Android中View的绘制过程 onMeasure方法简述附有自定义View例子Android中View的绘制过程 onMeasure方法简述附有自定义View例子

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我是靠谱客的博主糊涂小鸭子，这篇文章主要介绍Android中View的绘制过程 onMeasure方法简述附有自定义View例子Android中View的绘制过程 onMeasure方法简述附有自定义View例子，现在分享给大家，希望可以做个参考。

Android中View的绘制过程 onMeasure方法简述附有自定义View例子

Android中View的绘制过程

　　当Activity获得焦点时，它将被要求绘制自己的布局，Android framework将会处理绘制过程，Activity只需提供它的布局的根节点。

　　绘制过程从布局的根节点开始，从根节点开始测量和绘制整个layout tree。

　　每一个ViewGroup 负责要求它的每一个孩子被绘制，每一个View负责绘制自己。

　　因为整个树是按顺序遍历的，所以父节点会先被绘制，而兄弟节点会按照它们在树中出现的顺序被绘制。

　　绘制是一个两遍（two pass）的过程：一个measure pass和一个layout pass。

　　测量过程（measuring pass）是在measure(int, int)中实现的，是从树的顶端由上到下进行的。

　　在这个递归过程中，每一个View会把自己的dimension specifications传递下去。

　　在measure pass的最后，每一个View都存储好了自己的measurements，即测量结果。

　　第二个是布局过程（layout pass），它发生在 layout(int, int, int, int)中，仍然是从上到下进行（top-down）。

　　在这一遍中，每一个parent都会负责用测量过程中得到的尺寸，把自己的所有孩子放在正确的地方。

尺寸的父子关系处理

　　当一个View对象的 measure() 方法返回时，它的 getMeasuredWidth() 和 getMeasuredHeight()值应该被设置好了，并且它的所有子孙的值也应该一起被设置好了。

　　一个View对象的measured width 和measured height的值必须考虑到它的父容器给它的限制。

　　这样就保证了在measure pass的最后，所有的parent都接受了它的所有孩子的measurements结果。

　　注意：一个parent可能会不止一次地对它的孩子调用measure()方法。

　　比如，第一遍的时候，一个parent可能测量它的每一个孩子，并没有指定尺寸，parent只是为了发现它们想要多大；

　　如果第一遍之后得知，所有孩子的无限制的尺寸总和太大或者太小，parent会再次对它的孩子调用measure()方法，这时候parent会设定规则，介入这个过程，使用实际的值。

　　（即，让孩子自由发展不成，于是家长介入）。

布局属性说明

　　LayoutParams是View用来告诉它的父容器它想要怎样被放置的参数。

　　最基本的LayoutParams基类仅仅描述了View想要多大，即指明了尺寸属性。

　　即View在XML布局时通常需要指明的宽度和高度属性。

　　每一个维度都可以指定成下列三种值之一：

　　1.FILL_PARENT (API Level 8之后重命名为MATCH_PARENT)，表示View想要尽量和它的parent一样大（减去边距）。

　　2.WRAP_CONTENT，表示View想要刚好大到可以包含它的内容（包括边距）。

　　3.具体的数值。

　　ViewGroup的不同子类（不同的布局类）有相应的LayoutParams子类，其中会包含更多的布局相关属性。

onMeasure方法

　　onMeasure方法是测量view和它的内容，决定measured width和measured height的，这个方法由 measure(int, int)方法唤起，子类可以覆写onMeasure来提供更加准确和有效的测量。

　　有一个约定：在覆写onMeasure方法的时候，必须调用 setMeasuredDimension(int,int)来存储这个View经过测量得到的measured width and height。

　　如果没有这么做，将会由measure(int, int)方法抛出一个IllegalStateException。

　　onMeasure方法的声明如下：

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1
protected void onMeasure (int widthMeasureSpec, int heightMeasureSpec)

　　其中两个输入参数：

　　widthMeasureSpec

　　heightMeasureSpec

　　分别是parent提出的水平和垂直的空间要求。

　　这两个要求是按照View.MeasureSpec类来进行编码的。

　　参见View.MeasureSpec这个类的说明：这个类包装了从parent传递下来的布局要求，传递给这个child。

　　每一个MeasureSpec代表了对宽度或者高度的一个要求。

　　每一个MeasureSpec有一个尺寸（size）和一个模式（mode）构成。

　　MeasureSpecs这个类提供了把一个<size, mode>的元组包装进一个int型的方法，从而减少对象分配。当然也提供了逆向的解析方法，从int值中解出size和mode。

　　有三种模式：

　　UNSPECIFIED

　　这说明parent没有对child强加任何限制，child可以是它想要的任何尺寸。

　　EXACTLY

　　Parent为child决定了一个绝对尺寸，child将会被赋予这些边界限制，不管child自己想要多大。

　　AT_MOST

　　Child可以是自己任意的大小，但是有个绝对尺寸的上限。

　　覆写onMeasure方法的时候，子类有责任确保measured height and width至少为这个View的最小height和width。

　　（getSuggestedMinimumHeight() and getSuggestedMinimumWidth()）。

onLayout

　　这个方法是在layout pass中被调用的，用于确定View的摆放位置和大小。方法声明：

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protected void onLayout (boolean changed, int left, int top, int right, int bottom)

　　其中的上下左右参数都是相对于parent的。

　　如果View含有child，那么onLayout中需要对每一个child进行布局。

自定义View Demo

　　API Demos中的LabelView类是一个继承自View的自定义类的例子：

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/*
 * Copyright (C) 2007 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.example.android.apis.view;

// Need the following import to get access to the app resources, since this
// class is in a sub-package.
import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.View;

import com.example.android.apis.R;

/**
 * Example of how to write a custom subclass of View. LabelView
 * is used to draw simple text views. Note that it does not handle
 * styled text or right-to-left writing systems.
 *
 */
public class LabelView extends View {
    private Paint mTextPaint;
    private String mText;
    private int mAscent;
    
    /**
     * Constructor.  This version is only needed if you will be instantiating
     * the object manually (not from a layout XML file).
     * @param context
     */
    public LabelView(Context context) {
        super(context);
        initLabelView();
    }

/**
     * Construct object, initializing with any attributes we understand from a
     * layout file. These attributes are defined in
     * SDK/assets/res/any/classes.xml.
     * 
     * @see android.view.View#View(android.content.Context, android.util.AttributeSet)
     */
    public LabelView(Context context, AttributeSet attrs) {
        super(context, attrs);
        initLabelView();

TypedArray a = context.obtainStyledAttributes(attrs,
                R.styleable.LabelView);

CharSequence s = a.getString(R.styleable.LabelView_text);
        if (s != null) {
            setText(s.toString());
        }

// Retrieve the color(s) to be used for this view and apply them.
        // Note, if you only care about supporting a single color, that you
        // can instead call a.getColor() and pass that to setTextColor().
        setTextColor(a.getColor(R.styleable.LabelView_textColor, 0xFF000000));

int textSize = a.getDimensionPixelOffset(R.styleable.LabelView_textSize, 0);
        if (textSize > 0) {
            setTextSize(textSize);
        }

a.recycle();
    }

private final void initLabelView() {
        mTextPaint = new Paint();
        mTextPaint.setAntiAlias(true);
        // Must manually scale the desired text size to match screen density
        mTextPaint.setTextSize(16 * getResources().getDisplayMetrics().density);
        mTextPaint.setColor(0xFF000000);
        setPadding(3, 3, 3, 3);
    }

/**
     * Sets the text to display in this label
     * @param text The text to display. This will be drawn as one line.
     */
    public void setText(String text) {
        mText = text;
        requestLayout();
        invalidate();
    }

/**
     * Sets the text size for this label
     * @param size Font size
     */
    public void setTextSize(int size) {
        // This text size has been pre-scaled by the getDimensionPixelOffset method
        mTextPaint.setTextSize(size);
        requestLayout();
        invalidate();
    }

/**
     * Sets the text color for this label.
     * @param color ARGB value for the text
     */
    public void setTextColor(int color) {
        mTextPaint.setColor(color);
        invalidate();
    }

/**
     * @see android.view.View#measure(int, int)
     */
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(measureWidth(widthMeasureSpec),
                measureHeight(heightMeasureSpec));
    }

/**
     * Determines the width of this view
     * @param measureSpec A measureSpec packed into an int
     * @return The width of the view, honoring constraints from measureSpec
     */
    private int measureWidth(int measureSpec) {
        int result = 0;
        int specMode = MeasureSpec.getMode(measureSpec);
        int specSize = MeasureSpec.getSize(measureSpec);

if (specMode == MeasureSpec.EXACTLY) {
            // We were told how big to be
            result = specSize;
        } else {
            // Measure the text
            result = (int) mTextPaint.measureText(mText) + getPaddingLeft()
                    + getPaddingRight();
            if (specMode == MeasureSpec.AT_MOST) {
                // Respect AT_MOST value if that was what is called for by measureSpec
                result = Math.min(result, specSize);
            }
        }

return result;
    }

/**
     * Determines the height of this view
     * @param measureSpec A measureSpec packed into an int
     * @return The height of the view, honoring constraints from measureSpec
     */
    private int measureHeight(int measureSpec) {
        int result = 0;
        int specMode = MeasureSpec.getMode(measureSpec);
        int specSize = MeasureSpec.getSize(measureSpec);

mAscent = (int) mTextPaint.ascent();
        if (specMode == MeasureSpec.EXACTLY) {
            // We were told how big to be
            result = specSize;
        } else {
            // Measure the text (beware: ascent is a negative number)
            result = (int) (-mAscent + mTextPaint.descent()) + getPaddingTop()
                    + getPaddingBottom();
            if (specMode == MeasureSpec.AT_MOST) {
                // Respect AT_MOST value if that was what is called for by measureSpec
                result = Math.min(result, specSize);
            }
        }
        return result;
    }

/**
     * Render the text
     * 
     * @see android.view.View#onDraw(android.graphics.Canvas)
     */
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.drawText(mText, getPaddingLeft(), getPaddingTop() - mAscent, mTextPaint);
    }
}

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/*
 * Copyright (C) 2007 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.example.android.apis.view;

// Need the following import to get access to the app resources, since this
// class is in a sub-package.
import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.View;

import com.example.android.apis.R;


/**
 * Example of how to write a custom subclass of View. LabelView
 * is used to draw simple text views. Note that it does not handle
 * styled text or right-to-left writing systems.
 *
 */
public class LabelView extends View {
    private Paint mTextPaint;
    private String mText;
    private int mAscent;
    
    /**
     * Constructor.  This version is only needed if you will be instantiating
     * the object manually (not from a layout XML file).
     * @param context
     */
    public LabelView(Context context) {
        super(context);
        initLabelView();
    }

    /**
     * Construct object, initializing with any attributes we understand from a
     * layout file. These attributes are defined in
     * SDK/assets/res/any/classes.xml.
     * 
     * @see android.view.View#View(android.content.Context, android.util.AttributeSet)
     */
    public LabelView(Context context, AttributeSet attrs) {
        super(context, attrs);
        initLabelView();

        TypedArray a = context.obtainStyledAttributes(attrs,
                R.styleable.LabelView);

        CharSequence s = a.getString(R.styleable.LabelView_text);
        if (s != null) {
            setText(s.toString());
        }

        // Retrieve the color(s) to be used for this view and apply them.
        // Note, if you only care about supporting a single color, that you
        // can instead call a.getColor() and pass that to setTextColor().
        setTextColor(a.getColor(R.styleable.LabelView_textColor, 0xFF000000));

        int textSize = a.getDimensionPixelOffset(R.styleable.LabelView_textSize, 0);
        if (textSize > 0) {
            setTextSize(textSize);
        }

        a.recycle();
    }

    private final void initLabelView() {
        mTextPaint = new Paint();
        mTextPaint.setAntiAlias(true);
        // Must manually scale the desired text size to match screen density
        mTextPaint.setTextSize(16 * getResources().getDisplayMetrics().density);
        mTextPaint.setColor(0xFF000000);
        setPadding(3, 3, 3, 3);
    }

    /**
     * Sets the text to display in this label
     * @param text The text to display. This will be drawn as one line.
     */
    public void setText(String text) {
        mText = text;
        requestLayout();
        invalidate();
    }

    /**
     * Sets the text size for this label
     * @param size Font size
     */
    public void setTextSize(int size) {
        // This text size has been pre-scaled by the getDimensionPixelOffset method
        mTextPaint.setTextSize(size);
        requestLayout();
        invalidate();
    }

    /**
     * Sets the text color for this label.
     * @param color ARGB value for the text
     */
    public void setTextColor(int color) {
        mTextPaint.setColor(color);
        invalidate();
    }

    /**
     * @see android.view.View#measure(int, int)
     */
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(measureWidth(widthMeasureSpec),
                measureHeight(heightMeasureSpec));
    }

    /**
     * Determines the width of this view
     * @param measureSpec A measureSpec packed into an int
     * @return The width of the view, honoring constraints from measureSpec
     */
    private int measureWidth(int measureSpec) {
        int result = 0;
        int specMode = MeasureSpec.getMode(measureSpec);
        int specSize = MeasureSpec.getSize(measureSpec);

        if (specMode == MeasureSpec.EXACTLY) {
            // We were told how big to be
            result = specSize;
        } else {
            // Measure the text
            result = (int) mTextPaint.measureText(mText) + getPaddingLeft()
                    + getPaddingRight();
            if (specMode == MeasureSpec.AT_MOST) {
                // Respect AT_MOST value if that was what is called for by measureSpec
                result = Math.min(result, specSize);
            }
        }

        return result;
    }

    /**
     * Determines the height of this view
     * @param measureSpec A measureSpec packed into an int
     * @return The height of the view, honoring constraints from measureSpec
     */
    private int measureHeight(int measureSpec) {
        int result = 0;
        int specMode = MeasureSpec.getMode(measureSpec);
        int specSize = MeasureSpec.getSize(measureSpec);

        mAscent = (int) mTextPaint.ascent();
        if (specMode == MeasureSpec.EXACTLY) {
            // We were told how big to be
            result = specSize;
        } else {
            // Measure the text (beware: ascent is a negative number)
            result = (int) (-mAscent + mTextPaint.descent()) + getPaddingTop()
                    + getPaddingBottom();
            if (specMode == MeasureSpec.AT_MOST) {
                // Respect AT_MOST value if that was what is called for by measureSpec
                result = Math.min(result, specSize);
            }
        }
        return result;
    }

    /**
     * Render the text
     * 
     * @see android.view.View#onDraw(android.graphics.Canvas)
     */
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.drawText(mText, getPaddingLeft(), getPaddingTop() - mAscent, mTextPaint);
    }
}