Android简明开发教程十九：线程 Bezier曲线
Android中使用线程Thread的方法和Java SE相同。和大多数OS系统一样，Android中也有称为UI Thread的主线程。UI Thread 主要用来给相应的Widget分发消息，包括绘制（Drawing）事件。UI Thread 也是用来处理用户交互事件的线程。比如：如果你按下屏幕上某个按钮，UI 线程则将Touch 事件通知对应的控件（Widgets)，Widget 则将其状态设置成“按下”，并把“重绘”（Invalidate）事件发到Event Queue中去。 UI线程从Event Queue中读取事件后通知Widgets重画自身。
如果你的应用设计不好的话， UI线程的这种单线程模式就会导致非常差的用户响应性能。特别是你将一些费时的操作如网络访问或数据库访问也放在UI线程中，这些操作会造成用户界面无反应，最糟糕的是，如果UI线程阻塞超过几秒（5秒），著名的ANR对话框就会出现：
所以在设计应用时，需要把一些费时的任务使用单独的工作线程来运行避免阻塞UI线程，但是如果在工作线程中想更新UI线程的话，不能直接在工作线程中更新UI，这是因为UI线程不是“Thread Safe”。因此所有UI相关的操作一般必须在UI Thread中进行。
Android OS提供了多种方法可以用在非UI线程访问UI线程。
- Activity.runOnUiThread(Runnable)
- View.post(Runnable)
- View.postDelayed(Runnable, long)
- Handler
Bezier 示例动态显示Bezier曲线，使用了Activity.runOnUiThread 来更新屏幕，完整代码如下：
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public
class
Bezier
extends
Graphics2DActivity
implements
OnClickListener,Runnable{
/**
* The animation thread.
*/
private
Thread thread;
private
volatile
boolean
stopThread=
false
;
private
boolean
stopOrNot=
false
;
boolean
drawn;
/**
* The random number generator.
*/
static
java.util.Random random =
new
java.util.Random();
/**
* The animated path
*/
Path path =
new
Path();
/**
* Red brush used to fill the path.
*/
SolidBrush brush =
new
SolidBrush(Color.RED);
private
static
final
int
NUMPTS =
6
;
private
int
animpts[] =
new
int
[NUMPTS *
2
];
private
int
deltas[] =
new
int
[NUMPTS *
2
];
long
startt, endt;
private
Button btnOptions;
@Override
protected
void
drawImage() {
drawDemo(
100
,
100
);
}
public
void
onCreate(Bundle savedInstanceState) {
super
.onCreate(savedInstanceState);
setContentView(R.layout.beziers);
graphic2dView
= (GuidebeeGraphics2DView) findViewById(R.id.graphics2dview);
btnOptions = (Button) findViewById(R.id.btnStopStart);
btnOptions.setOnClickListener(
this
);
reset(
100
,
100
);
if
(thread ==
null
) {
thread =
new
Thread(
this
);
thread.start();
}
}
@Override
public
void
onClick(View view) {
if
(!stopOrNot){
btnOptions.setText(
"Start"
);
stopThread=
true
;
}
else
{
stopThread=
false
;
btnOptions.setText(
"Stop"
);
if
(thread ==
null
) {
thread =
new
Thread(
this
);
thread.start();
}
}
stopOrNot=!stopOrNot;
}
/**
* Generates new points for the path.
*/
private
void
animate(
int
[] pts,
int
[] deltas,
int
i,
int
limit) {
int
newpt = pts[i] + deltas[i];
if
(newpt <=
0
) {
newpt = -newpt;
deltas[i] = (random.nextInt() &
0x00000003
)
+
2
;
}
else
if
(newpt >= limit) {
newpt =
2
* limit - newpt;
deltas[i] = -((random.nextInt() &
0x00000003
)
+
2
);
}
pts[i] = newpt;
}
/**
* Resets the animation data.
*/
private
void
reset(
int
w,
int
h) {
for
(
int
i =
0
; i < animpts.length; i +=
2
) {
animpts[i +
0
]
= (random.nextInt() &
0x00000003
)
* w /
2
;
animpts[i +
1
]
= (random.nextInt() &
0x00000003
)
* h /
2
;
deltas[i +
0
]
= (random.nextInt() &
0x00000003
)
*
6
+
4
;
deltas[i +
1
]
= (random.nextInt() &
0x00000003
)
*
6
+
4
;
if
(animpts[i +
0
] > w /
2
) {
deltas[i +
0
] = -deltas[i +
0
];
}
if
(animpts[i +
1
] > h /
2
) {
deltas[i +
1
] = -deltas[i +
1
];
}
}
}
final
Runnable updateCanvas =
new
Runnable() {
public
void
run() {
int
offsetX = (graphic2dView.getWidth() -
SharedGraphics2DInstance.CANVAS_WIDTH) /
2
;
int
offsetY = (graphic2dView.getHeight()
- SharedGraphics2DInstance.CANVAS_HEIGHT) /
2
;
graphic2dView.invalidate(offsetX,offsetY,
offsetX+
100
,offsetY+
100
);
}
};
/**
* Sets the points of the path and draws and fills the path.
*/
private
void
drawDemo(
int
w,
int
h) {
for
(
int
i =
0
; i < animpts.length; i +=
2
) {
animate(animpts, deltas, i +
0
, w);
animate(animpts, deltas, i +
1
, h);
}
//Generates the new pata data.
path.reset();
int
[] ctrlpts = animpts;
int
len = ctrlpts.length;
int
prevx = ctrlpts[len -
2
];
int
prevy = ctrlpts[len -
1
];
int
curx = ctrlpts[
0
];
int
cury = ctrlpts[
1
];
int
midx = (curx + prevx) /
2
;
int
midy = (cury + prevy) /
2
;
path.moveTo(midx, midy);
for
(
int
i =
2
; i <= ctrlpts.length; i +=
2
) {
int
x1 = (curx + midx) /
2
;
int
y1 = (cury + midy) /
2
;
prevx = curx;
prevy = cury;
if
(i < ctrlpts.length) {
curx = ctrlpts[i +
0
];
cury = ctrlpts[i +
1
];
}
else
{
curx = ctrlpts[
0
];
cury = ctrlpts[
1
];
}
midx = (curx + prevx) /
2
;
midy = (cury + prevy) /
2
;
int
x2 = (prevx + midx) /
2
;
int
y2 = (prevy + midy) /
2
;
path.curveTo(x1, y1, x2, y2, midx, midy);
}
path.closePath();
// clear the clipRect area before production
graphics2D.clear(Color.WHITE);
graphics2D.fill(brush, path);
this
.runOnUiThread(updateCanvas);
}
public
void
run() {
Thread me = Thread.currentThread();
if
(!drawn) {
synchronized
(
this
) {
graphics2D.clear(Color.WHITE);
graphics2D.fill(brush, path);
graphic2dView.refreshCanvas();
drawn =
true
;
}
}
while
(thread == me && !stopThread) {
drawDemo(
100
,
100
);
}
thread =
null
;
}
}
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除了上述的方法外，Android还提供了AsyncTask类以简化工作线程与UI线程之间的通信。这里不详述。此外，上面Bezier曲线动画在屏幕上显示时有闪烁的现象，这是动态显示图像的一个常见问题，后面将专门讨论。