百分比布局适配

一、简述

百分比布局适配，就是以父容器的尺寸作为基准，在view的加载过程中，根据当前父容器实际尺寸换算出目标尺寸，再作用在view上。

百分比布局，实际是对容器的一种扩展，即对宽高百分比的设置。

二、google推荐的百分比布局的使用方式

首先要引入：implementation ‘com.android.support:percent:28.0.0’
然后将布局引用为：android.support.percent.PercentRelativeLayout
给控件设置：app:layout_widthPercent=“50%”
百分比布局，实际是对容器的一种扩展，即对宽高百分比的设置。

三、实现

3.1 创建属性文件

attrs.xml中来定义必备的属性

        <attr name="widthPercent" format="float" />
        <attr name="heightPercent" format="float" />
        <attr name="marginLeftPercent" format="float" />
        <attr name="marginRightPercent" format="float" />
        <attr name="marginTopPercent" format="float" />
        <attr name="marginBottomPercent" format="float" />

3.2 解析

解析在哪做？以RelativeLayout为例，重要属性都封装在静态类LayoutParams中。这个静态类中就定义了一些自定义属性，也是RelativeLayout特有的属性（相对布局中的独有属性，不能作用于线性布局中）：

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 MismatchedReadAndWriteOfArray
    final 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();
}

通过TypedArray来进行属性的解析，然后switch case中一堆特有属性的获取、解析。解析完成后，就可以在onMeasure方法中用到这些自定义属性了。
这里涉及到了View的加载过程-setContentView方法，简单说就是：我们一个控件的布局属性的创建，是通过父容器来完成的。

创建静态内部类LayoutParams并继承RelativeLayout的LayoutParams来保证相对布局的属性都能用

    public static class LayoutParams extends RelativeLayout.LayoutParams{

		//在arrts.xml中定义的属性
        private float widthPercent;
        private float heightPercent;
        private float marginLeftPercent;
        private float marginRightPercent;
        private float marginTopPercent;
        private float marginBottomPercent;

        public LayoutParams(Context c, AttributeSet attrs) {
            super(c, attrs);
            //解析自定义属性
            TypedArray a = c.obtainStyledAttributes(attrs,R.styleable.PercentLayout);
            widthPercent = a.getFloat(R.styleable.PercentLayout_widthPercent, 0);
            heightPercent = a.getFloat(R.styleable.PercentLayout_heightPercent, 0);
            marginLeftPercent = a.getFloat(R.styleable.PercentLayout_marginLeftPercent, 0);
            marginRightPercent = a.getFloat(R.styleable.PercentLayout_marginRightPercent, 0);
            marginTopPercent = a.getFloat(R.styleable.PercentLayout_marginTopPercent, 0);
            marginBottomPercent = a.getFloat(R.styleable.PercentLayout_marginBottomPercent, 0);
            a.recycle();
        }
    }

总结一下就是先在attrs.xml中定义的属性，然后再解析自定义属性（宽、高、四周间距）

3.3 计算并设置百分比布局

先通过MeasureSpec.getSize()方法获取父容器尺寸：

//获取父容器的尺寸
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);

根据view加载的源码分析，还得重写父容器generateLayoutParams方法，return内部的LayoutParams：

    public LayoutParams generateLayoutParams(AttributeSet attrs){
        return new LayoutParams(getContext(), attrs);
    }

在onMeasure中判断，这个params是否属于当前类的LayoutParams

    @Override
    protected boolean checkLayoutParams(ViewGroup.LayoutParams p) {
        return p instanceof LayoutParams;
    }

然后就可以进行百分比布局的设置了，从params中取出百分比的值后赋值。如果各个属性已经设置好了百分比的值，就能进行换算了

float widthPercent = lp.widthPercent;
params.width = (int) (widthSize * widthPercent);

四、使用

在xml的控件中，直接使用这些属性，并给定浮点型的值

五、完整代码

attrs.xml

<resources>

    <declare-styleable name="PercentLayout">
        <attr name="widthPercent" format="float" />
        <attr name="heightPercent" format="float" />
        <attr name="marginLeftPercent" format="float" />
        <attr name="marginRightPercent" format="float" />
        <attr name="marginTopPercent" format="float" />
        <attr name="marginBottomPercent" format="float" />
    </declare-styleable>

</resources>

PercentLayout.xml

public class PercentLayout extends RelativeLayout {

    public PercentLayout(Context context) {
        super(context);
    }

    public PercentLayout(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public PercentLayout(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        //获取父容器的尺寸
        int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightSize = MeasureSpec.getSize(heightMeasureSpec);
        int count = getChildCount();
		//遍历子控件
        for (int i = 0; i < count; i++) {
            View child = getChildAt(i);
            ViewGroup.LayoutParams params = child.getLayoutParams();
            //如果说是百分比布局属性
            if (checkLayoutParams(params)){
                LayoutParams lp = (LayoutParams)params;
                 float widthPercent = lp.widthPercent;
                 float heightPercent = lp.heightPercent;
                 float marginLeftPercent = lp.marginLeftPercent;
                 float marginRightPercent= lp.marginRightPercent;
                 float marginTopPercent= lp.marginTopPercent;
                 float marginBottomPercent = lp.marginBottomPercent;

				//如果各属性已经设置好了百分比的值，就进行换算
                 if (widthPercent > 0){
                     params.width = (int) (widthSize * widthPercent);
                 }

                if (heightPercent > 0){
                    params.height = (int) (heightSize * heightPercent);
                }

                if (marginLeftPercent > 0){
                    ((LayoutParams) params).leftMargin = (int) (widthSize * marginLeftPercent);
                }

                if (marginRightPercent > 0){
                    ((LayoutParams) params).rightMargin = (int) (widthSize * marginRightPercent);
                }

                if (marginTopPercent > 0){
                    ((LayoutParams) params).topMargin = (int) (heightSize * marginTopPercent);
                }

                if (marginBottomPercent > 0){
                    ((LayoutParams) params).bottomMargin = (int) (heightSize * marginBottomPercent);
                }

            }
        }

        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }

    @Override
    protected boolean checkLayoutParams(ViewGroup.LayoutParams p) {
        return p instanceof LayoutParams;
    }

    public LayoutParams generateLayoutParams(AttributeSet attrs){
        return new LayoutParams(getContext(), attrs);
    }

    public static class LayoutParams extends RelativeLayout.LayoutParams{

		//在arrts.xml中定义的属性
        private float widthPercent;
        private float heightPercent;
        private float marginLeftPercent;
        private float marginRightPercent;
        private float marginTopPercent;
        private float marginBottomPercent;

        public LayoutParams(Context c, AttributeSet attrs) {
            super(c, attrs);
            //解析自定义属性
            TypedArray a = c.obtainStyledAttributes(attrs,R.styleable.PercentLayout);
            widthPercent = a.getFloat(R.styleable.PercentLayout_widthPercent, 0);
            heightPercent = a.getFloat(R.styleable.PercentLayout_heightPercent, 0);
            marginLeftPercent = a.getFloat(R.styleable.PercentLayout_marginLeftPercent, 0);
            marginRightPercent = a.getFloat(R.styleable.PercentLayout_marginRightPercent, 0);
            marginTopPercent = a.getFloat(R.styleable.PercentLayout_marginTopPercent, 0);
            marginBottomPercent = a.getFloat(R.styleable.PercentLayout_marginBottomPercent, 0);
            a.recycle();
        }
    }
}

六、注意

这些属性的设定，对原有属性没有任何影响，我们只是扩展了它的使用。