Fix the SDDisplayLink issue of default value, add test cases `testSDDisplayLink`

SDWebImage/Private/SDDisplayLink.m

@@ -80,14 +80,18 @@ static CVReturn DisplayLinkCallback(CVDisplayLinkRef displayLink, const CVTimeSt
 - (CFTimeInterval)duration {
 #if SD_MAC
     CVTimeStamp outputTime = self.outputTime;
-    NSTimeInterval duration = (double)outputTime.videoRefreshPeriod / ((double)outputTime.videoTimeScale * outputTime.rateScalar);
+    NSTimeInterval duration = 0;
+    double periodPerSecond = (double)outputTime.videoTimeScale * outputTime.rateScalar;
+    if (periodPerSecond > 0) {
+        duration = (double)outputTime.videoRefreshPeriod / periodPerSecond;
+    }
 #elif SD_IOS || SD_TV
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wdeprecated-declarations"
     NSTimeInterval duration = self.displayLink.duration * self.displayLink.frameInterval;
 #pragma clang diagnostic pop
 #else
-    NSTimeInterval duration;
+    NSTimeInterval duration = 0;
     if (self.displayLink.isValid && self.currentFireDate != 0) {
         NSTimeInterval nextFireDate = CFRunLoopTimerGetNextFireDate((__bridge CFRunLoopTimerRef)self.displayLink);
         duration = nextFireDate - self.currentFireDate;

Tests/Tests/SDUtilsTests.m

@@ -9,6 +9,7 @@
 
 #import "SDTestCase.h"
 #import "SDWeakProxy.h"
+#import "SDDisplayLink.h"
 #import "SDInternalMacros.h"
 
 @interface SDUtilsTests : SDTestCase
@@ -41,6 +42,37 @@
     expect([proxy.debugDescription isEqualToString:object.debugDescription]).beTruthy();
 }
 
+- (void)testSDDisplayLink {
+    XCTestExpectation *expectation1 = [self expectationWithDescription:@"Display Link Stop"];
+    XCTestExpectation *expectation2 = [self expectationWithDescription:@"Display Link Start"];
+    SDDisplayLink *displayLink = [SDDisplayLink displayLinkWithTarget:self selector:@selector(displayLinkDidRefresh:)];
+    NSTimeInterval duration = displayLink.duration; // Initial value
+    expect(duration).equal(1.0 / 60);
+    [displayLink addToRunLoop:NSRunLoop.mainRunLoop forMode:NSRunLoopCommonModes];
+    [displayLink start];
+    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
+        expect(displayLink.isRunning).beTruthy();
+        [displayLink stop];
+    });
+    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
+        expect(displayLink.isRunning).beFalsy();
+        [displayLink start];
+        [expectation1 fulfill];
+    });
+    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
+        expect(displayLink.isRunning).beTruthy();
+        [displayLink stop];
+        [expectation2 fulfill];
+    });
+    [self waitForExpectationsWithCommonTimeout];
+}
+
+- (void)displayLinkDidRefresh:(SDDisplayLink *)displayLink {
+    NSTimeInterval duration = displayLink.duration; // Running value
+    expect(duration).beGreaterThan(0.01);
+    expect(duration).beLessThan(0.02);
+}
+
 - (void)testSDScaledImageForKey {
     // Test nil
     expect(SDScaledImageForKey(nil, nil)).beNil();