关于下拉刷新的实现原理我在上篇文章Android自定义控件之仿美团下拉刷新中已经详细介绍过了，这篇文章主要介绍表盘的动画实现原理
汽车之家的下拉刷新分为三个状态：
第一个状态为下拉刷新状态(pull to refresh)，在这个状态下是一个表盘随着下拉的距离动态改变指针的角度
第二个状态为放开刷新状态(release to refresh)，在这个状态下是指针角度变化的一个动画
第一个状态的实现：
这个效果我们使用自定义View来实现，我们从汽车之家apk中拿到下拉刷新所用到的两张图片：
我们将第一张图片画在画布上作为背景，接着我们根据当前进度值来动态旋转画布，然后再将第二章图片画在画布上，我们看到表针的旋转实则是画布在旋转。
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
//现将第一张图片画在画布上
canvas.drawBitmap(finalBackGroundBitmap,0,0,null);
//旋转画布
canvas.rotate(mCurrentProgress*2.7f,x/2,y/2);
//将第二张图片画在旋转后的画布上
canvas.drawBitmap(finalPointerBitmap, 0, 0, null);
}
自定义View的完整代码如下：
/**
* Created by zhangqi on 15/10/17.
*/
public class AutoHome extends View{
private Bitmap backGroundBitmap;
public Bitmap pointerBitmap;
private int x;
private int y;
private Bitmap finalBackGroundBitmap;
private Bitmap finalPointerBitmap;
private float mCurrentProgress;
public AutoHome(Context context) {
super(context);
init(context);
}
public AutoHome(Context context, AttributeSet attrs) {
super(context, attrs);
init(context);
}
public AutoHome(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
init(context);
}
private void init(Context context) {
backGroundBitmap = Bitmap.createBitmap(BitmapFactory.decodeResource(context.getResources(), R.drawable.load_icon_dial2x));
pointerBitmap = Bitmap.createBitmap(BitmapFactory.decodeResource(context.getResources(), R.drawable.load_icon_pointer2x));
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(measureWidth(widthMeasureSpec),measureWidth(heightMeasureSpec));
x = getMeasuredWidth();
y = getMeasuredHeight();
finalBackGroundBitmap = Bitmap.createScaledBitmap(backGroundBitmap, x, y, false);
finalPointerBitmap = Bitmap.createScaledBitmap(pointerBitmap, x, y, false);
}
private int measureWidth(int widMeasureSpec){
int result = 0;
int size = MeasureSpec.getSize(widMeasureSpec);
int mode = MeasureSpec.getMode(widMeasureSpec);
if (mode == MeasureSpec.EXACTLY){
result = size;
}else{
result = backGroundBitmap.getWidth();
if (mode == MeasureSpec.AT_MOST){
result = Math.min(result,size);
}
}
return result;
}
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
canvas.drawBitmap(finalBackGroundBitmap,0,0,null);
canvas.rotate(mCurrentProgress*2.7f,x/2,y/2);
canvas.drawBitmap(finalPointerBitmap, 0, 0, null);
}
public void setCurrentProgress(float progress){
mCurrentProgress = progress*100;
}
}
接着我们在Activity中用SeekBar来模拟一个进度值，从而传递给我们自定义View
public class MainActivity extends AppCompatActivity {
private SeekBar mSeekBar;
private AutoHome mAutoHome;
private float mCurrentProgress;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mSeekBar = (SeekBar) findViewById(R.id.seekbar);
mAutoHome = (AutoHome) findViewById(R.id.autohome);
mSeekBar.setOnSeekBarChangeListener(new SeekBar.OnSeekBarChangeListener() {
@Override
public void onProgressChanged(SeekBar seekBar, int i, boolean b) {
mCurrentProgress = (float)seekBar.getProgress()/(float)seekBar.getMax();
mAutoHome.setCurrentProgress(mCurrentProgress);
mAutoHome.invalidate();
}
@Override
public void onStartTrackingTouch(SeekBar seekBar) {
}
@Override
public void onStopTrackingTouch(SeekBar seekBar) {
}
});
}
第二个状态的实现：
第二个状态是表针在执行一个旋转的动画，我们可以将表针写成一个自定义View，然后表盘作为背景图片，然后表针View来执行rotate动画即可
/**
* Created by zhangqi on 15/10/27.
*/
public class PointerView extends View {
private int x;
private int y;
private Bitmap finalPointerBitmap;
private Bitmap pointerBitmap;
public PointerView(Context context) {
super(context);
init();
}
public PointerView(Context context, AttributeSet attrs) {
super(context, attrs);
init();
}
public PointerView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
init();
}
private void init() {
pointerBitmap = Bitmap.createBitmap(BitmapFactory.decodeResource(getResources(), R.drawable.load_icon_pointer2x));
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(measureWidth(widthMeasureSpec),measureWidth(heightMeasureSpec));
x = getMeasuredWidth();
y = getMeasuredHeight();
finalPointerBitmap = Bitmap.createScaledBitmap(pointerBitmap, x, y, false);
}
private int measureWidth(int widMeasureSpec){
int result = 0;
int size = MeasureSpec.getSize(widMeasureSpec);
int mode = MeasureSpec.getMode(widMeasureSpec);
if (mode == MeasureSpec.EXACTLY){
result = size;
}else{
result = pointerBitmap.getWidth();
if (mode == MeasureSpec.AT_MOST){
result = Math.min(result,size);
}
}
return result;
}
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
//目的是让表针初始位置为270度！
canvas.rotate(270,x/2,y/2);
canvas.drawBitmap(finalPointerBitmap,0,0,null);
}
}
然后我们在xml文件中这样写：
android:layout_width="45dp"
android:layout_height="45dp"
android:layout_margin="15dp"
android:visibility="gone" >
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center" />
android:layout_height="wrap_content"
android:src="@drawable/load_icon_dial2x" />
这样就将表盘作为背景了，我们可以操作表针来执行rotate动画
android:fromDegrees="0"
android:toDegrees="-150"
android:pivotY="50%"
android:pivotX="50%"
android:duration="1000"
android:repeatCount="infinite"
android:repeatMode="reverse"
>
mAutoHomeAnim = (PointerView) headerView.findViewById(R.id.anim_pointer);
animation = AnimationUtils.loadAnimation(context, R.anim.pointer_rotate);
//执行动画
mAutoHomeAnim.startAnimation(animation);
在listview中
由于下拉刷新核心代码和美团下拉刷新是一样的，这里我只截取不一样的部分
private void changeHeaderByState(int state){
switch (state) {
case DONE:
headerView.setPadding(0, -headerViewHeight, 0, 0);
//第一状态的view显示出来
mAutoHome.setVisibility(View.VISIBLE);
//先停止一下第二阶段view的动画
mAutoHomeAnim.clearAnimation();
//将第二阶段view隐藏起来
mAnimContainer.setVisibility(View.GONE);
break;
case RELEASE_TO_REFRESH:
tv_pull_to_refresh.setText("放开刷新");
break;
case PULL_TO_REFRESH:
tv_pull_to_refresh.setText("下拉刷新");
//第一状态view显示出来
mAutoHome.setVisibility(View.VISIBLE);
//停止第二阶段动画
mAutoHomeAnim.clearAnimation();
//将第二阶段view隐藏
mAnimContainer.setVisibility(View.GONE);
break;
case REFRESHING:
tv_pull_to_refresh.setText("正在刷新");
//将第一阶段view隐藏
mAutoHome.setVisibility(View.GONE);
//将第二阶段view显示出来
mAnimContainer.setVisibility(View.VISIBLE);
//先停止第二阶段动画
mAutoHomeAnim.clearAnimation();
//启动第二阶段动画
mAutoHomeAnim.startAnimation(animation);
break;
default:
break;
}
}
完整代码：
大家可以到我的GitHub上下载
