安卓百分比佈局解析

1.引入：compile 'com.android.support:percent:24.0.0'

2.點開源碼能夠看到，主要有兩個佈局類PercentFrameLayout和PercentRelativeLayout，一個工具類PercentLayoutHelper。

3.點開佈局類好比PercentRelativeLayout的源碼，能夠看到實現的很簡單。

public class PercentRelativeLayout extends RelativeLayout {
    private final PercentLayoutHelper mHelper = new PercentLayoutHelper(this);

   /**省略若干行構造方法之類的代碼**/

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
      
        //重點在這

        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);
        }

         /**省略若干行構造方法之類的代碼**/

        @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裏面調用了PercentLayoutHelper 的一些方法，另外在裏面定義了本身的LayoutParams ，而這個LayoutParams 也至關簡單。
這裏關鍵的一行代碼是在onMeasure方法裏面，mHelper.adjustChildren(widthMeasureSpec, heightMeasureSpec);經過PercentLayoutHelper 的adjustChildren 遍歷子view來設置 子view的寬高，寬高在PercentLayoutHelper 的內部類PercentLayoutInfo經過在佈局文件中設置的值計算好了。

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++) {

            //遍歷子view來設置 子view的寬高

            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.佈局中的使用方法：以PercentRelativeLayout爲例

<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>

引入xmlns:app的命名空間，而後app:layout_widthPercent="50%"就能夠設置寬高的百分比了。至關簡單 。

5.不過在使用的過程當中，可能會有一些其餘的需求，好比app:layout_widthPercent="50%",app:layout_heightPercent="50%"都是相對於屏幕的寬高的，假如要顯示一張正方形的圖片，以寬的50%爲準呢？這個時候就能夠這樣寫了：

<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"
        />

使用layout_aspectRatio屬性，設置app:layout_aspectRatio="100%"，layout_aspectRatio就是寬高比。這個時候就不要設置app:layout_heightPercent屬性了。在PercentLayoutHelper 裏面， 源代碼以下：

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);
            }

            //這一段代碼是關鍵，若是aspectRatio >=0，aspectRatio是寬高比

            if (aspectRatio >= 0) {
                if (widthNotSet) {

                     //若是寬沒有設置，就以高爲準

                    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) {

                    //若是高沒有設置，就以寬爲準

                    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，假如咱們要定義本身的PercentLinearLayout，基本能夠直接改一下名字，繼承自LinearLayout就行了：public class PercentLinearLayout extends LinearLayout ,在仿照PercentRelativeLayout裏面的LayoutParams定義一個本身的LayoutParams就行了。
不過官方爲何沒有直接提供一個PercentLinearLayout 類，而只提供了兩個PercentFrameLayout和PercentRelativeLayout呢？或許是因爲LinearLayout 自己就有權重屬性，自帶百分比效果了。