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);
    }
  }
}

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);
          }
        }
      }
    }
  }

<android.support.percent.PercentRelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:app="http://schemas.android.com/apk/res-auto"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:background="@color/background_color"
  >

  <TextView
    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"
    />

  <ImageView
    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"
    />

</android.support.percent.PercentRelativeLayout>

<ImageView
    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"
    />

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 + ")");
      }
    }