Android自定义View2--onMeasure，onLayout源码分析和自定义流式布局

onMeasure() 测量View的大小
onLayout() 确定子布局
onDraw() 实际绘制内容

自定义View主要是实现onMeasure+onDraw
自定义ViewGroup主要是实现onMeasure+onLayout

一、onMeasure源码分析

onMeasure的时候是先量子View，再计算和保存自己的尺寸，以FrameLayout的源码为例：

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int count = getChildCount();

        final boolean measureMatchParentChildren =
                MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
                MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
        mMatchParentChildren.clear();

        int maxHeight = 0;
        int maxWidth = 0;
        int childState = 0;

        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (mMeasureAllChildren || child.getVisibility() != GONE) {
            **//1、测量子View**
                measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                maxWidth = Math.max(maxWidth,
                        child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
                maxHeight = Math.max(maxHeight,
                        child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
                childState = combineMeasuredStates(childState, child.getMeasuredState());
                if (measureMatchParentChildren) {
                    if (lp.width == LayoutParams.MATCH_PARENT ||
                            lp.height == LayoutParams.MATCH_PARENT) {
                        mMatchParentChildren.add(child);
                    }
                }
            }
        }

         **//2、计算子View的最大宽高**
        maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
        maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

        // Check against our minimum height and width
        maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

        // Check against our foreground's minimum height and width
        final Drawable drawable = getForeground();
        if (drawable != null) {
            maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
            maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
        }
**//3、测量自己，确定自己的宽高**
        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                resolveSizeAndState(maxHeight, heightMeasureSpec,
                        childState << MEASURED_HEIGHT_STATE_SHIFT));
                        
        count = mMatchParentChildren.size();
        if (count > 1) {
            for (int i = 0; i < count; i++) {
                final View child = mMatchParentChildren.get(i);
                final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

                final int childWidthMeasureSpec;
                if (lp.width == LayoutParams.MATCH_PARENT) {
                    final int width = Math.max(0, getMeasuredWidth()
                            - getPaddingLeftWithForeground() - getPaddingRightWithForeground()
                            - lp.leftMargin - lp.rightMargin);
                    childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
                            width, MeasureSpec.EXACTLY);
                } else {
                    childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                            getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
                            lp.leftMargin + lp.rightMargin,
                            lp.width);
                }

                final int childHeightMeasureSpec;
                if (lp.height == LayoutParams.MATCH_PARENT) {
                    final int height = Math.max(0, getMeasuredHeight()
                            - getPaddingTopWithForeground() - getPaddingBottomWithForeground()
                            - lp.topMargin - lp.bottomMargin);
                    childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                            height, MeasureSpec.EXACTLY);
                } else {
                    childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
                            getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
                            lp.topMargin + lp.bottomMargin,
                            lp.height);
                }
**//4、当子View是MATCH_PARENT属性，再将子View设置成测量出来的parent的宽高，也就实现了View的MATCH_PARENT**
                child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
            }
        }
    }

可以看到这四步总结onMeasure:首先，它会遍历所有子View，并记录下子View的最大宽高，最大宽高加上padding margin作为FrameLayout自身的尺寸。然后，如果FrameLayout是不确定大小的模式，子View又是MATCH_PARENT属性的，就需要为这些子View重新度量。

二、onLayout源码分析

@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}

void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
    final int count = getChildCount();

    final int parentLeft = getPaddingLeftWithForeground();
    final int parentRight = right - left - getPaddingRightWithForeground();

    final int parentTop = getPaddingTopWithForeground();
    final int parentBottom = bottom - top - getPaddingBottomWithForeground();

    for (int i = 0; i < count; i++) {
        final View child = getChildAt(i);
        if (child.getVisibility() != GONE) {
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();

            final int width = child.getMeasuredWidth();
            final int height = child.getMeasuredHeight();

            int childLeft;
            int childTop;

            int gravity = lp.gravity;
            if (gravity == -1) {
                gravity = DEFAULT_CHILD_GRAVITY;
            }

            final int layoutDirection = getLayoutDirection();
            final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
            final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;

            switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                case Gravity.CENTER_HORIZONTAL:
                    childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
                    lp.leftMargin - lp.rightMargin;
                    break;
                case Gravity.RIGHT:
                    if (!forceLeftGravity) {
                        childLeft = parentRight - width - lp.rightMargin;
                        break;
                    }
                case Gravity.LEFT:
                default:
                    childLeft = parentLeft + lp.leftMargin;
            }

            switch (verticalGravity) {
                case Gravity.TOP:
                    childTop = parentTop + lp.topMargin;
                    break;
                case Gravity.CENTER_VERTICAL:
                    childTop = parentTop + (parentBottom - parentTop - height) / 2 +
                    lp.topMargin - lp.bottomMargin;
                    break;
                case Gravity.BOTTOM:
                    childTop = parentBottom - height - lp.bottomMargin;
                    break;
                default:
                    childTop = parentTop + lp.topMargin;
            }

            child.layout(childLeft, childTop, childLeft + width, childTop + height);
        }
    }
}

onLayout:FrameLayout对每个子View的layout过程是相同的，它会遍历所有的子View，通过子View的gravity属性进行xy轴偏移量的计算，最后调用child.layout()对子View进行布局。

小知识：
getMeasuredWidth:
1、在measure()过程结束后就可以获取对应的值。
2、通过setMeasuredDimension()方法来进行设置的。

getWidth:
1、在layout()过程结束后才能获取到；
2、通过视图右边的坐标减去左边的坐标计算出来的。

三、 自定义流式布局的代码：

FlowLayout.java:

import android.content.Context;
import android.graphics.Canvas;
import android.os.Build;
import android.support.annotation.RequiresApi;
import android.util.AttributeSet;
import android.view.View;
import android.view.ViewGroup;


import com.lz.langyabi.util.utilcode.ConvertUtils;

import java.util.ArrayList;
import java.util.List;

public class FlowLayout extends ViewGroup {
    private static final String TAG = "FlowLayout";
    private int mHorizontalSpacing = ConvertUtils.dp2px(16);
    private int mVertialSpacing = ConvertUtils.dp2px(8);

    private List<List<View>> allLines = new ArrayList<>();//记录所有的行，一行一行的存储，用于layout
    List<Integer> lineHeights = new ArrayList<>();//记录每一行的行高，用于layout

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

    public FlowLayout(Context context, AttributeSet attrs) {
        super(context, attrs);
    }
    
    //主题style
    public FlowLayout(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
    public FlowLayout(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
    }




    private void initMeasuredParams() {
        allLines = new ArrayList<>();
        lineHeights = new ArrayList<>();
    }

    //度量，测量View的大小
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        //initMeasuredParams不可以放到构造函数里面，放到这里可以是因为构造函数的生命周期中只调用一次，但onMeasure可能会多次触发
        initMeasuredParams();
        int paddingLeft = getPaddingLeft();
        int paddingRight = getPaddingRight();
        int paddingTop = getPaddingTop();
        int paddingBottom = getPaddingBottom();

        List<View> lineViews = new ArrayList<>();//保存一行中的所有view
        int lineWidthUsed = 0; //记录这行已经使用了多宽的size
        int lineHeight = 0;  // 一行的高

        int selfWidth = MeasureSpec.getSize(widthMeasureSpec);//ViewGroup解析的父亲给我的宽度
        int selfHeight = MeasureSpec.getSize(heightMeasureSpec);//ViewGroup解析的父亲给我的高度

        int parentNeedWidth = 0;//measure过程中，子View要求的父ViewGroup的宽
        int parentNeedHeight = 0;//measure过程中，子View要求的父ViewGroup的高

        //度量子View
        for (int i = 0; i < getChildCount(); i++) {
            View childView = getChildAt(i);
            LayoutParams childLP = childView.getLayoutParams();
            //将layoutParams转变成measureSpec
            int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec, paddingLeft + paddingRight, childLP.width);
            int childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec, paddingTop + paddingBottom, childLP.height);
            childView.measure(childWidthMeasureSpec, childHeightMeasureSpec);

            //获取子View的度量宽高
            int childMeasuredWidth = childView.getMeasuredWidth();
            int childMeasuredHeight = childView.getMeasuredHeight();
            //当新一个要加入的item的宽度加上现在累积的width的宽度大于行宽
            if (childMeasuredWidth + lineWidthUsed > selfWidth) {
                //所有行的数据和所有行的行高记录下来，用于layout
                allLines.add(lineViews);
                lineHeights.add(lineHeight);
                //一旦换行，我们就可以判断当前行需要的宽和高了，所以要记录下来
                parentNeedHeight = parentNeedHeight + lineHeight + mVertialSpacing;
                //宽度要记录，因为很多时候并不是布满全局
                parentNeedWidth = Math.max(parentNeedWidth, lineWidthUsed + mHorizontalSpacing);

                lineViews = new ArrayList<>();
                lineWidthUsed = 0;
                lineHeight = 0;
            }

            //View是分行layout的，所以要记录每一行有哪些View，这样可以方便layout布局
            lineViews.add(childView);
            //每行都有自己的宽高
            lineWidthUsed = lineWidthUsed + childMeasuredWidth + mHorizontalSpacing;
            lineHeight = Math.max(lineHeight, childMeasuredHeight);

        }

        //度量自己
        //根据子View的度量结果，来重新度量自己的ViewGroup
        //作为一个ViewGroup，它自己也是一个View，它的大小也需要根据它的父亲给他提供的宽高来度量
        int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        //明确了大小就是父亲（ViewGroup）给的
        int realWidth = (widthMode == MeasureSpec.EXACTLY) ? selfWidth : parentNeedWidth;
        int realHeight = (heightMode == MeasureSpec.EXACTLY) ? selfHeight : parentNeedHeight;
        setMeasuredDimension(realWidth, realHeight);
    }


    //布局，确定子View布局
    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        //一共有多少行
        int lineCount = allLines.size();

        int curL = getPaddingLeft();
        int curT = getPaddingTop();
        for (int i = 0; i < lineCount; i++) {
            //这一行的view的集合
            List<View> lineViews = allLines.get(i);
            int lineHight = lineHeights.get(i);
            for (int j = 0; j < lineViews.size(); j++) {
                View view = lineViews.get(j);

                int left = curL;
                int top = curT;
                int right = left + view.getMeasuredWidth();
                int bottom = top + view.getMeasuredHeight();

                view.layout(left, top, right, bottom);
                curL = right + mHorizontalSpacing;
            }
            curT = curT + mVertialSpacing + lineHight;
            curL = getPaddingLeft();
        }
    }

    //实际绘制内容
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
    }

}

设置数据的代码，我这里没用adpter去管理，主要是梳理自定义View：

import android.content.Intent;
import android.os.Bundle;
import androidx.appcompat.app.AppCompatActivity;
import android.util.TypedValue;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import com.nex3z.flowlayout.FlowLayout;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
       fillAutoSpacingLayout();
    }

    private void fillAutoSpacingLayout() {
        FlowLayout flowLayout = findViewById(R.id.flow);
        //R.array.lorem_ipsum是values文件夹下的本地数据
        String[] dummyTexts = getResources().getStringArray(R.array.lorem_ipsum);

        for (String text : dummyTexts) {
            TextView textView = buildLabel(text);
            flowLayout.addView(textView);
        }
    }

    private TextView buildLabel(String text) {
        TextView textView = new TextView(this);
        textView.setText(text);
        textView.setTextSize(TypedValue.COMPLEX_UNIT_SP, 16);
        textView.setPadding((int)dpToPx(16), (int)dpToPx(8), (int)dpToPx(16), (int)dpToPx(8));
        textView.setBackgroundResource(R.drawable.label_bg);

        return textView;
    }

    private float dpToPx(float dp){
        return TypedValue.applyDimension(
                TypedValue.COMPLEX_UNIT_DIP, dp, getResources().getDisplayMetrics());
    }
}

四、拓展

1、自定义布局的时候，遇到Error inflating class错误，先看看是否重写了至少两个构造方法。
2、通过adapter管理ViewGroup：
https://www.cnblogs.com/best-hym/p/12683944.html
3、自定义View基础：https://github.com/GcsSloop/AndroidNote
4、点赞头像层叠：https://github.com/LineChen/PileLayout

本文地址：https://blog.csdn.net/u013750244/article/details/107927552