android开发之做一个竖着的seekbar

大家好，又见面了，我是全栈君。

工作繁忙，总想有机会停下来喘口气，整理一下思路，做一些积累，但是这样的机会很少。要做android的界面，基础的要求是熟悉那些内置的组件（prebuilt componments），而有时候这些组件是不够用的，这时候我们就需要自己写一个新的，或者修改内置的。这就要求我们了解更多知识，比如各个View是怎么画出来的，UI上各种事件是如何分发和处理的等等。这些我都计划写出来和大家分享，网络上中文资料，说得清楚的太少了，就那么几篇被广泛的转载。今天说一说如何做一个vertical seekbar，竖起来的seekbar。实现它的过程中就会涉及一些View是如何被画出来的知识。通过本文你可以实现一个verticalseekbar，你也可以学到一些使用seekbar的知识。

我的一个同事很牛，需要自定义的组件时，从framework中抠出来相关（功能相似、在继承关系的hierarchy中地位相当）的代码（类），在自己的应用程序中做修改，实现自己想要的功能。需要的style、drawable等资源，有些是在应用程序里用不了的，就也自己做一份。我们公司做整个系统，从硬件到android到android上的应用系统， framework我们也修改，主要是功能上的增减。在framework里增加一个verticalseekbar我们也能做到，但是这样兼容性就被破坏了，所以这位仁兄把需要的拿出来在自己的程序中实现。

我做vertical seekbar也采取这个策略，不和大家啰嗦实现过程中太多因果逻辑，我想很多读者读这篇文章就是为了做出来一个vertical seekbar。为了方便手头没有代码和开发环境的读者，我会贴上framework中的相关代码。

vertical seekbar当然要实现seekbar一样的功能，所以首先seekbar有的vertical seekbar都要有，直接在你的工程中新建一个类vertical seekbar，从framework中把seekbar的代码复制过来，做一些必要的修改。我们来看看seekbar做了些什么.

seekbar代码：

/*
 * Copyright (C) 2006 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 android.widget;
import android.content.Context;
import android.util.AttributeSet;
/**
 * A SeekBar is an extension of ProgressBar that adds a draggable thumb. The user can touch
 * the thumb and drag left or right to set the current progress level or use the arrow keys.
 * Placing focusable widgets to the left or right of a SeekBar is discouraged. 
 * <p>
 * Clients of the SeekBar can attach a {@link SeekBar.OnSeekBarChangeListener} to
 * be notified of the user's actions.
 *
 * @attr ref android.R.styleable#SeekBar_thumb
 */
public class SeekBar extends AbsSeekBar {
    /**
     * A callback that notifies clients when the progress level has been
     * changed. This includes changes that were initiated by the user through a
     * touch gesture or arrow key/trackball as well as changes that were initiated
     * programmatically.
     */
    public interface OnSeekBarChangeListener {
        
        /**
         * Notification that the progress level has changed. Clients can use the fromUser parameter
         * to distinguish user-initiated changes from those that occurred programmatically.
         * 
         * @param seekBar The SeekBar whose progress has changed
         * @param progress The current progress level. This will be in the range 0..max where max
         *        was set by {@link ProgressBar#setMax(int)}. (The default value for max is 100.)
         * @param fromUser True if the progress change was initiated by the user.
         */
        void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser);
    
        /**
         * Notification that the user has started a touch gesture. Clients may want to use this
         * to disable advancing the seekbar. 
         * @param seekBar The SeekBar in which the touch gesture began
         */
        void onStartTrackingTouch(SeekBar seekBar);
        
        /**
         * Notification that the user has finished a touch gesture. Clients may want to use this
         * to re-enable advancing the seekbar. 
         * @param seekBar The SeekBar in which the touch gesture began
         */
        void onStopTrackingTouch(SeekBar seekBar);
    }
    private OnSeekBarChangeListener mOnSeekBarChangeListener;
    
    public SeekBar(Context context) {
        this(context, null);
    }
    
    public SeekBar(Context context, AttributeSet attrs) {
        this(context, attrs, com.android.internal.R.attr.seekBarStyle);
    }
    public SeekBar(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }
    @Override
    void onProgressRefresh(float scale, boolean fromUser) {
        super.onProgressRefresh(scale, fromUser);
        if (mOnSeekBarChangeListener != null) {
            mOnSeekBarChangeListener.onProgressChanged(this, getProgress(), fromUser);
        }
    }
    /**
     * Sets a listener to receive notifications of changes to the SeekBar's progress level. Also
     * provides notifications of when the user starts and stops a touch gesture within the SeekBar.
     * 
     * @param l The seek bar notification listener
     * 
     * @see SeekBar.OnSeekBarChangeListener
     */
    public void setOnSeekBarChangeListener(OnSeekBarChangeListener l) {
        mOnSeekBarChangeListener = l;
    }
    
    @Override
    void onStartTrackingTouch() {
        if (mOnSeekBarChangeListener != null) {
            mOnSeekBarChangeListener.onStartTrackingTouch(this);
        }
    }
    
    @Override
    void onStopTrackingTouch() {
        if (mOnSeekBarChangeListener != null) {
            mOnSeekBarChangeListener.onStopTrackingTouch(this);
        }
    }
    
}

它定义了一个接口OnSeekBarChangeListener，接口中定义了三个方法：

        void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser);
        void onStartTrackingTouch(SeekBar seekBar);
        void onStopTrackingTouch(SeekBar seekBar);

看名字也知道它们是做什么的，代码也给了注释。那么它们是怎么做到的呢？seekbar中声明了一个该接口的对象mOnSeekBarChangeListener，方法setOnSeekBarChangeListener()将给它赋值。在你的程序中定义一个对象，实现OnSeekBarChangeListener，并调用setOnSeekBarChangeListener()把这个对象传给mOnSeekBarChangeListener。在seekbar中还有几个override的方法：

void onProgressRefresh(float scale, boolean fromUser)

void onStartTrackingTouch()
void onStopTrackingTouch()

这些方法会在发生了相应事件的时候被调用，而它们又相应地去调用mOnSeekBarChangeListener中的方法，于是你的程序的定义的响应各种事件的方法就被执行了。

但是这些还不够，我们要让seekbar竖起来，要让thumb上下滑动，实现这些特性的代码在什么地方呢？我们顺着seekbar的继承关系向上找。来看看absseekbar。

absseekbar的代码：

/*
* 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 android.widget;
import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.Rect;
import android.graphics.drawable.Drawable;
import android.util.AttributeSet;
import android.view.KeyEvent;
import android.view.MotionEvent;
public abstract class AbsSeekBar extends ProgressBar {
private Drawable mThumb;
private int mThumbOffset;
/**
* On touch, this offset plus the scaled value from the position of the
* touch will form the progress value. Usually 0.
*/
float mTouchProgressOffset;
/**
* Whether this is user seekable.
*/
boolean mIsUserSeekable = true;
/**
* On key presses (right or left), the amount to increment/decrement the
* progress.
*/
private int mKeyProgressIncrement = 1;
private static final int NO_ALPHA = 0xFF;
private float mDisabledAlpha;
public AbsSeekBar(Context context) {
super(context);
}
public AbsSeekBar(Context context, AttributeSet attrs) {
super(context, attrs);
}
public AbsSeekBar(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
TypedArray a = context.obtainStyledAttributes(attrs,
com.android.internal.R.styleable.SeekBar, defStyle, 0);
Drawable thumb = a.getDrawable(com.android.internal.R.styleable.SeekBar_thumb);
setThumb(thumb);
int thumbOffset =
a.getDimensionPixelOffset(com.android.internal.R.styleable.SeekBar_thumbOffset, 0);
setThumbOffset(thumbOffset);
a.recycle();
a = context.obtainStyledAttributes(attrs,
com.android.internal.R.styleable.Theme, 0, 0);
mDisabledAlpha = a.getFloat(com.android.internal.R.styleable.Theme_disabledAlpha, 0.5f);
a.recycle();
}
/**
* Sets the thumb that will be drawn at the end of the progress meter within the SeekBar
* 
* @param thumb Drawable representing the thumb
*/
public void setThumb(Drawable thumb) {
if (thumb != null) {
thumb.setCallback(this);
}
mThumb = thumb;
invalidate();
}
/**
* @see #setThumbOffset(int)
*/
public int getThumbOffset() {
return mThumbOffset;
}
/**
* Sets the thumb offset that allows the thumb to extend out of the range of
* the track.
* 
* @param thumbOffset The offset amount in pixels.
*/
public void setThumbOffset(int thumbOffset) {
mThumbOffset = thumbOffset;
invalidate();
}
/**
* Sets the amount of progress changed via the arrow keys.
* 
* @param increment The amount to increment or decrement when the user
*            presses the arrow keys.
*/
public void setKeyProgressIncrement(int increment) {
mKeyProgressIncrement = increment < 0 ? -increment : increment;
}
/**
* Returns the amount of progress changed via the arrow keys.
* <p>
* By default, this will be a value that is derived from the max progress.
* 
* @return The amount to increment or decrement when the user presses the
*         arrow keys. This will be positive.
*/
public int getKeyProgressIncrement() {
return mKeyProgressIncrement;
}
@Override
public synchronized void setMax(int max) {
super.setMax(max);
if ((mKeyProgressIncrement == 0) || (getMax() / mKeyProgressIncrement > 20)) {
// It will take the user too long to change this via keys, change it
// to something more reasonable
setKeyProgressIncrement(Math.max(1, Math.round((float) getMax() / 20)));
}
}
@Override
protected boolean verifyDrawable(Drawable who) {
return who == mThumb || super.verifyDrawable(who);
}
@Override
protected void drawableStateChanged() {
super.drawableStateChanged();
Drawable progressDrawable = getProgressDrawable();
if (progressDrawable != null) {
progressDrawable.setAlpha(isEnabled() ? NO_ALPHA : (int) (NO_ALPHA * mDisabledAlpha));
}
if (mThumb != null && mThumb.isStateful()) {
int[] state = getDrawableState();
mThumb.setState(state);
}
}
@Override
void onProgressRefresh(float scale, boolean fromUser) { 
Drawable thumb = mThumb;
if (thumb != null) {
setThumbPos(getWidth(), thumb, scale, Integer.MIN_VALUE);
/*
* Since we draw translated, the drawable's bounds that it signals
* for invalidation won't be the actual bounds we want invalidated,
* so just invalidate this whole view.
*/
invalidate();
}
}
@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
Drawable d = getCurrentDrawable();
Drawable thumb = mThumb;
int thumbHeight = thumb == null ? 0 : thumb.getIntrinsicHeight();
// The max height does not incorporate padding, whereas the height
// parameter does
int trackHeight = Math.min(mMaxHeight, h - mPaddingTop - mPaddingBottom);
int max = getMax();
float scale = max > 0 ? (float) getProgress() / (float) max : 0;
if (thumbHeight > trackHeight) {
if (thumb != null) {
setThumbPos(w, thumb, scale, 0);
}
int gapForCenteringTrack = (thumbHeight - trackHeight) / 2;
if (d != null) {
// Canvas will be translated by the padding, so 0,0 is where we start drawing
d.setBounds(0, gapForCenteringTrack, 
w - mPaddingRight - mPaddingLeft, h - mPaddingBottom - gapForCenteringTrack
- mPaddingTop);
}
} else {
if (d != null) {
// Canvas will be translated by the padding, so 0,0 is where we start drawing
d.setBounds(0, 0, w - mPaddingRight - mPaddingLeft, h - mPaddingBottom
- mPaddingTop);
}
int gap = (trackHeight - thumbHeight) / 2;
if (thumb != null) {
setThumbPos(w, thumb, scale, gap);
}
}
}
/**
* @param gap If set to {@link Integer#MIN_VALUE}, this will be ignored and
*/
private void setThumbPos(int w, Drawable thumb, float scale, int gap) {
int available = w - mPaddingLeft - mPaddingRight;
int thumbWidth = thumb.getIntrinsicWidth();
int thumbHeight = thumb.getIntrinsicHeight();
available -= thumbWidth;
// The extra space for the thumb to move on the track
available += mThumbOffset * 2;
int thumbPos = (int) (scale * available);
int topBound, bottomBound;
if (gap == Integer.MIN_VALUE) {
Rect oldBounds = thumb.getBounds();
topBound = oldBounds.top;
bottomBound = oldBounds.bottom;
} else {
topBound = gap;
bottomBound = gap + thumbHeight;
}
// Canvas will be translated, so 0,0 is where we start drawing
thumb.setBounds(thumbPos, topBound, thumbPos + thumbWidth, bottomBound);
}
@Override
protected synchronized void onDraw(Canvas canvas) {
super.onDraw(canvas);
if (mThumb != null) {
canvas.save();
// Translate the padding. For the x, we need to allow the thumb to
// draw in its extra space
canvas.translate(mPaddingLeft - mThumbOffset, mPaddingTop);
mThumb.draw(canvas);
canvas.restore();
}
}
@Override
protected synchronized void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
Drawable d = getCurrentDrawable();
int thumbHeight = mThumb == null ? 0 : mThumb.getIntrinsicHeight();
int dw = 0;
int dh = 0;
if (d != null) {
dw = Math.max(mMinWidth, Math.min(mMaxWidth, d.getIntrinsicWidth()));
dh = Math.max(mMinHeight, Math.min(mMaxHeight, d.getIntrinsicHeight()));
dh = Math.max(thumbHeight, dh);
}
dw += mPaddingLeft + mPaddingRight;
dh += mPaddingTop + mPaddingBottom;
setMeasuredDimension(resolveSize(dw, widthMeasureSpec),
resolveSize(dh, heightMeasureSpec));
}
@Override
public boolean onTouchEvent(MotionEvent event) {
if (!mIsUserSeekable || !isEnabled()) {
return false;
}
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
setPressed(true);
onStartTrackingTouch();
trackTouchEvent(event);
break;
case MotionEvent.ACTION_MOVE:
trackTouchEvent(event);
attemptClaimDrag();
break;
case MotionEvent.ACTION_UP:
trackTouchEvent(event);
onStopTrackingTouch();
setPressed(false);
break;
case MotionEvent.ACTION_CANCEL:
onStopTrackingTouch();
setPressed(false);
break;
}
return true;
}
private void trackTouchEvent(MotionEvent event) {
final int width = getWidth();
final int available = width - mPaddingLeft - mPaddingRight;
int x = (int)event.getX();
float scale;
float progress = 0;
if (x < mPaddingLeft) {
scale = 0.0f;
} else if (x > width - mPaddingRight) {
scale = 1.0f;
} else {
scale = (float)(x - mPaddingLeft) / (float)available;
progress = mTouchProgressOffset;
}
final int max = getMax();
progress += scale * max;
setProgress((int) progress, true);
}
/**
* Tries to claim the user's drag motion, and requests disallowing any
* ancestors from stealing events in the drag.
*/
private void attemptClaimDrag() {
if (mParent != null) {
mParent.requestDisallowInterceptTouchEvent(true);
}
}
/**
* This is called when the user has started touching this widget.
*/
void onStartTrackingTouch() {
}
/**
* This is called when the user either releases his touch or the touch is
* canceled.
*/
void onStopTrackingTouch() {
}
/**
* Called when the user changes the seekbar's progress by using a key event.
*/
void onKeyChange() {
}
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
int progress = getProgress();
switch (keyCode) {
case KeyEvent.KEYCODE_DPAD_LEFT:
if (progress <= 0) break;
setProgress(progress - mKeyProgressIncrement, true);
onKeyChange();
return true;
case KeyEvent.KEYCODE_DPAD_RIGHT:
if (progress >= getMax()) break;
setProgress(progress + mKeyProgressIncrement, true);
onKeyChange();
return true;
}
return super.onKeyDown(keyCode, event);
}
}

连上注释372行，不多。我们看到有onTouchEvent()，trackTouchEvent()，onProgressRefresh()，setThumbPos()等一些方法，这些都是和seekbar功能相关的方法，去看看它们的注释。

onTouchEvent()是在View中就有定义的方法，（touch）事件处理我日后有空我会再发文章讨论，现在我们只要知道，在用户触摸seekbar（absseekbar）时，这个方法会响应touch事件就够了。onTouchEvent()会判断用户的动作，然后调用前述的一些方法（注意去看代码），注意它调用的顺序，判断是ACTION_DOWN时先调用onStartTrackingTouch()再调用trackTouchEvent(event)，于是在你的程序中定义的 onStartTrackingTouch()先执行了；判断是ACTION_UP时先调用trackTouchEvent(event)再调用onStopTrackingTouch()，至于为什么我就不啰嗦了，有时候这个顺序是很重要的。

trackTouchEvent()根据touch的位置来设置进度，它计算出progress后调用setProgress()，setProgress()是progressbar中实现的方法。

当seekbar的progress改变时onProgressRefresh()会被调用，它会调用setThumbPos()来设置thumb的位置。

大致过程就是这样，要更深入细致的了解还是得认真得看看代码。

现在我们知道，seekbar拥有的progressbar之外的功能，都是在absseekbar中实现的。系统默认seekbar是横着的，在absseekbar中计算progress、计算thumb的位置，用到了seekbar的宽和高，我们只要改变一下计算方法就能计算出竖着的seekbar所需要的各个数值。例如：

    private void trackTouchEvent(MotionEvent event) {

        final int Height = getHeight();
        Log.d(“demon”, “Y:”+event.getY()+” height:”+Height);
        final int available = Height – getPaddingBottom() – getPaddingTop();
        int Y = (int)event.getY();
        float scale;
        float progress = 0;
        if (Y > Height – getPaddingBottom()) {

            scale = 0.0f;
        } else if (Y < getPaddingTop()) {

            scale = 1.0f;
        } else {

            scale = (float)(Height – getPaddingBottom()-Y) / (float)available;
        }

        final int max = getMax();
        progress = scale * max;
        setProgress((int) progress);
    }

其他的地方你可以仿造这个写。

功能上分析告一段落，我们如何画出来一个竖着的seekbar呢？

我想了两个办法，一是自己画，也就是自己写个draw，太难，而且很难和系统风格统一。另外一个办法是让系统给我们画。verticalseekbar只是seekbar转了90度或-90度，我们可以把画布转一个角度，然后交给系统去画，具体的做法就是在ondraw()时调整画布，然后调用super.onDraw()。

这个调整也就是旋转和平移。至于旋转和平移的具体实现，我跟到native部分就没有看下去了，也没有看到具体的介绍。我的理解，你要保证画布的左上角在旋转平移以后坐标不变，否者会出现很多问题。就我们的seekbar而言，如果你要获得一个向上增长的seekbar，那么代码应该是：

protected void onDraw(Canvas c)
        {

                c.rotate(-90);
                c.translate(-height,0);//height是你的verticalseekbar的高
                super.onDraw(c);
        }

如果是向下的seekbar则应该是：

protected void onDraw(Canvas c)
        {

                c.rotate(90);
                c.translate(0,-width);//width是你的verticalseekbar的宽
                super.onDraw(c);
        }

你也可以自己计算一下来验证。

一个View在屏幕上画出来，首先measure()会被调用，这是个final方法，measure()会调用onMeasure()。你可以重写onMeasure()，但是重写的onMeasure()中必须调用setMeasuredDimension(int,int)， setMeasuredDimension()会保存该View的长和宽。我们可以重写onMeasure()方法给我们的seekbar分配一块竖直的，长宽由我们设定的区域。在画seekbar之前，我们把这个区域旋转了90度交给系统，系统给我们画一个标准的seekbar，在整个layout中，这个seekbar就成竖直的了。

实际操作时，我们在工程中新建了类verticalSeekbar，把framework中seekbar的代码拷贝过来，做一些必须的修改(类名、构造方法什么的)。注意如果我们的verticalseekbar是在XML中定义的，在代码中使用findViewById()方法来获得，系统将使用第二个构造方法(我简单地试了一下，没有看到官方资料或权威的解释)，也就是

    public SeekBar(Context context, AttributeSet attrs) {

        this(context, attrs, com.android.internal.R.attr.seekBarStyle);
    }

com.android.internal.R.attr.seekBarStyle在我我们的应用程序中是无法使用的，你可以换成android.R.attr.seekBarStyle，你也可以自己定义一个，但是必须包含系统本身的。

然后我们要加一些方法，也就是重写absseekbar中的一些方法，大致如下：

verticalSeekbar.java的大致内容