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本文介绍了如何在C++中使用SDL库创建一个计时器类，包括构造函数、开始、停止、暂停、恢复、重置及获取经过时间等方法的实现。通过示例代码展示了如何在程序中使用该计时器，并提供了一个简单的应用，显示计时器的运行状态和经过时间。

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#ifndef TIMER_H

#define TIMER_H

/**

* What we think our timer should look like

class Timer {

public:

Timer();

///Start the timer

void Start();

///Stop the timer

void Stop();

///Pause the timer

void Pause();

///Unpause the timer

void Unpause();

/**

* Restart the timer and return the elapsed ticks

* @return The elapsed ticks

int Restart();

/**

* Get the elapsed ticks

* @return The elapsed ticks

int Ticks() const;

///Check if Timer is started

bool Started() const;

///Check if Timer is paused

bool Paused() const;

private:

int mStartTicks, mPausedTicks;

bool mStarted, mPaused;

};

#endif

That looks pretty good, so let’s get started implementing these functions in timer.cpp, beginning with the constructor.

看起来挺好，于是我们就着手在timer.cpp里实现它吧。首先是构造函数。

When we construct a new timer we want it to be off, ie. not started or paused. We can do this like so:

当我们构造一个新计时器的时候，它应该是关闭的，或者应该说是既没有开始也没有暂停。我们可以通过以下方式实现：

Timer::Timer()

: mStartTicks(0), mPausedTicks(0), mStarted(false), mPaused(false)

{

}

In our Start function we’ll want to tag the timer as started and record the value of SDL_GetTicks when Start was called, so that we can properly return the elapsed time, which is the difference between the current SDL_GetTicks and the value of mStartTicks. In our Stop function we simply want to tag the timer as stopped, by setting mStarted and mPaused to false.

在我们的Start函数内，我们需要将计时器标记为started已经开始，并记下开始时SDL_GetTicks中的值，以便于之后能正确地返回经过的时间。而经过的时间就是当前使用SDL_GetTicks获得的时间与mStartTicks的差。在我们的Stop函数中，我们只需要通过把mStarted与mPaused设为false，将计时器标记为停止。

void Timer::Start(){

mStarted = true;

mPaused = false;

mStartTicks = SDL_GetTicks();

}

void Timer::Stop(){

mStarted = false;

mPaused = false;

}

For our Pause and Unpause functions we’ll need to do a bit more work. We wouldn’t want to pause the timer if it wasn’t started or was already paused as both operations don’t really make sense. When we pause the timer we’ll also need to store the elapsed time so that we can preserve the timer’s value so that when we unpause we continue adding on to the measured time instead of resetting it. We can do this with the method discussed above, were we take SDL_GetTicks - mStartTicks as the elapsed time, where mStartTicks is the value of SDL_GetTicks when the timer was started. So Pause should look like this:

暂停和取消暂停的函数还需要一点。如果计时器压根没有开始或者已经暂停了，我们就没必要让它暂停，因为那没有意义。当我们暂停计时的时候，我们还需要将经过的时间保存下来，以便于能恢复计时器经过的时间。这样之后在取消暂停的时候，我们可以继续在这个计时器上计数，而不是重新开始计数。我们可以用前面提到的方法做到这一点。我们用SDL_GetTicks - mStartTicks 作为经过的时间，而mSartTicks是计时器开始的时候用SDL_GetTicks 获得的时间。所以说，Pause函数应该是这样的：

void Timer::Pause(){

if (mStarted && !mPaused){

mPaused = true;

mPausedTicks = SDL_GetTicks() - mStartTicks;

}

So now mPausedTicks stores the elapsed ticks, so we can resume timing when we call Unpause. In Unpause we’ll want to mark the timer as not paused and use the value of mPausedTicks to set the value of mStartTicks to have some offset from the value of SDL_GetTicks at the time Unpause is called so that we preserve the value of the timer. We can do this by setting mStartTicks to SDL_GetTicks - mPausedTicks, so Unpause also ends up being quite simple:

所以现在，mPausedTicks里保存的是经过的时间，这样我们就可以在调用Unpause函数的时候恢复计时。在Unpause中，我们要把计时器标记为没有暂停，并把mPausedTicks的值赋给mStartTicks

void Timer::Unpause(){

if (mPaused){

mPaused = false;

mStartTicks = SDL_GetTicks() - mPausedTicks;

mPausedTicks = 0;

}

Our function Restart, to restart the timer and return the elapsed time turns out to be quit simple. The function Ticks is used to get the elapsed time, then the timer is restarted and the value is returned. This ordering is very important as we need to get the elapsed time before we reset the timer, or else we’ll return 0 every time.

我们的Restart函数，想要重新开始并返回

int Timer::Restart(){

int elapsedTicks = Ticks();

Start();

return elapsedTicks;

}

Finally we arrive at the most important function of our Timer, the one that actually tells us how much time has elapsed! We’ll only want to return a value if the timer has been started, if the timer is paused we’ll want to return the value of mPausedTicks (the elapsed time between Start and Pause), if the timer is running we’ll want to return return the elapsed time, with SDL_GetTicks - mStartTicks. If it’s not started then we’ll want to return 0, as no time has been measured.

int Timer::Ticks() const {

if (mStarted){

if (mPaused)

return mPausedTicks;

else

return SDL_GetTicks() - mStartTicks;

}

return 0;

}

We finish off our timer with some simple getters to check if the Timer is started or paused:

bool Timer::Started() const {

return mStarted;

}

bool Timer::Paused() const {

return mPaused;

}

And there we have it! A useful, simple timer. Now let’s try using it in a program to make sure it works. We’ll make a very simple program that will display the elapsed time and read some input for starting/stopping/pausing/unpausing the timer. For this program we’ll also need the Window class that we wrote in Lesson 7 to provide the various graphics functions we’ll need.

I’ll only be posting code relevant to the specifics of using the timer in the lesson, but if you have difficulty with some code that isn’t posted you can always find the full source and assets for each lesson on Github.

After opening our Window we’ll want to create an instance of the Timer class and some SDL_Textures to hold our messages.

    //Our timer:

Timer timer;

//Textures to display a message and ticks elapsed

SDL_Texture *msg = nullptr, *ticks = nullptr;

//Color for the text

SDL_Color white = { 255, 255, 255 };

//Rects for the text

SDL_Rect msgBox, ticksBox;

//Setup msg text

msg = Window::RenderText("Ticks Elapsed: ", "../res/Lesson8/SourceSansPro-Regular.ttf",

white, 30);

//Setup msg dstRect

msgBox.x = 0;

msgBox.y = Window::Box().h / 2;

//Query w & h from texture

SDL_QueryTexture(msg, NULL, NULL, &msgBox.w, &msgBox.h);

Here we also setup the message box positioning to stick a bit below the middle of the window height (recall that y is the y position of the top-left corner) and set its width and height equal to the texture’s width and height.

We also want to display the value of the timer’s Ticks function, the elapsed time, after the end of the “Ticks Elapsed: ” message. There’s one small issue though, Ticks returns an int, but we need a string to render a message with. This can be resolved by using a stringstream; we write Ticks to the stream and then pass it to the message creation function as a string, like so:

    //Setup ticks message

//We must use a stringstream to convert int to string

std::stringstream ssTicks;

ssTicks << timer.Ticks();

ticks = Window::RenderText(ssTicks.str(), "../res/Lesson8/SourceSansPro-Regular.ttf",

white, 30);

//clear the stream

ssTicks.str("");

//Setup the ticks dstRect

ticksBox.x = msgBox.w + 20;

ticksBox.y = Window::Box().h / 2;

SDL_QueryTexture(ticks, NULL, NULL, &ticksBox.w, &ticksBox.h);

We then clear the stringstream by filling it with a blank string and set the positioning of the message to be a bit after the text message.

Within our event polling loop we’ll want to check for some key presses to tell the timer to start/stop/pause/unpause as desired.

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Please, don't. https://github.com/styleguide/templates/2.0

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<html>

<head>

<title>Page not found · GitHub</title>

body {

background: #f1f1f1;

font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;

text-rendering: optimizeLegibility;

margin: 0; }

.container { margin: 30px auto 40px auto; width: 800px; text-align: center; }

a { color: #4183c4; text-decoration: none; font-weight: bold; }

h3 { color: #666; }

ul { list-style: none; padding: 25px 0; }

li {

display: inline;

margin: 10px 50px 10px 0px;

}

input[type=text], input[type=password] {

font-size: 13px;

min-height: 32px;

padding: 7px 8px;

outline: none;

color: #333;

background-color: #fff;

background-repeat: no-repeat;

background-position: right center;

border: 1px solid #ccc;

border-radius: 3px;

box-shadow: inset 0 1px 2px rgba(0,0,0,0.075);

-moz-box-sizing: border-box;

box-sizing: border-box;

transition: all 0.15s ease-in;

-webkit-transition: all 0.15s ease-in 0;

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.button {

font-family: Helvetica, sans-serif;

position: relative;

display: inline-block;

padding: 7px 15px;

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font-weight: bold;

color: #333;

text-shadow: 0 1px 0 rgba(255,255,255,0.9);

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border-bottom-color: #c5c5c5;

box-shadow: 0 1px 3px rgba(0,0,0,0.075);

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-webkit-touch-callout: none;

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user-select: none;

-webkit-appearance: none;

}

#auth {

position: absolute;

top: 0;

right: 0;

z-index: 50;

background-color: rgba(53,95,120,.4);

padding: 0 10px 5px 10px;

-webkit-border-bottom-right-radius: 10px;

-webkit-border-bottom-left-radius: 10px;

-moz-border-radius-bottomright: 10px;

-moz-border-radius-bottomleft: 10px;

border-bottom-right-radius: 10px;

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box-shadow: 0 3px 0 rgba(0, 0, 0, 0.28);

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#auth h1, #auth p, #auth label {

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.auth-form-body {

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#auth input[type=text], input[type=password] {

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margin-top: 7px;

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#auth .button {

clear: none;

margin-top: 7px;

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border: 1px solid #427696;

border-bottom-color: #386580;

}

label[for=search] {

display: block;

text-align: left;

}

#search label {

font-weight: 200;

padding: 5px;

}

#search input[type=text] {

font-size: 18px;

width: 705px;

padding: 8px;

}

#search .button {

padding: 10px;

width: 90px;

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#logo {

margin-top: 35px;

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#suggestions {

margin-top: 35px;

color: #ccc;

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#suggestions a {

color: #666666;

font-weight: 200;

font-size: 14px;

margin: 0 10px;

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#parallax_wrapper {

position: relative;

z-index: 0;

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#parallax_field {

overflow: hidden;

position: absolute;

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width: 100%;

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#parallax_field #parallax_bg {

position: absolute;

top: -20px;

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height: 425px;

z-index: 1;

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#parallax_illustration {

display: block;

margin: 0 auto;

width: 940px;

height: 370px;

position: relative;

overflow: hidden;

clear: both;

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#parallax_illustration img {

position: absolute;

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#parallax_illustration #parallax_error_text {

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z-index: 10;

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#parallax_illustration #parallax_octocat {

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z-index: 9;

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#parallax_illustration #parallax_speeder {

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#parallax_illustration #parallax_octocatshadow {

top: 297px;

left: 371px;

z-index: 7;

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#parallax_illustration #parallax_speedershadow {

top: 263px;

left: 442px;

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#parallax_illustration #parallax_building_1 {

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z-index: 5;

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#parallax_illustration #parallax_building_2 {

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z-index: 4;

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</style>

</head>

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<img alt="building" class="js-plaxify" data-invert="true" data-xrange="0" data-yrange="20" height="415" id="parallax_bg" width="940"

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<img alt="land speeder's shadow" class="js-plaxify" data-xrange="10" data-yrange="10" height="75" id="parallax_speedershadow" width="430"

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<img alt="building" class="js-plaxify" data-invert="true" data-xrange="50" data-yrange="20" height="123" id="parallax_building_1" width="304"

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<img alt="building" class="js-plaxify" data-invert="true" data-xrange="75" data-yrange="30" height="50" id="parallax_building_2" width="116"

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