use JavaScript To make a real-time clock

The renderings are as follows ： Next , Step by step, I'll show you how to make , And put forward some noteworthy matters .

The first is to build up the framework ,

```<div>
<canvas id="canvas" width="500px" height="500px"></canvas>
</div>
<!--
Put one on the outside div The main function is to center the canvas , It looks better this way
-->```

Then give div and canvas The style of ：

```div{
text-align: center;
margin-top: 150px;
}
/*canvas{
border: 1px solid #000;
}*/
/* Give the canvas a border , Mainly used to adjust the position of the canvas */```

Get the width and height of the canvas , In this way, when we change later, the following content can be changed automatically , Write the code alive ,

```var canvas = document.getElementById("canvas");
var context = canvas.getContext("2d");
// Get the width and height of the canvas
var width = canvas.width;
var height = canvas.height;
// Get the radius of the circle
var r = width / 2;```

And the origin of the entire canvas (0,0) The position of , This is conducive to the calculation of the back position .

translate（x,y）; take x And y The coordinates of the points are redefined as the origin of the canvas .

`context.translate(r,r);`

After the origin has been determined, you can draw a circle

`context.arc(0,0,r - 6,0,Math.PI * 2);`
`context.arc(x,y,r,sAngle,eAngle,counterclockwise);`
Parameters describe
x The center of a circle x coordinate .
y The center of a circle y coordinate .
r The radius of the circle .
sAngle Starting angle , In radians .（ The three o'clock position of the circle of the arc is 0 degree ）.
eAngle End angle , In radians .
counterclockwise Optional . Specify whether the drawing should be counter clockwise or clockwise .False = Clockwise ,true = Anti-clockwise .

Then choose the drawing method , There are two ways to draw ：

1 context.stroke()----- Stroke

2 context.fill()----- fill

```// Stroke
context.arc(100,100,50,0,2* Math.PI);
context.strokeStyle = "#00FFFF";
context.stroke();
// fill
context.beginPath();
context.arc(300,300,50,0,2* Math.PI);
context.closePath();
context.fillStyle = "#FF00FF";
context.fill();```

design sketch ： It is worth noting that , At the beginning of the second drawing path , Have to add

context.beginPath();     and

context.closePath();

otherwise , It will affect the drawing of the following styles .

The next step is to 12 Put a number in the corresponding position of the circle , thus , We're going to calculate the... Of each position x Coordinates and y coordinate .

That's what we need Math.sin() and Math.cos() function .

``` // obtain x,y coordinate , And will 12 Write the number to the corresponding position
var hournum = [3,4,5,6,7,8,9,10,11,12,1,2];
for(var i = 0; i< hournum.length; i++){
// The angle of clockwise rotation （ Radian system ）
var rad = 2 * Math.PI / 12 * i;
var x = (r - 30) * Math.cos(rad); // obtain x coordinate
var y = (r - 30) * Math.sin(rad); // obtain y coordinate
context.fillText(hournum[i],x,y);
}
// draw 60 A little bit
for(var j = 0; j< 60; j++){
// The rotation angle of the minute hand （ Radian system ）
var rad = 2 * Math.PI / 60 * j;
var x = (r - 18) * Math.cos(rad); // obtain x coordinate
var y = (r - 18) * Math.sin(rad); // obtain y coordinate
context.beginPath();
if(j % 5 === 0){ // stay 3 6 9 12 Location , Set the color of the dot to a darker color , It can be distinguished from other colors
context.fillStyle = "#0f0f0f";
}else{
context.fillStyle = "#f0883a";
}
context.arc(x,y,3,0,Math.PI * 2);
context.closePath();
context.fill();
}```

Behind it is the hour hand , minute hand , The drawing of the second hand , The three are basically the same , I'll give you the specific drawing method of the hour hand ：

```// Draw the hour hand
function drawHour(hour,minu){
context.save(); // Save the current environment
context.beginPath();
var rad_h = 2 * Math.PI / 12 * hour;
var rad_m = 2 * Math.PI / 12 / 60 * minu; // The rotation of the hour hand is affected by the minute hand
context.lineWidth = 6; // The width of the hour hand
context.moveTo(0,7); // The starting point of the hour hand
context.lineTo(0,-r / 2); // The end of the hour hand
context.lineCap = "round"; // Set the end shape If the cap is set , There can't be a closed path
context.strokeStyle = "#000";
context.stroke();
}```

The way to draw a nail is to draw a circle , There's already a way to draw circles , I won't give a detailed introduction here .

Then we get the system time and , The call to the above function .

```// Get real time
function timer(){
context.clearRect(0,0,width,height); // Clear the previous drawing
var now_time = new Date(); // Get current system time
var hour = now_time.getHours(); // Get the current time
var minu = now_time.getMinutes(); // Get the current score
var sec = now_time.getSeconds(); // Get the current second
fun();
drawHour(hour,minu);
drawMinu(minu,sec);
drawSec(sec);
draw();
}```

Finally, set a timer , The time interval is set to 1000ms Call once , In this way, the clock is realized .

`setInterval(timer,1000);`

Here's what we should pay attention to ：

1. Application of two functions ：

save()   ----    Save the current environment state

restore()   -----    Return the saved environmental state between

2. When calculating coordinates , Of two functions "()" It's a radian system , That is to say π In the form of .

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