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Android学习笔记——自定义View

2018-05-11 | Comments

1、自定义View
(1)整体结构及工作流程
(2)measure
(3)layout
(4)draw
2、measure过程
(1)MeasureSpec(measureSpec/mode/size)
(2)父View宽高的测量方法
(3)子View宽高的测量方法
3、layout过程
(1)left/top/right/bottom含义
(2)left/top/right/bottom计算
4、draw过程
(1)绘制顺序
5、源码举例
(1)LinearLayout
(2)ListView
(3)ScrollView
6、参考文档

1、自定义View

(1)整体结构及工作流程

Activity、Window、DecorView之间的关系:

Activity:相当于一个Controller,具备生命周期。

Window:相当于窗口,是视图的承载器,PhoneWindow是唯一实现类。

DecorView:顶级View,是一个Framelayout,包含StatusBar、TitleBar+ContentView、NavigationBar三个部分。
StatusBar是状态栏;
TitleBar对应各种ActionBar;
ContentView对应R.id.content,setContentView设置的View被添加到R.id.content对应的View上,可通过findViewById(android.id.content)得到ContentView,findViewById(android.id.content).getChildAt(0)得到设置进去的View;
NavigationBar是虚拟按键。

延伸:沉浸式状态栏本质是将状态栏、导航栏隐藏,实现应用界面全屏化。 状态栏及导航栏相关知识小结

ViewRoot:实现类是ViewRootImpl,它是连接WindowManager和DecorView的纽带,View的三大流程是通过ViewRoot来完成的。

View的工作流程:

  1. 最先从ViewRoot.performTraversals()方法开始
  2. DecorView的绘制:调用DecorView的performMeasure,performLayout,performDraw三个方法。DecorView的measure顺序:performMeasure -> measure -> onMeasure;DecorView的layout顺序:performLayout -> layout -> onLayout;DecorView的draw顺序:performDraw -> draw -> onDraw。然后onMeasure/onLayout/onDraw又会调用child.measure()/child.layout()/child.onDraw()将这个过程传给child。
  3. 子View的绘制:measure过程 measure -> onMeasure;layout -> onLayout;draw -> onDraw
  4. 依次往下传递。

measure过程:得到所有View体系的宽高
layout过程:得到所有View的坐标
draw过程:绘制所有View

关于requestLayout()/invalidate():
requestLayout()会调用到ViewRoot.performTraversals(),触发measure、layout和draw过程;postInvalidate()和invalidate()会出发当前View的draw过程,但是前者在非UI线程,后者在UI线程调用。
Android View 深度分析requestLayout、invalidate与postInvalidate

整个过程的简略版代码,以LinearLayout为例:

(2)measure

measure(int widthMeasureSpec, int heightMeasureSpec)做的事情:

  1. 保存widthMeasureSpec和heightMeasureSpec
  2. 调用onMeasure(widthMeasureSpec, heightMeasureSpec)方法

onMeasure(int widthMeasureSpec, int heightMeasureSpec)做的事情:

  1. 调用child.measure(childWidthMeasureSpec, childHeightMeasureSpec)测量子View的宽高
  2. 得到当前View的总宽高,并调用setMeasuredDimension(totalWidth, totalHeight)保存总宽高

setMeasuredDimension(int measuredWidth, int measuredHeight)做的事情:

  1. 保存当前View的总宽高

具体代码如下:

//LinearLayout

@Override
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
    ...
    mOldWidthMeasureSpec = widthMeasureSpec;
    mOldHeightMeasureSpec = heightMeasureSpec;
    ...
    onMeasure(widthMeasureSpec, heightMeasureSpec);
    ...
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    if (mOrientation == VERTICAL) {
        measureVertical(widthMeasureSpec, heightMeasureSpec);
    } else {
        measureHorizontal(widthMeasureSpec, heightMeasureSpec);
    }
}

void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
    int totalWidth = ...;
    int totalHeight = 0;
    for (int i = 0; i < getChildCount(); i++) {
        measureChild(child, widthMeasureSpec, heightMeasureSpec);
        totalHeight += child.getMeasureHeight();
    }
    ...
    setMeasuredDimension(totalWidth, totalHeight);
}

protected void measureChild(View child, int parentWidthMeasureSpec,
        int parentHeightMeasureSpec) {
    final LayoutParams lp = child.getLayoutParams();

    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
            mPaddingLeft + mPaddingRight, lp.width);
    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
            mPaddingTop + mPaddingBottom, lp.height);

    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

private void setMeasuredDimension(int measuredWidth, int measuredHeight) {
    mMeasuredWidth = measuredWidth;
    mMeasuredHeight = measuredHeight;
}

(3)layout

layout(int l, int t, int r, int b)做的事情:

  1. 保存当前View的坐标
  2. 调用onLayout(changed, l, t, r, b)方法

onLayout(boolean changed, int l, int t, int r, int b)做的事情:

  1. 计算子View的坐标
  2. 调用child.layout(childLeft, childTop, childRight, childBottom)将子View的坐标传给子View

具体代码如下:

//LinearLayout

@Override
public void layout(int l, int t, int r, int b) {
    ...        
    mLeft = l;
    mTop = t;
    mRight = r;
    mBottom = b;
    ...
    onLayout(changed, l, t, r, b);    
    ...
}

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
    if (mOrientation == VERTICAL) {
        layoutVertical(l, t, r, b);
    } else {
        layoutHorizontal(l, t, r, b);
    }
}

void layoutVertical(int left, int top, int right, int bottom) {
    for (int i = 0; i < getChildCount(); i++) {
        ...
        childLeft = ...;
        childTop = ...;
        childRight = childLeft + child.getMeasuredWidth();            
        childBottom = childTop + child.getMeasuredHeight();

        child.layout(childLeft, childTop, childRight, childBottom);
    }

}

(4)draw

draw(Canvas canvas)做的事情:

  1. 调用drawBackground(canvas)绘制背景
  2. save canvas‘ layers
  3. 调用onDraw(canvas)绘制内容
  4. 调用dispatchDraw(canvas)绘制子View
  5. 绘制fade effect和restore layers
  6. 调用onDrawScrollBars(canvas)绘制滚动条

onDraw(canvas)做的事情:

  1. 绘制当前View的内容

dispatchDraw(Canvas canvas)做的事情:

  1. 调用child.draw(canvas)绘制子View

具体代码如下:

//LinearLayout

public void draw(Canvas canvas) {
    // Step 1, draw the background, if needed
    drawBackground(canvas);
    
    // Step 2, save the canvas' layers(非必需)
    
    // Step 3, draw the content
    onDraw(canvas);

    // Step 4, draw the children
    dispatchDraw(canvas);

    // Step 5, draw the fade effect and restore layers(非必需)
    
    // Step 6, draw decorations (scrollbars)
    onDrawScrollBars(canvas);      
}

@Override
protected void dispatchDraw(Canvas canvas) {
    for (int i = 0; i < getChildCount(); i++) {
        drawChild(canvas, child);
    }
}

protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
    return child.draw(canvas, this, drawingTime);
}

2、measure过程

(1)MeasureSpec(measureSpec/mode/size)

measureSpec:32位的int值,高2位代表mode,低30位 代表size。

mode是测量模式,包括:
UNSPECIFIED:父View对当前View没啥限制,当前View可以是任意大小
EXACTLY:父View决定了当前View的精确大小
AT_MOST:当前View最高可以到某个大小

size指某种测量模式下的规格大小。

public static class MeasureSpec {
    private static final int MODE_SHIFT = 30;
    private static final int MODE_MASK  = 0x3 << MODE_SHIFT;

    public static final int UNSPECIFIED = 0 << MODE_SHIFT;
    public static final int EXACTLY     = 1 << MODE_SHIFT;
    public static final int AT_MOST     = 2 << MODE_SHIFT;

    public static int makeMeasureSpec(int size, int mode) {
        return (size & ~MODE_MASK) | (mode & MODE_MASK);
    }

    public static int getMode(int measureSpec) {
        return (measureSpec & MODE_MASK);
    }

    public static int getSize(int measureSpec) {
        return (measureSpec & ~MODE_MASK);
    }
}

(2)父View宽高的测量方法

根据父View中子View的布局样式,可能受子View的宽高影响。
如布局为水平方向,则宽度为子View宽度的累加;布局为垂直方向,则高度为子View高度的累加。其它根据实际情况而定。
宽高计算出来之后之后,调用setMeasuredDimension()设置宽高的值。

宽高的计算过程中,可能需要累加各种padding、margin、分割线之类的。

(3)子View宽高的测量方法

测量子View的方法measureChild:
(1)先计算出子View的measureSpec,即childMeasureSpec
(2)再根据child.measure(childWidthMeasureSpec, childHeightMeasureSpec)测量子View的大小

测量完成之后,可以根据getMeasuredWidth()和getMeasuredHeight()得到测量出来的宽高值

protected void measureChild(View child, int parentWidthMeasureSpec,
        int parentHeightMeasureSpec) {
    final LayoutParams lp = child.getLayoutParams();

    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
            mPaddingLeft + mPaddingRight, lp.width);
    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
            mPaddingTop + mPaddingBottom, lp.height);

    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

childMeasureSpec的计算:
由parentMeasureSpec和childDimension共同约束。
childDimension是LayoutParams中的width和height,要得到childMeasureSpec,需要先得到childSpecMode和childSpecSize。

public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
    //父View的mode和size
    int specMode = MeasureSpec.getMode(spec);
    int specSize = MeasureSpec.getSize(spec);

    //子View最大大小(父View的大小-内边距)
    int size = Math.max(0, specSize - padding);

    int resultSize = 0;
    int resultMode = 0;

    switch (specMode) {
    // Parent has imposed an exact size on us
    case MeasureSpec.EXACTLY:
        if (childDimension >= 0) {
            //有精确值的情况
            resultSize = childDimension;
            resultMode = MeasureSpec.EXACTLY;
        } else if (childDimension == LayoutParams.MATCH_PARENT) {
            //填满父容器,而父容器又是一个精确值,因此子View也是精确模式
            // Child wants to be our size. So be it.
            resultSize = size;
            resultMode = MeasureSpec.EXACTLY;
        } else if (childDimension == LayoutParams.WRAP_CONTENT) {
            //内容自适应,而父容器又是一个精确值,因此子View有最大值
            // Child wants to determine its own size. It can't be bigger than us.
            resultSize = size;
            resultMode = MeasureSpec.AT_MOST;
        }
        break;

    // Parent has imposed a maximum size on us
    case MeasureSpec.AT_MOST:
        if (childDimension >= 0) { 
            //有精确值的情况
            // Child wants a specific size... so be it
            resultSize = childDimension;
            resultMode = MeasureSpec.EXACTLY;
        } else if (childDimension == LayoutParams.MATCH_PARENT) {
            //填满父容器,而父容器又是最大值模式,因此子View也是最大值模式
            // Child wants to be our size, but our size is not fixed.
            // Constrain child to not be bigger than us.
            resultSize = size;
            resultMode = MeasureSpec.AT_MOST;
        } else if (childDimension == LayoutParams.WRAP_CONTENT) {
            //内容自适应,而父容器又是最大值模式,因此子View也是最大值模式
            // Child wants to determine its own size. It can't be bigger than us.
            resultSize = size;
            resultMode = MeasureSpec.AT_MOST;
        }
        break;

    // Parent asked to see how big we want to be
    case MeasureSpec.UNSPECIFIED:
        if (childDimension >= 0) {
            //有精确值的情况
            // Child wants a specific size... let him have it
            resultSize = childDimension;
            resultMode = MeasureSpec.EXACTLY;
        } else if (childDimension == LayoutParams.MATCH_PARENT) {
            //填满父容器,而父容器是未指定模式,因此子View也是未指定模式
            // Child wants to be our size... find out how big it should be
            resultSize = 0;
            resultMode = MeasureSpec.UNSPECIFIED;
        } else if (childDimension == LayoutParams.WRAP_CONTENT) {
            //内容自适应,而父容器是未指定模式,因此子View也是未指定模式
            // Child wants to determine its own size.... find out how
            // big it should be
            resultSize = 0;
            resultMode = MeasureSpec.UNSPECIFIED;
        }
        break;
    }
    //根据mode和size得到measureSpec
    return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

前面举的例子是LinearLayout,它复写了onMeasure方法。
如果不复写onMeasure()的话,当前View的默认宽高的实现如图:

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
            getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}

//默认大小,由自身的最小宽高和自身的measureSpec共同决定
public static int getDefaultSize(int size, int measureSpec) {
    int result = size;
    int specMode = MeasureSpec.getMode(measureSpec);
    int specSize = MeasureSpec.getSize(measureSpec);

    switch (specMode) {
    case MeasureSpec.UNSPECIFIED:
        result = size;
        break;
    case MeasureSpec.AT_MOST:
    case MeasureSpec.EXACTLY:
        result = specSize;
        break;
    }
    return result;
}

//建议的最小宽度,由layout:minWidth属性和mBackground的最小宽度共同决定
protected int getSuggestedMinimumWidth() {
    return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}

3、layout过程

(1)left/top/right/bottom含义

是相对坐标,相对于父View的坐标。

(2)left/top/right/bottom计算

无非是结合各种padding、margin、divider来计算。

还是以LinearLayout为例(简化版):

void layoutVertical(int left, int top, int right, int bottom) {
    //父View的内边距
    int childTop = mPaddingTop;
    for (int i = 0; i < count; i++) {
        //分割线
        childTop += mDividerHeight;
        //topMargin
        childTop += lp.topMargin;
        
        childBottom = childTop + childHeight;
        
        //父View的内边距和子View的leftMargin
        childLeft = mPaddingLeft + lp.leftMargin;
        
        childRight = childLeft + childWidth;
        
        child.layout(childLeft, childTop, childRight, childBottom);

        childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
    }
}

4、draw过程

(1)绘制顺序

上面已经介绍了。

5、源码举例

自定义View的套路:
(1)复写的方法无非onMeasure、onLayout、onDraw/dispatchDraw,根据情况,有可能复写其中的某几个。
(2)onMeasure方法中调用child.measure()测量子View大小,并计算当前View的大小。如果当前View的大小是默认计算方式,则不需要复写该方法。
(3)onLayout方法中调用child.layout()设置子View的坐标位置,并保存当前View的位置坐标。
(4)onDraw绘制当前View的内容,dispatchDraw绘制子View的内容,一般不复写dispatchDraw。

下面几个View源码的套路都差不多,ListView在该套路基础上增加了Recycler复用机制。

(1)LinearLayout

(2)ListView

(3)ScrollView

可以参考FlowLayout的源码:https://github.com/Mr-YangCheng/ForAndroidInterview/blob/master/android/Android%20%E8%87%AA%E5%AE%9A%E4%B9%89ViewGroup%E5%85%A5%E9%97%A8%E5%AE%9E%E8%B7%B5.md

6、参考文档

(1)setContentView 背后那些事儿
(2)Window、Activity、DecorView以及ViewRoot之间的关系
(3)View测量、布局及绘制原理

本文原文发自 某学姐, 转载请保留出处, 谢谢.

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