heightHint = View.MeasureSpec.getSize(heightMeasureSpec) - mHost.getPaddingTop()
- mHost.getPaddingBottom();
for (int i = 0, N = mHost.getChildCount(); i N; i++) {
//遍历子view来设置 子view的宽高
View view = mHost.getChildAt(i);
ViewGroup.LayoutParams params = view.getLayoutParams();
if (DEBUG) {
Log.d(TAG, should adjust + view + + params);
}
if (params instanceof PercentLayoutParams) {
PercentLayoutInfo info =
((PercentLayoutParams) params).getPercentLayoutInfo();
if (info != null) {
if (params instanceof ViewGroup.MarginLayoutParams) {
info.fillMarginLayoutParams(view, (ViewGroup.MarginLayoutParams) params,
widthHint, heightHint);
} else {
info.fillLayoutParams(params, widthHint, heightHint);
}
}
}
}
}
4.布局中的使用方法：以PercentRelativeLayout为例
xmlns:app=http://schemas.android.com/apk/res-auto
android:layout_width=match_parent
android:layout_height=match_parent
android:background=@color/background_color
>
android:id=@+id/mian_tab_name
android:layout_width=wrap_content
android:layout_height=wrap_content
android:gravity=center
android:textColor=@color/back_green
android:textSize=15sp
android:text=name
/>
app:layout_widthPercent=50%
app:layout_heightPercent=50%
android:src=@drawable/ic_launcher_icon
android:scaleType=fitXY
android:layout_below=@+id/mian_tab_name
android:background=@color/black
/>
引入xmlns:app的命名空间，然后app:layout_widthPercent=50%就可以设置宽高的百分比了。相当简单 。
5.不过在使用的过程中，可能会有一些其他的需求，比如app:layout_widthPercent=50%,app:layout_heightPercent=50%都是相对于屏幕的宽高的，假如要显示一张正方形的图片，以宽的50%为准呢?这个时候就可以这样写了：
app:layout_widthPercent=50%
app:layout_aspectRatio=100%
android:src=@drawable/ic_launcher_icon
android:scaleType=fitXY
android:layout_below=@+id/mian_tab_name
android:background=@color/CS_black
/>
使用layout_aspectRatio属性，设置app:layout_aspectRatio=100%，layout_aspectRatio就是宽高比。这个时候就不要设置app:layout_heightPercent属性了。在PercentLayoutHelper 里面， 源代码如下：
public void fillLayoutParams(ViewGroup.LayoutParams params, int widthHint,
int heightHint) {
// Preserve the original layout params, so we can restore them after the measure step.
mPreservedParams.width = params.width;
mPreservedParams.height = params.height;
// We assume that width/height set to 0 means that value was unset. This might not
// necessarily be true, as the user might explicitly set it to 0. However, we use this
// information only for the aspect ratio. If the user set the aspect ratio attribute,
// it means they accept or soon discover that it will be disregarded.
final boolean widthNotSet =
(mPreservedParams.mIsWidthComputedFromAspectRatio
|| mPreservedParams.width == 0) (widthPercent 0);
final boolean heightNotSet =
(mPreservedParams.mIsHeightComputedFromAspectRatio
|| mPreservedParams.height == 0) (heightPercent 0);
if (widthPercent >= 0) {
params.width = (int) (widthHint * widthPercent);
}
if (heightPercent >= 0) {
params.height = (int) (heightHint * heightPercent);
}
//这一段代码是关键，如果aspectRatio >=0，aspectRatio是宽高比
if (aspectRatio >= 0) {
if (widthNotSet) {
//如果宽没有设置，就以高为准
params.width = (int) (params.height * aspectRatio);
// Keep track that we've filled the width based on the height and aspect ratio.
mPreservedParams.mIsWidthComputedFromAspectRatio = true;
}
if (heightNotSet) {
//如果高没有设置，就以宽为准
params.height = (int) (params.width / aspectRatio);
// Keep track that we've filled the height based on the width and aspect ratio.
mPreservedParams.mIsHeightComputedFromAspectRatio = true;
}
}
if (DEBUG) {
Log.d(TAG, after fillLayoutParams: ( + params.width + , + params.height + ));
}
}
6.另外，假如我们要定义自己的PercentLinearLayout，基本可以直接改一下名字，继承自LinearLayout就好了：public class PercentLinearLayout extends LinearLayout ,在仿照PercentRelativeLayout里面的LayoutParams定义一个自己的LayoutParams就好了。
不过官方为什么没有直接提供一个PercentLinearLayout 类，而只提供了两个PercentFrameLayout和PercentRelativeLayout呢?
